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 A PHYSIOLOGICAL STUDY OF THE CLI 
 
 MATIC CONDITIONS OF MARYLAND 
 
 AS MEASURED BY PLANT 
 
 GROWTH 
 
 (A second contribution from data obtained under the auspices of the 
 Maryland State Weather Service, in 1914) 
 
 Dissertation submitted to the Board of University 
 
 Studies of the Johns Hopkins University in 
 
 conformity with the requirements 
 
 for the degree of Doctor of 
 
 Philosophy 
 
 By 
 F. MERRILL HILDEBRANDT 
 
 
 in 
 
 BALTIMORE 
 June, 1917 
 
 IRbprinted from Physiological Reseaeches, 2: 341-405. 19211 
 
A PHYSIOLOGICAL STUDY OF THE CLI 
 
 MATIC CONDITIONS OF MARYLAND 
 
 AS MEASURED BY PLANT 
 
 GROWTH 
 
 (A second contribution from data obtained under the auspices of the 
 Maryland State Weather Service, in 1914) 
 
 Dissertation submitted to the Board of University 
 
 Studies of the Johns Hopkins University in 
 
 conformity with the requirements 
 
 for the degree of Doctor of 
 
 Philosophy 
 
 By 
 MERRILL HILDEBRANDT 
 
 BALTIMORE 
 
 June, 1917 
 
 I&EPRINTED FROM PHYSIOLOGICAL RESEARCHES, 2: 341-405. 1921] 
 
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A PHYSIOLOGICAL STUDY OF THE CLIMATIC CONDITIONS OF 
 MARYLAND, AS MEASURED BY PLANT GROWTH 1 
 
 A Second Contribution from Data Obtained Under, the Auspices of 
 the Maryland State Weather Service, in 1914 
 
 F. MERRILL HILDEBRANDT 
 
 ABSTRACT 2 
 
 The present paper presents the results obtained from a study of a series of obser- 
 vations on the climatic complexes for nine different stations in Maryland for the sum- 
 mer of 1914, as the effectiveness of each complex was automatically integrated by soy- 
 bean plants grown for a period of 4 weeks from the seed, new seeds being planted every 
 2 weeks. Corresponding instrumental observations were also studied. 
 
 The field work was carried out by Dr. Forman T. McLean, under the joint auspices 
 of the Maryland State Weather Service and the Laboratory of Plant physiology of the 
 Johns Hopkins University. McLean 3 has presented an account of the plan and methods 
 by which the observational data were obtained and he also made a thorough study of 
 the growth data for soy-bean, for the stations at Oakland and Easton, his main results 
 and conclusions having been set forth in his publications. 
 
 The study with which the present paper deals involved all the climatic and soy-bean 
 data obtained by McLean; it thus included data for nine different localities well dis- 
 tributed throughout the state. The plants for all stations and for all periods were 
 treated practically alike, excepting for the climatic conditions of the several localities. 
 The same soil was used for all cultures. The values obtained from the plant measure- 
 ments, made after 2 and after 4 weeks of growth, therefore constitute comparative and 
 quantitative descriptions of the total influence exerted by all the climatic conditions 
 upon the plants, and they exhibit the seasonal march of this climatic resultant in terms 
 of the responses of the different sets of cultures. The plant measurements themselves, 
 and the values derived from them, form perhaps the most important contribution in 
 the present paper. The graphs of these values depict the seasonal march of each cli- 
 matic complex, not after the manner of the instrumental readings usually employed in 
 the study of climate, but as measures of the effectiveness of the climatic complex to 
 favor or retard the growth of the standard plant, the latter being employed as a "living 
 instrument" or indicator of climatic effectiveness. 4 
 
 The plant measurements employed are: (1) stem height, (2) leaf area, (3) leaf- 
 product (length multiplied by width), (4) dry weight. The climatic values used are: 
 (1) air temperature, (2) the evaporating power of the air, (3) sunlight intensity and 
 duration. Values to represent the temperature efficiency for growth are derived from 
 the temperature records by means of the Livingston physiological temperature indices 
 
 1 Botanical contribution from the Johns Hopkins Hospital University, no. 60. This work was practically 
 completed in 1917, but war conditions delayed its publication. 
 
 2 This abstract was preprinted, without change, from these types and was issued as Physiological Researches 
 Preliminary Abstracts, vol. 2, no. 8, May, 1921. 
 
 3 McLean, Forman T. A preliminary study of climatic conditions in Maryland, as related to plant growth 
 Physiol. Res. 2: 129-208. 1917. A brief account had appeared earlier: — Idem. Relation of climate to plant 
 growth in Maryland. Monthly Weather Rev. 43: 65-72. 1915. 
 
 * Livingston, B. E-, and McLean, F. T. A living climatological instrument. Science 43: 362. 1916. 
 
 341 
 
 PHYSIOLOGICAL RESEARCHES, VOL. 2, NO. 8 
 SERIAL NO. 18, MAY, 1921 
 
342 F. Mebbill Hildebeandt 
 
 All of the measurements, both plant and climatic, are expressed relatively, so that they 
 may be compared for different stations and for different periods. 
 
 Considering the soy-bean plant (as here employed) as a standard plant or indicator 
 for the measurement- of climatic efficiency to produce plant growth, it appears that, 
 for the season of 1914, the climatic complexes for some culture periods were more efficient 
 in this sense than might be expected from an attempt to interpret the corresponding 
 climatic data, while the complexes for other periods were actually less efficient to pro- 
 duce soy-bean growth than might be surmised from the corresponding climatic values. 
 No successful method has yet been brought forward by which the value of a climatic 
 complex, to produce growth in any plant form, may be deduced from instrumental 
 data, and the plant measurements of this study furnish a means by which climatic 
 efficiency as a whole may be directly compared for different periods and for different 
 stations. The seasonal means derived from the plant data give relatively low efficiency 
 values for the climatic complexes of the three western stations (Oakland, Chewsville 
 and Monrovia), high values for the complexes of Baltimore and Darlington, and inter- 
 mediate values for the complexes of the remaining stations (College, Coleman, Easton, 
 and Princess Anne). 
 
 If indices for total seasonal climatic efficiency are derived by multiplying the seasonal 
 average growth rate per day by the normal length (days) of the growing season for the 
 station in question, these indices have the following values for the several stations: 
 Oakland, 9009; Chewsville, 12480; College, 16867; Easton, 17688; Princess Anne, 19005; 
 Coleman, 21115; Darlington, 236SS; Baltimore, 25422. In considering these relative 
 climatic indices it should be emphasized that the data of this study do not involve pre- 
 cipitation as an influential climatic feature; the culture plants were automatically 
 irrigated so that they never suffered from lack of soil moisture. 
 
 A study was made of the interrelations of the different kinds of plant measurements, 
 dealing therefore with some aspects of growth correlation in soy-bean. It appears 
 that the rate of stem elongation was greater than the rate of leaf expansion when both 
 were relatively small, while the former rate was the smaller of the two when both were 
 relatively large. The rate of production of dry weight appears to have been nearly pro- 
 portional to the rate of increase of leaf surface ; the relative values of these two growth 
 criteria are generally about equal numerically. 
 
 The statements just made apply to the data for plants quite openly exposed, but 
 some observations on cultures somewhat protected above by glass were available, and 
 also a set of observations on cultures in forest at Baltimore. These all indicate that 
 the height rate was relatively greater than the rate of leaf expansion for these more or 
 less shaded conditions, while the rate of dry-weight production was smaller than the 
 corresponding rate of leaf expansion. The outcome of this part of the study may throw 
 some light on the general problem as to what sort of plant measurements may be best 
 suited to quantitative comparisons of the efficiencies of different climatic complexes. 
 An interesting, and probably valuable result of this study is that the calculated leaf- 
 product (length times breadth, which can be obtained without injury to the plants, if 
 that is desirable) is generally proportional to the leaf area; of course for these soy-bean 
 plants. 
 
 The climatic data themselves showed a pronounced general agreement between the 
 graph for sunlight and the corresponding one for evaporation (standardized white 
 cylindrical porous-cup atmometer), this being probably due to the relatively great 
 importance of solar radiation in determining the evaporation rate. The climatic 
 values indicate a general seasonal march, which is very evident for temperature, less 
 so for sunlight and rather obscure for evaporation. For details regarding the climatic 
 values, as well as for the plant values, reference must be made to the tables and graphs 
 and to the text of the paper. 
 
Climatic Conditions op Maryland 343 
 
 CONTENTS 
 
 Introduction 344 
 
 The observational data and the averages derived from them 
 
 The plant measurements 349 
 
 The climatic measurements 352 
 
 Temperature 353 
 
 Light 357 
 
 Evaporation 359 
 
 Results and discussion 
 
 Introductory 360 
 
 Results from stations in the open 
 The 2-week values 
 The 2-week plant data for stations in the open 
 
 Correlations between the 2-week plant graphs '. . 371 
 
 Trends of the 2-week plant values and their seasonal ranges for the sev- 
 eral stations 376 
 
 The 2-week climatic data for stations in the open 377 
 
 The 2-week temperature data 378 
 
 Chewsville and Monrovia 378 
 
 Baltimore, Darlington and Coleman 379 
 
 Easton and Princess. Anne 379 
 
 College 379 
 
 Oakland 379 
 
 The generalized graph 380 
 
 Light and the evaporating power of the air, 2-week data 380 
 
 Variability of temperature and evaporation values 382 
 
 Correlation of the 2-week plant and climatic values 383 
 
 The 4-week values 385 
 
 The 4-week plant data for stations in the open 386 
 
 The 4-week climatic data for stations in the open 390 
 
 Results for the three covered stations 
 
 Introductory 391 
 
 The plant data, covered stations 391 
 
 The Oakland covered station 393 
 
 The Baltimore covered station 394 
 
 The Easton covered station 394 
 
 The climatic conditions, covered stations , 394 
 
 Results for the Baltimore forest station 395 
 
 The plant data as measures of the climatic efficiency for growth of the standard 
 plants 
 
 Introductory 396 
 
 Seasonal averages of mean daily intensity values for the several stations. . . . 398 
 
 Total seasonal efficiencies for the several stations 403 
 
 General conclusion 405 
 
344 F. Merrill Hildebrandt 
 
 INTRODUCTION 
 
 During the summer of 1914 an elaborate investigation was undertaken by 
 the Maryland State Weather Service in cooperation with the Laboratory of 
 Plant Physiology of the Johns Hopkins University, with the object of ascer- 
 taining some of the relations between climatic conditions and the growth of 
 certain plants exposed at different stations in Maryland. Detailed informa- 
 tion as to the growth of the plants used is of course necessary in such a study, 
 as is also corresponding knowledge of those environmental conditions that 
 are considered as climatic. The plant records were secured in this case by 
 growing cultures of certain plants under the environmental conditions to be 
 studied, and noting the amount of growth accomplished during definite peri- 
 ods of time. In order that a corresponding series of measurements of some 
 of the environmental conditions might be available for comparison with 
 these growth measurements, the cultures were located at certain of the 
 regular observation stations of the U. S. Weather Bureau, at nine different 
 places in the state. The general plan of the study and a detailed considera- 
 tion of the methods used has already been published by McLean, 5 who did 
 all of the field work personally. The original data dealt with in the present 
 paper were secured from McLeans' records, and it will be necessary to give 
 here only so much description of the ways in which these measurements were 
 obtained as is needed to render their use in the present publication intelligible. 
 The following description is taken mainly from McLean's paper, which also 
 deals with the growth of soy-bean plants, but for only two of the stations, 
 Easton and Oakland. The present paper gives the main results for soy-bean 
 plants for all of the nine stations, together with some attempts at interpreta- 
 tion. 
 
 This study has been carried out partly through financial aid furnished by 
 the Maryland State Weather Service. It was suggested by Prof. B. E. Liv- 
 ingston and carried out under his direction. The writer wishes to express 
 his indebtedness to Prof. Livingston for much assistance in carrying out the 
 study and for helpful criticism in the preparation of manuscript. The writer 
 also wishes to thank Dr. H. E. Pulling for valuable suggestions made during 
 the course of the study. 
 
 The stations employed were Oakland, Chewsville, Monrovia, College Park, 
 Baltimore, Darlington, Coleman, Easton, and Princess Anne. One station, 
 Oakland, is in the Allegheny plateau. Four stations are in the Piedmont 
 plateau; one of these (Chewsville) being in the Hagerstown valley, two (Darl- 
 ington and Monrovia) in the hilly country north and west of Baltimore, and 
 one (Baltimore) at the lower edge of the plateau near Chesapeake bay. Four 
 stations, College Park, Coleman, Easton and Princess Anne, are in the coastal 
 
 6 McLean, F. T. A Preliminary study of climatic conditions in Maryland, as related to plant growth. 
 Physiol. Res. 2: 129-208. 1917. 
 
Climatic Conditions of Maryland 345 
 
 plain. Coleman, Easton, and Princess Anne, are east of Chesapeake bay, 
 while College Park is west of it and much farther inland, near the line of de- 
 marcation between the coastal plain and the Piedmont plateau. All of the 
 stations except Oakland are at comparatively low elevations — less than 310 
 meters (1000 feet) above sea-level. Oakland has an elevation of 775 meters 
 (2500 feet). The geographical distribution (see fig. 1) of these stations is 
 such as to insure considerable differences in climatic conditions. 
 
 At each of the nine places referred to, a series of cultures was grown in 
 the open with no covering other than a screen of large-meshed wire netting. 
 These have been termed the exposed stations. Also, at Oakland, Baltimore, 
 and Easton series of cultures were grown under glazed cold-frame sash sup- 
 ported horizontally 1 meter (3.3 feet) above the surface of the soil. These 
 have been termed the covered stations. The plants were freely exposed at 
 the sides, the sash being supported merely by corner posts. They were 
 located within a very few meters of the enclosures containing the plants of 
 the exposed stations and were subjected to the same climatic conditions as 
 the exposed plants, except in so far as these conditions were modified by the 
 glass covers. Furthermore, a series of cultures was exposed in the woods 
 hear the Laboratory of Plant Physiology of the Johns Hopkins University at 
 Baltimore. This has been termed the Baltimore forest station. As in the 
 case of the exposed stations, these forest cultures were covered by a pro- 
 tective screen of wire netting. They were, of course, subjected to a complex 
 of climatic conditions quite different from those of the exposed and covered 
 stations at Baltimore. The forest station was distant about 150 meters (490 
 feet) from the exposed and covered stations, at Baltimore. There are thus 
 plant data. available from 13 series of cultures in all, each series having been 
 exposed to a different set of environmental conditions throughout the season. 
 
 The cultures were so planned that the plants might be considered as stand- 
 ard plants for the comparative measurement of climatic conditions in accord- 
 ance with a suggestion made by Livingston and McLean. 6 Since the prob- 
 lem of expressing plant growth in terms of the intrumental measurements of 
 the climatic conditions that control it is rendered exceedingly complex by the 
 number of these conditions and by their continual variation, as well as by 
 the changing internal conditions of the plant itself, a detailed analysis of 
 the control of plant growth is very difficult. Livingston and McLean sug- 
 gest that the rate of growth of any plant is itself an expression of the summa- 
 tion of all the effects of the external conditions acting during the growing 
 period, and that a standard plant may be employed as an automatically weight- 
 ing, integrating, and recording instrument for the comparative measurement 
 of environmental conditions as these are effective in growth control. Thus 
 several environments may be measured and compared in terms of their sev- 
 
 6 Livingston, B. E., and McLean, F. T. A living climate-logical instrument. Science 43: 362. 1916, 
 
346 
 
Climatic Conditions of Maryland 347 
 
 eral capacities for producing growth in the standard plant. This method of 
 measuring the environmental complex in terms of plant growth can be applied, 
 of course, only when it may be assumed that all the standard plants are alike 
 at the beginnings of the several periods of exposure. In this study the re- 
 quirement just stated was fulfilled by employing the seed as the starting point 
 for the plants of the various cultures. It was apparent that if the cultures 
 were always started from the seed, the plants might be considered as more 
 nearly alike at the beginning of the several culture periods than would have 
 been the case if an attempt had been made to obtain like plants in any other 
 phase of their development. 
 
 Growth rates were measured and compared in terms of size and weight of 
 the plants. Each culture consisted of six plants grown for a period of four 
 weeks from, the seed. Cultures were started approximately every two weeks 
 during the growing season, at each of the stations employed, and growth 
 measurements were made after about two weeks and again after about a 
 month. The plants were harvested at the end of the longer period. 
 
 As in other problems in which a number of conditions enter into the control 
 of a process, the relations between conditions and process rate are more easily 
 detected the smaller is the number of conditions involved, and conditions 
 may be temporarily left out of consideration if they are the same in several 
 experiments. Just as the internal conditions of the standard plant at the 
 start of the experiment are left out of the argument by the simple device of 
 having them all alike at the beginning of the exposure period (the instrument 
 being thus set at the zero of its scale, in the words of Livingston and McLean), 
 so selected conditions of the surroundings may be left out of consideration by 
 having them alike throughout all of the periods. According to this principle, 
 most of the environmental conditions that acted on the plants below the soil 
 surface were kept nearly the same for all times and at all stations. Assuming 
 that this artificial control of the subterranean environmental conditions kept 
 them practically constant, the differences observed in the growth rates of 
 the standard plants were taken to be related almost entirely to the aerial 
 conditions of the surroundings. These are the ones referred to by McLean 
 as climatic, and this term will be used with the same meaning in the present 
 paper. To accomplish this control of the subterranean conditions, the soil 
 was always the same at the beginning of all cultures and its moisture content 
 was generally kept approximately the same throughout all culture periods, 
 by means of the Livingston auto-irrigator. This arrangement and its opera- 
 tion have been described by McLean and will also receive some attention 
 below. 
 
 While several different plant species were employed throughout the experi- 
 mental work, the present paper deals only with the data obtained from soy- 
 bean. A variety of this plant called "Peking," was used. The seed was of 
 pure strain obtained from the 1913 crop of the Maryland Agricultural Experi- 
 
348 F. Merrill Hildebrandt 
 
 ment Station. All the seeds were first treated with carbon bisulphide vapor 
 for one week, to destroy insects, after which they were placed in paraffined 
 paper cylinders with tight-fitting covers and stored until ready for use. 
 
 A rather light soil, of the type classified as Norfolk Sand by Bonsteel, 7 
 obtained from an untilled field near College Park, Maryland, was used in 
 all of the plant cultures. This soil was chosen for three reasons: first, its 
 water-holding power is comparatively low, so there was little danger of long- 
 continued excessive water-logging after a rain ; second, it was not a very fertile 
 soil and yet was capable of giving good growth under favorable climatic con- 
 ditions; and finally, it was conveniently obtained. The method of obtaining 
 it was to remove the top soil to a depth of 15 cm. from a small area of the 
 field. This top soil was then thoroughly mixed, sifted, and placed in cloth 
 sacks for shipment to the various stations where it was stored in air-dry 
 condition in covered, water-tight, galvanized iron cans until needed for use 
 in the cultures. The soil containers for the cultures were ordinary "6 inch" 
 porous clay flower-pots, in form like the frustrum of a cone, being smaller at 
 the bottom, and of a cubic capacity of approximately 1980 cc. 
 
 In order to secure uniform soil conditions in the various cultures, it was 
 necessary, not only that the soil should be of the same character in all of 
 them, but also that it should be brought into the same physical condition for 
 the beginning of all cultures. Furthermore, it was desirable that this physical 
 condition be such that it might be retained with as little change as possible 
 during the growth periods of the plant. To put the soil into a state of aggre- 
 gation to be least altered by varying weather conditions (especially heavy 
 rains which pack it more or less) it was saturated with water immediately 
 after being put into pots. This was accomplished by plunging the filled 
 pots into a bucket of water and allowing them to remain submerged until 
 air bubbles ceased to rise. The pots were then drained and a two-week inter- 
 val was allowed to intervene before the seeds were planted. 
 
 The soil moisture in the cultures was maintained always above a certain 
 minimum by means of auto-irrigators. 8 This device, as here used, consisted 
 of two cylindrical porous clay cups (of the regular form supplied by the "Plant 
 World") connected with each other and with a water reservoir by glass tubes 
 each having the form of an inverted J. The cups were placed vertically in 
 the pot, their rubber-stoppered tops level with the soil surface, and were so 
 arranged as to supply water to the soil against a pressure of 35 cm., or some- 
 what more, of a water column. The moisture content of the soil was thus 
 maintained so that it was never less than about 10 or 11 per cent on the basis 
 
 'Bonsteel, J. A. The Soils of Prince George's County [Maryland]. Maryland Geological Survey: Balti- 
 more, 1911. 
 
 8 Livingston, B. E. A method for controlling plant rrioisture. Plant World 11: 39-40. 1908. Livingston, 
 B. E. Auto-irrigation of pots of soil for experimental cultures. Carnegie Inst. Washington Year Book 14: 76. 
 1916. 
 
Climatic Conditions of Maryland 349 
 
 of dry weight. With this water content this particular soil was rather too 
 wet than too dry for the best growth of the plants. 
 
 After preparing the pots and arranging the watering devices the pots were 
 allowed to remain fallow for about two weeks before planting, as has been 
 mentioned. Thus the soil was fully drained after the preliminary saturation 
 and had settled into a condition somewhat approaching that of structure 
 equilibrium, before the seeds were planted. The seeds were placed 2.5 cm. 
 deep, six seeds in each pot. Care was taken to space them uniformly from 
 each other, from the auto-irrigator cups and from the sides of the pots, so 
 that all should have, as nearly as possible under the general conditions of the 
 experiments, the same soil moisture conditions. When the plants were 
 removed from a pot (about six weeks after that pot had been filled) the soil 
 was discarded and fresh soil from the stored supply was used in refilling for the 
 next following culture. 
 
 THE OBSERVATIONAL DATA AND THE AVERAGES DERIVED 
 
 FROM THEM 
 
 The Plant Measurements 
 
 The first plant measurements were made after approximately two weeks of 
 growth from the seed. At that time the length of each leaflet, from the tip 
 to the junction of blade and petiole, was determined, as was also the greatest 
 width of each leaflet, measured at right angles to the long axis. The height 
 of each plant was also measured, from the soil surface to the base of the termi- 
 nal bud. At the end of approximately four weeks of growth the height 
 measurement was repeated, after which the plants were cut off at the soil 
 surface and photographic prints were prepared of the fresh leaves. By 
 means of these leaf-prints the leaf area (one side) was afterwards determined 
 planimetrically, for each leaf and for each plant. Finally, the dry weight of 
 tops for each culture was determined. All linear measurements were made 
 to the nearest millimeter, areal measurement to the nearest 0.1 sq. cm., and 
 weight measurements to the nearest 0.01 g. 
 
 For convenience, these five kinds of observational data on growth rates are 
 summarized below. 
 
 After about 2 weeks of growth: — 
 
 (1) Stem, height, millimeters. 
 
 (2) Length and breadth of all leaflets, millimeters. 
 After about 4 weeks of groivth: — 
 
 (1) Stem height, millimeters. 
 
 (2) Leaf area (one side), square millimeters. 
 
 (3) Dry weight (of tops), milligrams. 
 
350 F. Merrill Hildeerandt 
 
 Of course the growth data obtained by actual observation require averag- 
 ing in some way so as to represent the various periods throughout the season. 
 The ordinary method of averaging was applied in all cases, but the observa- 
 tional data for leaflet dimensions required special preb'minary treatment. 
 The two dimensions derived from the measurements were multiplied together 
 for each leaflet, giving the leaflet-product and these products were summed to 
 give the total leaf-product of each plant. As McLean pointed out, the mean 
 daily rate of increase in total leaf-product for a period of 4 weeks is very nearly 
 proportional to the corresponding rate of increase in actual leaf area, and it 
 seems safe to suppose, as McLean did, that the 2-week leaf-product values 
 may be regarded as indices of increase in the area of the leaves. These 
 product values are therefore used, for the 2-week periods of this study, as 
 indicators of the rates of leaf expansion. Leaf areas were of course not deter- 
 mined for the 2-week periods, although they are available for the 4-week 
 periods. 
 
 There were six plants started in each culture, but in many cases the num- 
 ber of plants from which records were actually taken was less than six (it 
 was never less than three and was usually four or five in such cases), on account 
 of noticeable injury due to other conditions than the ones here studied, such 
 as insect attack, etc. All plant data have therefore been reduced to averages 
 per plant. Also, in many cases the length of the period differed slightly from 
 14 days for the 2-week periods, and from 28 days for the 4-week periods, and 
 the averages per plant have consequently been expressed as mean daily values 
 for the respective periods. This method renders the plant measurements for 
 the different periods more strictly comparable. It should be noted, however, 
 that the growing periods were 14 and 28 days long in the majority of cases, 
 and that variations in the length of the culture period were slight. Consid- 
 ering the 2-week and 4-week plant values as measures of the results of plant 
 processes acting through these periods, the mean daily values represent mean 
 daily increments or process rates, and they will be termed "daily increments," 
 for their respective periods, in the discussion that follows. Thus, for a 
 plant 10 cm. high at the end of a 13-day period, 10/13 (or 0.77) cm. is regarded 
 as the mean daily increment of increase in height, for that period. Letting 
 the word growth represent the particular process to which the given kind of 
 measurement refers (as increase in height, increase in leaf area, etc.), these 
 may be spoken as growth increments. 
 
 The mean daily growth increments, and also the mean daily climatic 
 values, for the respective periods have been expressed in terms of the corre- 
 sponding average of all the periods considered, for all exposed stations. This 
 procedure renders all the values directly comparable. To obtain this unit 
 for any land of value, all of the corresponding values (as all 2-week daily 
 mean increments in height, for example, for all exposed stations) were summed 
 and the sum was divided by the number of values summed. Then each indi- 
 
Climatic Conditions of Maryland 351 
 
 victual value was divided by the unit thus obtained. The data were expressed 
 as these ratio values, which will be termed relative values in the following 
 discussion. To avoid decimals, these relative values have all been multiplied 
 by 100, and are thus given in the tables and in the text of this paper. The 
 absolute magnitude of the unit used for expressing each kind of value is of 
 course not important; it is essential only that all comparative values be ex- 
 pressed in terms of the same unit. The unit here employed represents in 
 every case simply the average of all similar quantities that are used in the 
 present study. If another station had been employed, or if the season had 
 been longer or shorter at any station, the values of these comparative units 
 would have been different. The magnitudes of these units thus depend to 
 some extent upon the climatic conditions encountered at the various stations 
 in the summer of 1914, to some extent upon the number and location of the 
 stations, to some extent upon the nature of the soil used in this investigation, 
 and to some extent upon the physiological nature of the soy-bean plant. 
 The actual values of the various growth units are given below. 
 For first 2 weeks of growth from seed 
 
 Average daily increment of stem height per plant, 3.56 mm. 
 
 Average daily increment of leaf -product per plant, 112.0 sq. mm. 
 For first 4 weeks of growth from seed 
 
 Average daily increment of stem height per plant, 3.20 mm. 
 
 Average daily increment of leaf area per plant, 122.0 sq. mm. 
 
 Average daily increment of dry weight (of tops) per plant, 6.29 g. 
 The use of the relative values described above simplifies the plotting of 
 the graphs upon which interpretation of such a study as this so largely de- 
 pends. It also renders possible a direct comparison between the values for 
 any two cultures irrespective of their dates or stations. Furthermore, it is 
 possible to tell from the magnitude of the relative value for any culture the 
 extent to which the plant, or climatic, measurement under consideration 
 departs from the mean of that measurement for all the cultures of the study. 
 To obtain the original or absolute plant,- or climatic, value from a given 
 relative value, it is necessary only to reverse the arithmetical procedure by 
 which the relative value was derived. For example, suppose it is desired to 
 get the actual mean daily rate of increase in leaf-area per plant for the four- 
 week period ending September 2, and for the station at Coleman. The relative 
 value given in the table is 108. The first operation is to divide by 100, which 
 gives 1.08 as the true relative value. The average daily increment in leaf 
 area per plant, for the period and station in question, was therefore 1.08 times 
 the value of the common unit employed for comparing the rates of increase 
 in leaf area. Multiplying this unit value (122 sq. mm., as given above), by 
 1.08 gives 132.0 sq. mm. as the actual mean daily increment required. To 
 obtain the average total leaf area per plant at the end of the period in ques- 
 tion, we multiply 132.0 sq. mm. by the number of clays in the period (28 in this 
 
352 F. Merrill Hildebrandt 
 
 case) and get 3698 sq. mm. Since there were 5 plants measured in this cul- 
 ture, the total leaf area for the entire culture at the end of the period is ob- 
 tained by multiplying 3698 sq. mm. by 5, which gives 18490 sq. mm. or 184.9 
 sq. cm., which is the actual areal value determined from the prints of these 
 leaves. All of the original absolute values may be obtained from the rela- 
 tive ones in a similar manner. It is of course evident from the above de- 
 scription of the manner in which the relative values have been derived that 
 they are proportional to the corresponding absolute values. In all subsequent 
 discussion, when plant (and also climatic) values are referred to, it will be 
 understood that these are the relative values rather than the absolute ones. 
 As in the case of the observational data, the five kinds of derived growth 
 values used in this study are listed below, for convenience. 
 After about 2 weeks pf growth from seed 
 
 (1) Relative mean daily increment of stem height per plant. 
 
 (2) Relative mean daily increment of total leaf-product per plant. 
 After about 4- weeks of growth from seed 
 
 (1) Relative mean daily increment of stem height per plant. 
 
 (2) Relative mean daily increment of total leaf area per plant. 
 
 (3) Relative mean daily increment of dry weight (tops) per plant. 
 
 The Climatic Measurements 
 
 The weather observations taken by the cooperative observers at the sev- 
 eral stations here employed consisted of daily readings of maximum and 
 minimum thermometers, daily ocular observations of cloudiness, daily meas- 
 urements of rainfall, and general notes as to storms, wind, etc. The records 
 of the Baltimore office of the Weather Bureau were used for the Baltimore 
 stations. In addition to these records of the weather observers, evaporation 
 was measured by means of Livingston standardized cylindrical porous cups 
 with non-rain absorbing mountings. 9 Of the five sets of climatic observa- 
 tions mentioned above, only three will be considered in this paper; namely 
 those of temperature, light, and evaporation. As was pointed out by McLean, 
 rainfall showed little or no relation to the growth of these plants, as was in- 
 deed to be expected, since the soil moisture of the cultures was always kept 
 sufficiently high (by the auto-irrigators) for the needs of the plants. Also, 
 the miscellaneous climatological observations reported by the weather observ- 
 ers will not be considered in this paper, since none of them have been found 
 to bear any discoverable relation to the growth rates of these plants. The 
 
 'Livingston, B. E. A rain-correcting atmometer for ecological instrumentation. Plant World 13: 79-82. 
 1910. — Livingston, B. E., and Shive, J. W. The non-absorbing atmometer. Carnegie Inst. Washington Year 
 Book 13: 93-94. 1915. — Shive, J. W. An improved non-absorbing porous cup atmometer. Plant World 18: 7-10. 
 1915. — Johnston, E. S. A simple non-absorbing atmometer mounting. Plant World 21: 257-260. 1918. — Liv- 
 ingston, B. E., and Thone, Frank. A simplified non-absorbing mounting for porous porcelain atmometers. 
 Science 52: 85-87. 1920. 
 
Climatic Conditions of Makyland 353 
 
 observational data were obtained daily throughout the entire season. For 
 convenience, these three kinds of observational data on climatic conditions 
 are listed below. 
 
 (1) Temperature. 
 
 Daily maximum and minimum air temperature (shade), de- 
 grees, Fahernheit. 
 
 (2) Light. 
 
 Daily light condition, whether clear, partly cloudy, or cloudy. 
 
 (3) Evaporation. 
 
 Daily evaporation from standardized cylindrical porous-cup 
 
 atmometer, cubic centimeters. 
 
 The climatic data, like the plant data, require special treatment before 
 
 they can be used in such a study as this. How the mean values for the 
 
 various periods were secured from the observational data will be described, 
 
 for each of the three kinds of climatic measurements, in the following sections. 
 
 TEMPERATURE 
 
 It is clear that the readings of a thermometer do not express the effective- 
 ness of various degrees of temperature to accelerate or retard plant growth, 
 and it therefore becomes desirable to replace the actual thermometer readings 
 by a series of weighted values, more or less directly proportional to the tem- 
 perature effect upon the growth of plants. Owing to lack of information of 
 a quantitative nature as to the relation between plant growth and environ- 
 mental temperature, this can be accomplished only in a tentative and approxi- 
 mate way at the present time. 
 
 The observational data for temperature were all obtained from maximum 
 and minimum thermometers read daily at sunset, these data being taken 
 from the published monthly reports of the U. S. Weather Bureau. 10 The 
 mean temperature for each day was determined by averaging the maximum 
 and minimum for that day. McLean has discussed some of the ways in 
 which daily maximum and minimum temperature data may be treated in 
 order to obtain weighted values that may tentatively represent temperature 
 effects upon plant growth rates. He emphasizes the fact that temperature 
 values, as shown by a thermometer, do not show a linear proportionality to 
 plant growth. If thermometer readings might be taken as expressing, even 
 in an approximate way, the effectiveness of temperature to produce plant 
 growth, such a relation could only be true up to the optimum temperature, 
 since beyond tins point increased temperature results in decreased growth. It 
 would therefore be desirable to replace each thermometer reading by an 
 index representing the effectiveness of that particular temperature for plant 
 
 10 Fassig, O. L. Climatologieal data, Maryland and Delaware Section. May to November, inclusive. U. S. 
 Weather Bureau. 1914. 
 
354 F. Merrill Hildebrandt 
 
 growth. Three ways of doing this, all of which have been considered by 
 McLean, may receive brief mention here. 
 
 (1) One way of expressing temperature, which has been used in ecological 
 studies, has been called the remainder-summation method. This is based on 
 the supposition that the growth activities of most plants stop when the 
 temperature falls below 40° F. 11 Above this temperature, growth increases 
 with increased temperature, to an optimum. For convenience, the growth 
 rate for 40°F. may be considered as unity: then it should be 2 for 41°, 5 for 
 44°, 20 for 59°, etc. If we subtract 39° from any given temperature, the 
 remainder will represent, according to this method, the efficiency of the given 
 temperature for producing growth. A total efficiency value for any period 
 of time, such as the 4-week growth periods of these studies, may be obtained 
 by subtracting 39° from each daily mean temperature and summing the 
 remainders for the period, thus obtaining the remainder summation. 
 
 (2) Another method of weighting temperature values for the purpose 
 before us, and one that apparently has a somewhat more rational basis, was 
 suggested by Livingston and Livingston. 12 They proposed a series of tem- 
 perature-efficiency indices based on the van't Hoff-Arrhenius law, which 
 states that the velocity of many chemical reactions approximately doubles 
 with a rise in the temperature of 10°C. (18° F.). If it is assumed that the 
 growth rate for plants follows this law above 40° F., at which temperature 
 the rate is taken to be unity, a series of values representing temperature 
 efficiencies for higher temperatures may be derived. When this scheme is 
 used, the efficiency value for any temperature is represented by the value of 
 the index that corresponds to the temperature value itself. Assuming the 
 growth rate to be unity for a temperature of 40° F., it should be 1.21 for a 
 temperature of 45°, 2.0 for 58°, etc. These indices are called by Livingston 
 and Livingston "exponential indices," and they have published a table of 
 these values. 
 
 Since most of the temperatures with which we have to deal are below the 
 optimum for plant growth, since temperature and the growth rate are related 
 in an approximately linear manner between 40° F. and the optimum (about 
 90° F.) , and since both the exponental and remainder series of index values 
 increase in a nearly linear way throughout this range, both of the methods 
 just considered give temperature efficiency numbers that are more or less 
 approximately proportional to plant growth as it is influenced by tempera- 
 tures between these limits. It is obvious, however, that neither of these 
 methods can properly express efficiencies for temperatures above the optimum, 
 
 11 The temperature data of this paper are all expressed in terms of temperature degreeson the Fahrenheit 
 scale, simply because the observational data had this old-fashioned form. By retaining the Fahrenheit values 
 much labor has been avoided, but it is not to be understood that the writer is really as conserva- 
 tive as this feature of the paper might seem to suggest. 
 
 12 Livingston, B. E., and Grace J. Livingston. Temperature coefficients in plant geography and clima- 
 tology. Bot. Gaz. 55: 349-375. 1913. 
 
Climatic Conditions of Maryland 355 
 
 since they give numbers winch continue to increase with increasing tempera- 
 ture, while growth increases with increasing temperature up to the optimum 
 and then decreases with higher temperature. Also, both these methods give 
 results that are approximately proportional to each other for ordinary sum- 
 mer temperatures. This fact has been noted by Livingston and Livingston 
 and again by Stevens, 13 and it is also obvious from the climatic data given by 
 McLean. But it must be remembered that these methods cannot be satis- 
 factory excepting when the temperatures dealt with he mainly between 40° 
 and 90° F. 
 
 When the exponential indices are employed each daily mean temperature 
 for any period is replaced by its index and the series of indices thus obtained 
 is summed for the period, giving the exponential summation. 
 
 (3) The third method of expressing temperature values as they affect plant 
 growth has been more recently suggested by Livingston. 14 It is based on the 
 results of Lehenbauer's experiments with maize seedlings. From. Lehen- 
 bauer's data, Livingston derived a series of coefficients giving the efficiencies 
 of various temperatures in terms of the growth of this plant. He has called 
 these "physiological temperature indices." The growth rates upon which 
 the index values were based are those shown by Lehenbauer's seedlings when 
 exposed for 12 hours to various maintained temperatures, the other condi- 
 tions of the experiment being approximately the same for all tests. Living- 
 ston suggests that the coefficients thus derived from the growth of maize 
 under controlled conditions, with different maintained temperatures, may 
 possibly express some approach toward a general relation between plant 
 growth and temperature and may thus be applicable to plants growing under 
 other conditions. The graph of these physiological indices exhibits the same 
 direction of slope between a low temperature and the optimum as do the graphs 
 of temperature efficiencies derived by the other two methods, but for this por- 
 tion of the temperature range the slope of the graph of physiological indices is 
 generally steeper than that of the graph of remainder indices, the latter graph 
 itself having a much steeper slope than that of the exponential indices. This 
 is shown by Livingston and also by Stevens, in the papers cited above. 
 Since they are derived from the actual growth rates of a plant, the physiological 
 temperature indices appear to have a more rational basis than either the 
 remainder or the exponential indices. For this reason, and for others that 
 will appear below, physiological indices have been used in this study for ex- 
 pressing the temperature values as they are to be compared with the plant 
 growth-rates. 
 
 13 Stevens, Neil E. Influence of temperature on the growth of Endothia parasitica. Amer. Jour. Bot. 
 4: 112-118. 1917. — Idem. Influence of certain climatic factors on the development of Endothia parasitica. 
 Ibid. 4: 1-33. 1917. 
 
 11 Livingston, B. E. Physiological temperature indices for the study of plant growth in relation to cli- 
 matic conditions. Physiol. Res. 1: 399-420. 1916. — Lehenbauer, P. A. Growth of maize seedlings in relation 
 to temperature. Physiol. Res. 1: 247-288. 1914. 
 
356 F. Merrill Hildebrandt 
 
 In using the physiological temperature indices for the purposes of this 
 study, the procedure has been as follows. Each daily mean temperature for 
 any period has been replaced by its corresponding index (taken from Living- 
 ston's table, — 1916) and then all the daily index values have been summed 
 to give the physiological summation for the period. This summation value 
 is finally divided by the number of days in the given period. 
 
 Two other series of temperature values are presented in the tables of this 
 paper, but neither has been found to be as satisfactory for expressing this 
 climatic condition as are the physiological-summation indices. These are 
 (a) the average daily mean temperature for each culture period (in degrees, 
 Fahrenheit) and (b) the remainder-summation index for each period. 
 
 As has been stated, the physiological-summation indices for all periods 
 have been represented as daily means, and these have been stated always as 
 relative values, in terms of the general average for all periods and stations. 
 The average value used as unity in expressing the relative index values for 
 temperature is 56.39. 
 
 The following considerations may be added to show the reason for using 
 the physiological-summation indices in this study, secured as above described, 
 rather than the remainder-summation or exponential-summation indices. 
 It will be necessary to anticipate somewhat the discussion that is to follow 
 this section. The three climatic conditions (temperature, evaporation and 
 light) each show a definite seasonal march for each of the places employed in 
 this study. The temperature rises from low values in the spring to a mid- 
 summer maximum, which is followed by a subsequent fall to low autumnal 
 values. On the other hand, the values representing light and evaporation 
 both decrease, in general, throughout the season. If, now, a generalized 
 curve representing the growth of the plants be drawn, employing average 
 values to represent all the stations together, and plotting them as ordinates 
 with the dates of the middles of the periods as abscissas, such a growth curve 
 follows the seasonal march of temperature and shows only secondary varia- 
 tions as related to the other two climatic indices. The growth of the plants 
 is thus apparently determined mainly by temperature. Obviously, also, the 
 seasonal march of the temperature values must show the same general form 
 of curve no matter what scheme is used in expressing temperature efficiency. 
 In view of these facts, and in consideration of the general comparative pur- 
 pose of the present study, a method should be used, for expressing tempera- 
 ture efficiency, that gives a seasonal march of the efficiency values in accord 
 with the corresponding march of generalized plant growth. Of the three 
 methods mentioned, the physiological-summation index fulfills tins require- 
 ment best, and this has accordingly been selected for use throughout the 
 entire study, as has been said. 
 
 An examination of the plant and climatic graphs (to be considered later) 
 shows that the plant values for most of the stations rise above the temperature 
 
Climatic Conditions of Maryland 357 
 
 efficiency values in the middle of the season, and fall below them, at its end. 
 Tins is probably due in part to the effect of light and evaporation but it may 
 also be related to an inadequacy of the temperature efficiency values to repre- 
 sent the actual effect of temperature on the growth of these plants. It 
 appears to be at least suggested that the actual temperature efficiency values 
 for these so3 r -bean plants increase more rapidly with increase in the tempera- 
 ture itself, for the range here encountered (between 40° and 85° F.), than to 
 the physiological index values derived from Lehenbauer's study of maize 
 seedlings. This whole question deserves much more experimental study. 
 It is a surprising fact that we have available only a single thoroughgoing in- 
 vestigation (Lehenbauer's) of the relation of temperature to the growth of 
 higher plants, in spite of the fact that the primary importance of the tem- 
 perature control of growth is obvious to every observer and has long been 
 qualitatively appreciated. A comparison, for any of the stations employed, 
 of the range of growth values for the plants with the remainder-summation 
 values for temperature (which are practically equivalent to the exponential- 
 summation values in this study) and with the physiological-summation in- 
 dices will furnish evidence for the verification of these statements. The 
 graphs of the physiological-summation indices of temperature efficiency show 
 much steeper slopes than do the corresponding graphs derived from the other 
 two kinds of temperature indices mentioned above, however, so that the physi- 
 ological indices are evidently more suitable to represent temperature effici- 
 encies than are either of the other two kinds of indices. 
 
 LIGHT 
 
 The only records of light conditions that were available for all of the sta- 
 tions of this study were the daily ocular estimates of cloudiness obtained by 
 the weather observers, and these alone were used. To make use of these 
 estimates it was, of course, first necessary to bring the dairy percentages of 
 clear "sky together for each culture period, so as to derive for each period a 
 single value that might be taken to represent the intensity of the light condi- 
 tion for that period. The method employed to accomplish this is presented 
 below. 15 
 
 The total heat equivalent of the actual sunshine for any given period at a 
 given station is primarily a function of three terms: (1) the maximum possible 
 number of hours of sunshine (determined by latitude and season); (2) the 
 mean daily intensity of full sunshine for the period and station, which may 
 be expressed in terms of units of heat received per unit of a horizontal sur- 
 face ; (3) the condition of the sky, whether overcast, partly overcast or clear. 
 The daily values for the first two of these terms vary in a regular manner 
 
 15 The presentation of this method is here practically the same as that previously published. See: Hilde- 
 brandt, F. M. A method for approximating sunshine intensity from ocular observations of cloudiness. Johns 
 Hopkins Univ. Circ, March, 1917. 
 
358 F. Merrill Hildebrandt 
 
 throughout the year for any given place, and the ones for the third term are 
 roughly stated in the observer's records, as just mentioned. 
 
 The first two terms are combined in the ordinates of the graph given by 
 Kimball 16 for the maximum possible total radiation received per day at 
 Mount Weather, Virginia. Since this station is at about the same latitude 
 as the stations here dealt with, the ordinates from Kimball's graph may be 
 taken as approximate measures of the total maximum possible light intensi- 
 ties for the corresponding dates for all of the Maryland stations. These 
 values represent the total amount of heat received from the sun and sky on 
 clear days at Mount Weather, in gram-calories per square centimer of a hori- 
 zontally exposed surface. The method of using this graph along with the 
 weather observer's reports, for estimating sunshine intensity for any station 
 and period, will be best shown by an example. Suppose it is desired to esti- 
 mate the average daily sunshine intensity for some station in the general 
 region of Mount Weather, for the first week of August. The average ordi- 
 nate value for this week is first obtained from Kimball's graph. For periods 
 as short as a week or two this may be done by averaging the values for the 
 first and last days of the period, since the curve may be taken as a straight 
 line for such short intervals. The ordinate values for August 1 and August 
 
 7 are ■ and , and their average is . From the report of 
 
 the weather observer at the place in question, the number of clear, partly 
 cloudy, and cloudy days is next determined for the days August 1 to August 
 7, inclusive, and some arbitrary weighting is given to each kind of day. This 
 was done in the present instance by regarding days reported "clear" as days 
 of full sunshine, those reported "partly cloudy" as half days of sunshine, 
 and those reported "cloudy" as without any sunshine. The same scheme 
 of weighting must of course be adhered to in all the estimates used for com- 
 parative purposes in any discussion. Suppose there were 2 clear days, 3 half- 
 cloudy days and 2 cloudy days. By summing these values as 2, 1.5 and 0, 
 we obtain 3.5, representing the equivalent number of wholly clear days for 
 the period considered. Now, 3.5 is 0.5 of the total number of days in the 
 week period, and the latter value may be termed "the coefficient of clear 
 weather." By multiplying the average daily intensity value for clear days, 
 
 , (obtained by the use of Kimball's graph) by this coefficient of clear 
 
 weather (0.5) we obtain g.-cal. as a rough approximation of the 
 
 average daily sunshine index for the week. 
 
 While it is certain that solar radiation affects plants in other ways than 
 through its heating effect, it is no less certain that by far the greater part of 
 the energy of sunshine absorbed by plants is converted into heat (largely as 
 latent heat of vaporization of water), and it seems probable that the other 
 effects produced upon the plant may be more or less proportional to the total 
 
 \' 16 Kimball, Herbert H. The total radiation received on a horizontal surface from the sun and sky at 
 Mount Weather. Monthly Weather Rev. 42: 474-487. 1914. (See especially fig. 8, p. 484.) 
 
Climatic Conditions of Maryland 359 
 
 energy equivalent of sunshine. This method of deriving sunshine indices is, 
 however, to be taken only as a rough approximation. 
 
 For each 2-week and for each 4-week period of this study an index of sun- 
 shine intensity was secured in the manner just described, and each of these 
 sunshine values was expressed in terms of the average of all values in the 
 series. The relative values thus obtained are quite parallel with the other 
 relative values already referred to. They are given in the tables of data and 
 the actual values may be obtained from them in a manner like that described 
 for the plant values. The general unit used in expressing these relative light 
 values (the average daily sunshine intensity for all stations and for all periods) 
 is 442 gram-calories per square centimeter of horizontal surface. 
 
 evaporation 
 
 The evaporating power of the air was measured as has been said, by means 
 of standardized cylindrical porous-cup atmometers, located so as to have about 
 the same exposure as the plant cultures. The instruments were read at inter- 
 vals of about two weeks, the dates of reading being the same as those on which 
 observations were made on the plants. After every reading each atmometer 
 cup was removed and replaced by another that had just been standardized. 
 The used cup was subsequently restandardized so as to detect any change in 
 the coefficient of the cup consequent upon its exposure. When the restand- 
 ardization showed a change in the coefficient, the mean of the original coef- 
 ficient and the coefficient found upon restandardization was employed to 
 reduce the reading to the Livingston cylindrical standard. The evaporation 
 readings should therefore be directly comparable to other measurements 
 related to the same standard, 
 
 As has been pointed out by Livingston, 17 the porous-cup atmometer is 
 somewhat similar to plant foliage in the way in which its evaporating surface 
 is exposed to the surroundings. It may therefore be supposed that the 
 transpiration from the plants for any period should be approximately propor- 
 tional to the evaporation from the atmometer, except in so far as the trans- 
 piration rates may be influenced by conditions within the plant. The work 
 of Briggs and Shantz indicates that evaporation from small open pans or 
 porous cups is influenced by the same external conditions, and in about 
 the same way, as is plant transpiration, if the comparison is made for periods 
 of a day or more. Of course the two rates do not vary proportionally within 
 the day period, since the internal conditions of the plant exhibit a peculiar 
 daily march, but with such details this study does not need to deal. It has 
 been supposed, therefore, that the effectiveness of the external conditions 
 to influence the transpiration rates of the plants of this study was approxi- 
 
 17 Livingston, B. E. The relation of desert plants to soil moisture and to evaporation. Carnegie Inst. 
 Wash. Pub. 50. 1906. 
 
360 F. Merrill Hildebrandt 
 
 mately measured by the corresponding corrected evaporation rates from the 
 atmometer. The atmometer readings have been reduced, in every case, to 
 mean daily rates for the 2-week and 4-week periods, and these rates have been 
 taken as indices of the evaporating power of the air as it affected transpiration 
 from the plants. Finally, all atmometric values have been expressed rela- 
 tively, in terms of the general average for all stations and for all periods, as 
 in the case of the other data. The general average used as unity for these 
 relative atmometric values is 16.2 cc. per day. 
 
 The three derived climatic values in this study may be brought together 
 here, for convenience. The list applies to the 2-week as well as to the 3-week 
 series of data. 
 
 (1) Temperature. Relative daily mean of the physiological-summation 
 indices for the period. 
 
 (2) Light. Relative daily mean of calculated light-intensity values for 
 the period, gram-calories per square centimeter of horizontal surface. 
 
 (3) Evaporation. Relative daily mean of atmometric indices for the per- 
 iod, cubic centimeters of loss from the Livingston standard cylindrical porous 
 cup. 
 
 RESULTS AND DISCUSSION 
 
 Introductory 
 
 The discussion of the data obtained in this study will be devoted in part 
 to descriptions of the growth changes observed in the plants at the various 
 stations, and for the various periods at each station, and in part to corre- 
 sponding descriptions of the climatic values. Owing to the complexity of 
 the problem and to the number and variety of the data to be dealt with, it 
 has been found necessary to depart frequently from a general logical order 
 and to treat matters that seem to be of seondary importance at greater 
 length than might appear necessary from a more restricted point of view. 
 Such physiological interpretations as are attempted in the course of the 
 presentation of the data are of interest partly for their own sake, but more 
 particularly because of the bearing they may have on the general problem of 
 the use of standard plants for the comparative integration of effective climatic 
 complexes. The work here reported was planned primarily to make a first 
 trial in the use of standard plants in this way. The discussion of the data 
 will be presented more in the form of a running narrative, with digressions at 
 many points, than is ideally desirable, but the newness of this land of study 
 and the fact that the fundamental principles and even the terms to be em- 
 ployed have yet to be developed, make anything approaching a true logical 
 sequence quite impossible now. 
 
 The various kinds of data to be considered will be brought forward in groups 
 corresponding to their sources. The 2-week plant data and the 2-week cli- 
 
Climatic Conditions of Maryland 361 
 
 matic data for the stations in the open will first be presented, followed by a 
 presentation of the 4-week plant and climatic data for these stations. Sub- 
 sequently, a special discussion of the data for the covered stations and a 
 similar treatment of the data for the forest station at Baltimore will be 
 given. 
 
 The relative numbers, or indices, derived as described above, are given in 
 tables I to VIII, together with the dates of the first and last days of each 
 culture period and other information, including the length of each period, 
 the number of plants in each culture, etc. Also, a set of figures is presented 
 showing graphically certain parts of the information given in the tables. 
 Tables I to VIII give twenty-six sets of data. Nine of these sets (tables I- 
 III) contain the plant and climatic measurements for the 2-week culture 
 periods for the exposed stations, nine others (tables IV-VI) give the data for 
 the four-week culture periods for the exposed stations, six others (tables VII- 
 VIII) give the data for the 2- and 4-week culture periods for the covered 
 stations at Oakland, Baltimore and Easton, and the two remaining (table 
 VIII) give the data for the 2- and 4-week culture periods for the Baltimore 
 forest station. 
 
 In each of the eight tables just mentioned, the first line gives the name 
 of the place referred to, the kind of culture period (whether 2- or 4-week), 
 the character of the exposure of the plants (whether the station is exposed, 
 covered, or forest, and the dates of the beginning and end of each culture 
 period. The second fine of each table gives the serial culture numbers. These 
 numbers being assigned to the various cultures for convenience of reference. 
 When several kinds of stations occur at one place, cultures of the same num- 
 ber cover approximately the same time period. For instance, for Baltimore 
 there is an exposed station, a covered station, and a forest station, and there 
 is a 2-week culture "8" for each of these three stations, the dates for each 
 of these being August 20 and September 3. In some cases, culture periods 
 of the same number for the exposed and covered stations show a difference 
 of a day in the lengths of their respective periods owing to the fact that it 
 was impossible to take measurements on both the exposed and covered 
 plants on the same day. The third fine of each table gives the length of each 
 culture period, in days. The fourth line gives the number of plants actually 
 used in obtaining the plant measurements. A dash appearing in place of a 
 relative value indicates that the data necessary for calculating this value 
 are lacking. An asterisk placed opposite a. climatic index value shows that 
 this particular value was not plotted in the graphs (to be described later). 
 (Points are omitted from the climatic graphs in the case of most cultures 
 where no plant data are available for comparison with the climatic values.) 
 The remainder of the table presents the relative plant and climatic values, 
 the derivation of which has already been made clear. The last column of 
 each of the tables gives the seasonal averages for the station considered. 
 
362 
 
 F. Merrill Hildebrandt 
 
 TABLE I 
 Two-week data for exposed stations, Oakland, Chewsville and Monrovia 
 
 OAKLAMD 
 
 e-wcek periods. 
 EX.P05EO 3TATIOM. 
 
 Z3 
 
 JUNE 
 
 5 
 
 JUME 
 J9 
 
 JUME 
 
 19 
 
 JULV 
 
 JULY 
 3 
 
 JULV 
 l& 
 
 JULV 
 l& 
 
 JULV 
 31 
 
 3" 
 
 AUG 
 
 If 
 
 AUG 
 AUG 
 
 2r 
 
 AUG SEPT 
 27 12. 
 
 SEPT SE:PT 
 12 25 
 
 
 
 
 AV. 
 
 Culture number 
 
 l 
 
 £ 
 
 3 
 
 4 
 
 3 
 
 6 
 
 7 
 
 o 
 
 9 
 
 
 
 
 
 Lenqtb of qrowinq period, daqs. 
 
 13 
 
 1** 
 
 14 
 
 13 
 
 15 
 
 14 
 
 i 3 
 
 l& 
 
 '3 
 
 
 
 
 
 Number of plants. 
 
 1 
 
 5" 
 
 5 
 
 & 
 
 .S 
 
 6 
 
 fc> 
 
 €» 
 
 4 
 
 
 
 
 
 ffemoinder summation Index.- 
 
 334 
 
 354 
 
 •lot 
 
 364 
 
 431 
 
 3to9 
 
 343 
 
 370 
 
 262 
 
 
 
 
 3fol 
 
 Average daily relative pbsisiolocjital 
 temperature, inden 
 
 C?ty 
 
 t>6 
 
 S5 
 
 78 
 
 ©7 
 
 67 
 
 64 
 
 66 
 
 43 
 
 
 
 
 S9 
 
 Avcraqc dai ly mean t^mperature.deq T 
 
 65 
 
 64 
 
 <=>e 
 
 far 
 
 6e 
 
 e»5 
 
 t,G 
 
 62. 
 
 foi 
 
 
 
 
 65 
 
 Averaqe doilu relative evaporation 
 index. , 
 
 153 
 
 ijq 
 
 9S 
 
 79 
 
 104 
 
 90 
 
 7i 
 
 5-7 
 
 6,9 
 
 
 
 
 96 
 
 Averaqe duilu relative, sunshine- 
 intensify 
 
 122 
 
 I2E 
 
 i09 
 
 103 
 
 ne. 
 
 102 
 
 MO 
 
 83 
 
 ai 
 
 
 
 
 >D5 
 
 Averaqc daily relative, increment' in 
 stem Ijcio, nt 
 
 7G 
 
 96 
 
 124 
 
 ai 
 
 96 
 
 qS 
 
 ior 
 
 S3 
 
 42 
 
 
 
 
 e& 
 
 Averaqc dailu relative increment" in 
 leaf- product 
 
 - 
 
 31 
 
 a. 7 
 
 74 
 
 at? 
 
 78 
 
 82 
 
 ■46 
 
 - 
 
 
 
 
 7 7 
 
 CHEWSVILLE 
 
 2-woeK periods. 
 
 EXPOSED STATION 
 
 19 
 
 June 
 
 JLVCE 
 
 JUNE 
 16 
 
 JUME 
 
 JUME 
 
 30 
 
 June 
 30 
 
 JUlV 
 
 JULY 
 14 
 
 JULV 
 
 as 
 
 JULY 
 28 
 AUG. 
 
 AUG 
 I I 
 
 AUG, 
 25 
 
 AUl3 
 Z5 
 
 5C.PI 
 
 a 
 
 SEPT 
 3 
 
 SEPT 
 
 22 
 
 SEPI 
 
 22 
 OCT 
 
 1 
 
 OCT. 
 
 7 
 OCT. 
 
 SO 
 
 
 Av 
 
 Culture number. 
 
 1 
 
 2 
 
 3 
 
 4 
 
 3 
 
 6 
 
 7 
 
 a 
 
 9 
 
 IO 
 
 1 1 
 
 
 
 Lenqtb o^- qrowinq period, du\js 
 
 ii 
 
 n 
 
 H 
 
 H 
 
 ,« 
 
 (1 
 
 H 
 
 14 
 
 i ^ 
 
 13" 
 
 13 
 
 
 
 Number of plants 
 
 6 
 
 <b 
 
 4 
 
 & 
 
 ^ 
 
 4 
 
 3 
 
 s 
 
 & 
 
 & 
 
 6 
 
 
 
 Remainder summation indcu. 
 
 116 
 
 •H3 
 
 472 
 
 1.53 
 
 523 
 
 450 
 
 .517 
 
 136 
 
 302 
 
 32© 
 
 301 
 
 
 4H.2 
 
 Averaqe daily relative pbvjsioloqical 
 tern pera+ure "index.. 
 
 9? 
 
 lOG 
 
 iicj 
 
 I iZ 
 
 i19 
 
 I iz. 
 
 145 
 
 I03 
 
 4e 
 
 51 
 
 S3 
 
 
 9S 
 
 Averaqc daiU mean temperature, deq ft 
 
 G.S 
 
 7t 
 
 71 
 
 7l 
 
 7G 
 
 - 
 
 TG 
 
 TO 
 
 ei 
 
 Gi 
 
 G2 
 
 
 69 
 
 Averaqc dail\j relative evaporation 
 "mde*.. 
 
 no 
 
 l 15 
 
 lOT 
 
 H5 
 
 ioe 
 
 67 
 
 io5 
 
 ao 
 
 7c» 
 
 73 
 
 61 
 
 
 9l 
 
 Averaqe daiKj relative sunshine 
 intensity 
 
 I2i 
 
 129 
 
 II& 
 
 lOG 
 
 iEl 
 
 91 
 
 qe 
 
 67 
 
 101 
 
 73 
 
 14 
 
 
 9S 
 
 Averaqc daily relative increment m 
 stem laeiqhr. 
 
 93 
 
 93 
 
 113 
 
 lie 
 
 I 26 
 
 9a 
 
 9S 
 
 76 
 
 ie 
 
 37 
 
 37 
 
 
 S7 
 
 Averaqc doilM relative, iiicrement" in 
 lea} — product 
 
 104 
 
 1 lO 
 
 I37 
 
 qe 
 
 I39 
 
 1 27 
 
 IOO 
 
 52 
 
 32 
 
 20 
 
 4 
 
 
 Si 
 
 MOMROVIA 
 
 E-ujeck periods. 
 
 EXPOSED iTATiOrt 
 
 MAY 
 IS 
 
 jurtE 
 
 i 
 
 jurtE 
 i 
 
 JUNE 
 15 
 
 JUNE. 
 15 
 
 JUNE 
 29 
 
 jurte 
 july 
 
 )3 
 
 JULY 
 
 13 
 
 JULY 
 Z7 
 
 JULY 
 Zl 
 
 AUG. 
 IO 
 
 AUG 
 IO 
 
 AUG. 
 2*» 
 
 AUG, 
 
 Z<J 
 
 5EPT 
 
 7 
 
 5EP7 
 
 7 
 
 SEiFI 
 
 2J 
 
 SEPT 
 £1 
 
 OCT. 
 
 a 
 
 OCT 
 
 a 
 
 OCT, 
 19 
 
 
 AV. 
 
 Cullurc number. 
 
 1 
 
 £. 
 
 3 
 
 4 
 
 5 
 
 6 
 
 7 
 
 6 
 
 9 
 
 IO 
 
 1 1 
 
 
 
 Lenqrn of qrounnq period, daus. 
 
 14 
 
 M 
 
 14 
 
 14 
 
 14 
 
 14 
 
 H 
 
 14 
 
 14 
 
 17 
 
 11 
 
 
 
 number of" plants 
 
 3 
 
 •5 
 
 G 
 
 1 
 
 fc> 
 
 3 
 
 6 
 
 5 
 
 & 
 
 £> 
 
 6 
 
 
 
 ^cma index- summation index. 
 
 44 2- 
 
 154 
 
 17a 
 
 4^f 
 
 J 36 
 
 4&3 
 
 SI I 
 
 433 
 
 3lM 
 
 36& 
 
 £*>Z. 
 
 
 431 
 
 Aueraqe dail«j relative pbqsialoq ical 
 temperotu'"e inde>. 
 
 105 
 
 MO 
 
 121 
 
 II2 
 
 >5& 
 
 H7 
 
 114 
 
 105 
 
 59 
 
 5"3 
 
 59 
 
 
 I04 
 
 Averaqe dailv, mean temperature, deq F 
 
 T\ 
 
 7l 
 
 73 
 
 7 I 
 
 77 
 
 72 
 
 7J 
 
 70 
 
 e>£ 
 
 6S 
 
 63 
 
 
 no 
 
 Averaqe daily relative evaporaTioo 
 
 > ndei. 
 
 - 
 
 I4G 
 
 !5£ 
 
 93 
 
 '17 
 
 ieo 
 
 MZ. 
 
 oa 
 
 96 
 
 82 
 
 76 
 
 
 IO& 
 
 Averaae dailv relative sunshine 
 i nte n s 1 1 ^ 
 
 I5Z. 
 
 I22. 
 
 101 
 
 IO£. 
 
 MO 
 
 I05 
 
 9S 
 
 62 
 
 t05 
 
 G© 
 
 J2 
 
 
 IOO 
 
 Pivera^e. JoiIlj relaTive increment" in 
 stem, heiqlit 
 
 73 
 
 Si 
 
 i£i 
 
 96 
 
 1 15" 
 
 90 
 
 107 
 
 76 
 
 20 
 
 67 
 
 25" 
 
 
 T9 
 
 Averaqe daily relative increment m 
 leaf- product 
 
 66 
 
 90 
 
 HO 
 
 83 I3S 
 
 I09 
 
 lOI 
 
 112. 
 
 6 
 
 3b 
 
 4 
 
 
 So 
 
Climatic Conditions of Maryland 
 
 3G3 
 
 TABLE II 
 Two-week data for ex-posed stations, College, Baltimore and Darlinylon 
 
 COLLEGE 
 
 £-weeK per loch? 
 
 EXPOSED £> rATlOM. 
 
 MAI 
 
 dune 
 
 6 
 
 JUNE 
 JUNL 
 
 JUMP 
 
 JULY 
 
 JU*-Y 
 3 
 
 JULY 
 
 17 
 
 JULY 
 
 n 
 
 JULY 
 31 
 
 July 
 31 
 
 AUG,, 
 14 
 
 AU<S 
 
 AUG. 
 
 acpr 
 
 IO 
 
 .ill 
 IO 
 
 31 1 I 
 
 _.i .■ 
 
 OCT 
 IO 
 
 OCT. 
 IO 
 
 ocr 
 
 24 
 
 
 AV. 
 
 Culture number 
 
 * 
 
 ^ 
 
 -I 
 
 •!> 
 
 & 
 
 7 
 
 a 
 
 9 
 
 IO 
 
 ii 
 
 12 
 
 
 
 Lenqth of- qrowmq period, da\(5. 
 
 14 
 
 13 
 
 H 
 
 l-l 
 
 11 
 
 14 
 
 u 
 
 14 
 
 IS 
 
 15 
 
 14 
 
 
 
 Number* of ple-vtts>. 
 
 G 
 
 •5 
 
 -s 
 
 fa 
 
 4 
 
 4 
 
 5 
 
 <b 
 
 - 
 
 & 
 
 & 
 
 
 
 Remainder summation index. 
 
 5-40, 
 
 •ton, 
 
 526 
 
 49 A 
 
 szo 
 
 'l"»4 
 
 4 78 
 
 441 
 
 370 
 
 .333- 
 
 -327 
 
 
 1iO 
 
 Averaqe dail^ relative physiological 
 temperarure index. 
 
 108 
 
 103 
 
 139 
 
 134 
 
 lie 
 
 133 
 
 143 
 
 US" 
 
 e>7 
 
 JO 
 
 J3 
 
 
 IQtJ 
 
 Averaqe aailv, mean temperature, dcq. h" 
 
 71 
 
 70 
 
 Tt. 
 
 H 
 
 76 
 
 *4 
 
 76 
 
 7l 
 
 o4 
 
 <5l 
 
 & a 
 
 
 TO 
 
 Avaraq»i daiKj relative evaporation 
 index. 
 
 1GO 
 
 (54 
 
 133 
 
 9G 
 
 143 
 
 147 
 
 i&S 
 
 107 
 
 73 
 
 34 
 
 ea 
 
 
 ■■*? 
 
 Averaqe dail\j relative sunshine 
 1 iTronoitv^. 
 
 - 
 
 - 
 
 - 
 
 - 
 
 - 
 
 
 - 
 
 - 
 
 
 
 - 
 
 
 
 Avcroqc Jaiw rclaT'wc increment in 
 stem heiqh-K 
 
 qs 
 
 132 
 
 113 
 
 MB 
 
 124 
 
 no 
 
 90 
 
 7(5 
 
 
 48 
 
 33 
 
 
 =)S 
 
 Averaqe dail, relative increment in 
 leaf- produol: 
 
 96 
 
 IZ3 
 
 SI 
 
 152. 
 
 203 
 
 150 
 
 1 
 
 117 
 
 G£ 
 
 - 
 
 13 
 
 IO 
 
 
 IOI 
 
 BALTIMORE 
 
 2-i-ueijk period ■=. 
 EXPOSE D STATION. 
 
 MAY 
 
 14 
 
 HAY 
 29 
 
 MAY 
 
 29 
 
 JUNE 
 IO 
 
 JUNE 
 IO 
 
 JUNE 
 25 
 
 JUNE 
 25 
 
 JULY 
 9 
 
 JULY 
 
 9 
 JULY 
 £3 
 
 JULY 
 £3 
 
 AUG. 
 lb 
 
 AUG. 
 AUG. 
 
 AU6 
 20 
 
 5EP7 
 
 3 
 
 5un 
 
 3 
 3EF7 
 
 SEPT 
 
 19 
 
 OC7. 
 
 J 
 
 OCT. 
 
 OCT. 
 14 
 
 
 Av. 
 
 CliItutg number: 
 
 1 
 
 a 
 
 "5 
 
 4 
 
 5 
 
 & 
 
 7 
 
 S 
 
 c t 
 
 to 
 
 II 
 
 
 
 Lenqtb of-qrowmq period, duuj.3 
 
 13 
 
 12 
 
 15 
 
 14 
 
 14 
 
 14 
 
 14 
 
 ■ 4 
 
 ie> 
 
 12 
 
 13 
 
 
 
 Number op plonts- 
 
 s- 
 
 - 
 
 -S 
 
 6 
 
 & 
 
 4 
 
 5 
 
 5 
 
 & 
 
 S 
 
 .5 
 
 
 
 'Rem cinder summah'on mde.x- 
 
 45i 
 
 370 
 
 SOB 
 
 4&9 
 
 ■946 
 
 -SO 6 
 
 543 
 
 hOC 
 
 593 
 
 300 
 
 37.9 
 
 
 •4 48 
 
 Aweraqe dailu relative phijs io logical 
 temperature mden- 
 
 9 1 
 
 49 
 
 IK3 
 
 I25 
 
 isa 
 
 iv 
 
 l&O 
 
 139 
 
 G2 
 
 "1" 
 
 
 IIS. 
 
 Averaqe daiitj mean "temperature, decj f: 
 
 G9 
 
 70 
 
 73 
 
 73 
 
 78 
 
 73" 
 
 78 
 
 73 
 
 G4 
 
 &« 
 
 q 
 
 
 72 
 
 Average da t l^ relative evaporation 
 
 injcu. 
 
 127 
 
 IOZ 
 
 US 
 
 60 
 
 92. 
 
 112 
 
 HO 
 
 di 
 
 »">9 
 
 53 
 
 G4 
 
 
 93 
 
 Averaqe da'i Nj relative sunshine 
 
 137 
 
 a? 
 
 ioa 
 
 go 
 
 q 1 ) 
 
 7a 
 
 79 
 
 Y4 
 
 82 
 
 89 
 
 59 
 
 — . 
 
 36 
 
 Averaqe am|\j relative increment m 
 item heiqht. ■ 
 
 9i 
 
 - 
 
 I4& 
 
 152. 
 
 183 
 
 152 
 
 152, 
 
 149 
 
 73 
 
 5G 
 
 96 
 
 125 
 
 Averaqe da,|-j relative increment in 
 leaf-prodtAch 
 
 IOG 
 
 - 
 
 125 
 
 163 
 
 194 
 
 1 IO 
 
 2.33 
 
 ISO 
 
 S-4 
 
 as 
 
 49 
 
 
 119 
 
 1 ■ r 
 
 DARLIPSGTOrs 
 
 2- uueeK pe riods. 
 
 EXPOSED STATIOM 
 
 MAY 
 
 15 
 MAY 
 
 30 
 
 MAY 
 30 
 
 JUNE 
 13 
 
 JUNE 
 13 
 
 JUNE, 
 26 
 
 JUME 
 
 JULY 
 IO 
 
 JULY 
 
 IO 
 
 JULY 
 
 24 
 
 JULY 
 
 24 
 
 A UQ. 
 
 7 
 
 AU3 
 
 7 
 AUG. 
 
 ^I 
 
 AUG 
 21 
 
 SEPT 
 4 
 
 3EP1 
 A 
 
 SSP7 
 18 
 
 13 
 
 OCT 
 
 2 
 
 OCT 
 
 OCT. 
 16 
 
 
 Av. 
 
 Culture number. 
 
 t 
 
 2 
 
 3 
 
 ■9. 
 
 3 
 
 & 
 
 X 
 
 3 
 
 9 
 
 IO 
 
 1 1 
 
 
 
 Lenqth of qrowmq period, do. ij.s. 
 
 1-5 
 
 14 
 
 ■ 3 
 
 14 
 
 14 
 
 14 
 
 14 
 
 14 
 
 M 
 
 14 
 
 14 
 
 
 
 Number of plants. 
 
 & 
 
 ■5 
 
 e> 
 
 4 
 
 S 
 
 S 
 
 " 
 
 •5 
 
 « 
 
 -5 
 
 4 
 
 
 
 Remainder sunn nation ii?dcx>. 
 
 430 
 
 432 
 
 391 
 
 432 
 
 tJIZ 
 
 456 
 
 506 
 
 471 
 
 309 
 
 3 IS 
 
 336 
 
 
 4IS 
 
 Averaqe do>l^j relative ph^sioloqieal 
 t"e m pe ratu. re index. 
 
 37 
 
 99 
 
 94 
 
 93 
 
 149 
 
 113 
 
 146 
 
 123 
 
 4e 
 
 S9 
 
 54 
 
 
 7l 
 
 Averaqe daiUj meon temperature, cleq. f: 
 
 Ge 
 
 TO 
 
 «9 
 
 70 
 
 76 
 
 72. 
 
 76. 
 
 72 
 
 61 
 
 ce 
 
 133. 
 
 
 69 
 
 Averaqe daiKj relqtive evaporation 
 index. 
 
 loo 
 
 123 
 
 90 
 
 62 
 
 74 
 
 77 
 
 79 
 
 e>4 
 
 82 
 
 39 
 
 39 
 
 
 e 7 
 
 Averaqe dailv, re\ative sunshine 
 intensity. 
 
 154 
 
 129 
 
 92 
 
 77 
 
 1 II 
 
 ,., 
 
 1 l& 
 
 73 
 
 90 
 
 9G 
 
 43 
 
 
 100 
 
 Averaqe daiKi relat'wa increment in 
 stem heiqhT. 
 
 MO 
 
 104 
 
 146 
 
 141 
 
 228 
 
 14G 
 
 - 
 
 107 
 
 42. 
 
 48- 
 
 59 
 
 
 n3 
 
 Averaqe da.t^ relative mcrementlw 
 leaf- product. 
 
 135 
 
 12<^ 
 
 153 
 
 115 
 
 £93' 
 
 I4B 
 
 
 ioa 
 
 29 
 
 43 
 
 " 
 
 
 us 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
364 
 
 F. Mebeill Hildebeandt 
 
 TABLE III 
 Two-week data for exposed stations, Coleman, Easton and Princess Anne 
 
 COLEMAN 
 
 s-week periods. 
 
 EX POSED 5TA TIOM. 
 
 MAY 
 
 MAi 
 23 
 
 MAY 
 28 
 
 JUNE 
 1 1 
 
 JUNE 
 
 II 
 
 JUKE 
 
 24 
 
 JUNE 
 
 zi 
 
 JULY 
 
 a 
 
 JUL1 
 
 a 
 
 JULY 
 Z2 
 
 JULY 
 
 AUG 
 
 5 
 
 AUG; 
 AUG. 
 
 iq 
 
 AUG 
 
 19 
 SEPT 
 
 2 
 
 SEPT SEPT 
 
 5EPT3EPT 
 Its SO 
 
 3XFT OCT 
 SO 13 
 
 OC T. oc r 
 13 26 
 
 AV. 
 
 Cu 1 ture n u m bar. 
 
 1 
 
 2. 
 
 J> 
 
 4 
 
 5 
 
 fa 
 
 7 
 
 a 
 
 9 
 
 IO 
 
 1 1 
 
 l£ 
 
 
 Lenqth of qrowinq period, do^s. 
 
 1*5 
 
 19 
 
 13 
 
 14 
 
 14 
 
 11 
 
 11 
 
 1 4 
 
 19 
 
 14 
 
 13 
 
 1.5 
 
 
 Numberr oj- plants. 
 
 G 
 
 <o • 
 
 1 
 
 4 
 
 & 
 
 <b 
 
 3" 
 
 fa 
 
 4 
 
 ^ 
 
 4 
 
 <s 
 
 
 Remainder bunitnut'ion index. 
 
 412 
 
 166 
 
 133 
 
 168 
 
 S60 
 
 ^5>7 
 
 S3S 
 
 520 
 
 3SO 
 
 399 
 
 - 
 
 - 
 
 na 
 
 Averaqe doiKj relative ph\jsioloqieal 
 temperature index. 
 
 SI 
 
 113 
 
 IIS 
 
 ISO 
 
 170 
 
 116 
 
 158 
 
 US 
 
 S1 
 
 BS 
 
 - 
 
 - 
 
 123 
 
 Averaqe daikj mean temperature, deq. rT 
 
 Go 
 
 72. 
 
 67, 
 
 79 
 
 T9 
 
 T6 
 
 77 
 
 76 
 
 G7, 
 
 G7 
 
 ~ 
 
 - 
 
 72. 
 
 Average doilvj relative evaporation 
 index. 
 
 i39 
 
 152 
 
 137 
 
 120 
 
 143 
 
 IJJ 
 
 licj 
 
 96 
 
 J32 
 
 I3S 
 
 97 
 
 93 
 
 12.7 
 
 Averaqe dailv rela+ive sunshine- 
 in+ensirhf. 
 
 Ml 
 
 IfaO 
 
 125 
 
 Gl 
 
 HI 
 
 I £4 
 
 MS 
 
 &5 
 
 - 
 
 - 
 
 - 
 
 - 
 
 120 
 
 Averoqe cici K) relative increment in 
 Stem heiqiit". 
 
 19 
 
 61 
 
 107 
 
 I3S 
 
 wq 
 
 I 57 
 
 12.1 
 
 107 
 
 G2 
 
 53 
 
 G7 
 
 2.S 
 
 96 
 
 Avcraqe dail^ relative increment in 
 leaf- product. 
 
 )OS 
 
 III 
 
 91 
 
 172 
 
 2.11 
 
 19a 
 
 itie 
 
 t IO 
 
 33 
 
 " 
 
 3a 
 
 4 
 
 IOT 
 
 EA5TOM 
 
 E- ujeek periods. 
 
 EXPOSED STATiO/H 
 
 MAY 
 6 
 
 MAY 
 
 Z5 
 
 MAY 
 25 
 
 JUNE 
 
 e 
 
 UUNE 
 
 a 
 
 jume 
 
 Z2 
 
 jurtE 
 £2 
 
 JULY 
 fa 
 
 JULY 
 & 
 
 JULY 
 
 30 
 
 JULY 
 20 
 
 AUG 
 
 3 
 
 AUG 
 
 3 
 
 AUG 
 
 17 
 
 17 
 AUG 
 31 
 
 AUG 
 31 
 
 5EPT 
 11 
 
 SEPT 
 "1 
 
 SEPT 
 zo 
 
 SEPT OCT. 
 2B II 
 
 OCT OCT 
 II Zfa 
 
 AV. 
 
 Cul+urc numboc 
 
 ' 
 
 £ 
 
 3- 
 
 4 
 
 -5 
 
 e 
 
 7 
 
 e 
 
 9 
 
 IO 
 
 It 
 
 I£ 
 
 
 Lenqth of qrowinq period, da^s. 
 
 17 
 
 11 
 
 i-l 
 
 I"* 
 
 ii 
 
 ii 
 
 M 
 
 11 
 
 11 
 
 H 
 
 13 
 
 15 
 
 
 Number of plants. 
 
 G 
 
 4 
 
 5 
 
 G 
 
 4 
 
 s- 
 
 S 
 
 S" 
 
 S 
 
 <£> 
 
 G 
 
 3 
 
 
 Remainder summation index. 
 
 13o 
 
 -1,53 
 
 15f 
 
 192 
 
 53Z 
 
 195 
 
 Si 3 
 
 JTI9 
 
 392 
 
 3BI 
 
 32 to 
 
 347 
 
 MS 
 
 Averacje dail\j relative physiological 
 tern pe rature i ndex 
 
 U7 
 
 112 
 
 MO 
 
 i3l 
 
 151 
 
 1 33 
 
 111 
 
 116 
 
 87 
 
 76 
 
 ei 
 
 1S 
 
 IO& 
 
 Averaqe dail\j mean temperafure 5 deq.F 
 
 65 
 
 71 
 
 7l 
 
 71 
 
 77 
 
 71 
 
 7G 
 
 76 
 
 G7 
 
 GG 
 
 61 
 
 62. 
 
 71 
 
 Averaqc dailv, relative evaporation 
 index. 
 
 95 
 
 130 
 
 I53 
 
 91 
 
 Qfa 
 
 I33 
 
 i as 
 
 II3 
 
 1 1 I 
 
 I04 
 
 ee 
 
 62. 
 
 ios 
 
 Averaqc Uail^j relative sunshine 
 i n tensity. 
 
 115 
 
 172 
 
 HI 
 
 120 
 
 12.1 
 
 too 
 
 H7 
 
 99 
 
 =37 
 
 9J 
 
 64 
 
 * 
 
 HI 
 
 Averaqe dail^ relative increment in 
 stem heiqht. 
 
 15 
 
 S7 
 
 I IS 
 
 132 
 
 IIO 
 
 116 
 
 1 32 
 
 US 
 
 8> 
 
 43 
 
 65 
 
 31 
 
 9S 
 
 Average dailv relative increment in 
 leaf- produc+r 
 
 71 
 
 97 
 
 LIZ. 
 
 99 
 
 ISZ 
 
 ISO 
 
 I63 
 
 I29 
 
 61 
 
 35 
 
 - 
 
 . - 
 
 IOS 
 
 PRIMCE55 AHNE 
 
 2- uj ee h pe rioclb 
 
 Exposed static r-< 
 
 may 
 
 M 
 MAY 
 26 
 
 MAY 
 
 JUME 
 
 B 
 
 juriE 
 
 23 
 
 JUHE 
 23 
 
 JULI 
 
 7 
 
 JULY 
 7 
 
 JULY 
 
 £l 
 
 2I 
 
 AUG. 
 1 
 
 1 
 
 AUm 
 
 IQ 
 
 IS 
 SEPT 
 
 5eP7 
 I 
 
 SEPT 
 15 
 
 5EFT 
 IS 
 
 SEPT 
 29 
 
 SEPI 
 29 
 
 OCT 
 12. 
 
 OCT. 
 
 12. 
 OCT 
 
 217 
 
 AV. 
 
 Culture number. 
 
 1 
 
 2 
 
 3 
 
 4 
 
 s 
 
 G 
 
 7 
 
 S 
 
 9 
 
 IO 
 
 II 
 
 12 
 
 
 Lenqtb o[ qrowmq period, days 
 
 15 
 
 13 
 
 IS 
 
 11 
 
 11 
 
 11 
 
 11 
 
 It 
 
 i1 
 
 19 
 
 13 
 
 IS 
 
 
 Number of plants. 
 
 6 
 
 5 
 
 6> 
 
 G 
 
 G 
 
 ta 
 
 3 
 
 fa 
 
 fa 
 
 S 
 
 S 
 
 ■4 
 
 
 Rcmaiuder Summation index. 
 
 358 
 
 -*o5 
 
 501 
 
 493 
 
 51^ 
 
 •J63 
 
 5i3 
 
 5ZO 
 
 3B7 
 
 3b3 
 
 315 
 
 32 S 
 
 132 
 
 Averoqe dailv relative pbvjsioloqical 
 tern pb'rotu re i nde X. 
 
 59 
 
 105 
 
 lOl 
 
 1 32 
 
 I44 
 
 I2C3 
 
 111 
 
 1 46 
 
 34 
 
 71 
 
 GO 
 
 13 
 
 IOI 
 
 Avcraqe dai 1^ mea n temperaTure.deq.ff 
 
 63 
 
 70 
 
 7S 
 
 T\ 
 
 7G 
 
 71 
 
 To 
 
 '76 
 
 67 
 
 63" 
 
 63 
 
 Gl 
 
 70 
 
 Averaqe dail\j relative evaporation 
 
 inde» 
 
 117 
 
 13-* 
 
 - 
 
 - 
 
 t>3 
 
 «3d 
 
 lOl 
 
 73 
 
 73 
 
 73 
 
 70 
 
 39 
 
 Q4 
 
 Averoqe daW^ relative, sunsnine 
 i ntfen-sitAj. 
 
 IOT 
 
 It© 
 
 103 
 
 as 
 
 as 
 
 9& 
 
 75" 
 
 So 
 
 79 
 
 S2. 
 
 &7 
 
 ^2 
 
 86 
 
 Averaqc dcuKj relative increment in 
 stem beicjbT: 
 
 62. 
 
 no 
 
 IOT 
 
 12-1 
 
 I«I7 
 
 113 
 
 ISS 
 
 iat 
 
 79 
 
 *5" 
 
 S3 
 
 45 
 
 I OS 
 
 Averoqe dail^ relative increment" in 
 leaf- product. 
 
 96 
 
 MS 
 
 l 04 
 
 MS 
 
 IG3 
 
 IOS 
 
 I77 
 
 I3S 
 
 61 
 
 31 
 
 IS 
 
 & 
 
 9& 
 
Climatic Conditions of Maryland 
 
 365 
 
 TABLE IV 
 Four-week data for exposed stations, Oakland, 
 
 Chewsville and Monrovia 
 
 OAKLAMD 
 
 4--A>eck periods 
 EXP05ED STATION 
 
 MAY 
 
 *» 
 
 June 
 
 June 
 5 
 
 rime 
 
 ii 
 
 JULY 
 16 
 
 JULT 
 
 3 
 
 JULY 
 31 
 
 JULY 
 
 AUG. 
 19 
 
 JULY 
 
 31 
 
 *uq. 
 Z7 
 
 4UG 
 
 M 
 SEPI 
 
 l£ 
 
 ■wc. 
 2i 
 
 
 
 
 
 AV^ 
 
 Culture number, 
 
 ' 
 
 z 
 
 3 
 
 9 
 
 5 
 
 <b 
 
 7 
 
 s 
 
 
 
 
 
 
 Le-nqtb of qrowu7q period , dai^E>, 
 
 27 
 
 2S 
 
 27 
 
 za 
 
 29 
 
 27 
 
 29 
 
 29 
 
 
 
 
 
 
 Number o r ' plants. 
 
 4 
 
 5 
 
 JJ 
 
 o 
 
 -5" 
 
 & 
 
 & 
 
 e 
 
 
 
 
 
 
 Remainder summation index. 
 
 663 
 
 TSS 
 
 765 
 
 795 
 
 BOO 
 
 713 
 
 719 
 
 1=52 
 
 
 
 
 
 73S 
 
 AvGracjc daM^ relative phvjsioloqicol 
 temperature index. 
 
 66 
 
 76 
 
 es 
 
 SJ 
 
 77 
 
 66 
 
 65 
 
 55 
 
 
 
 
 
 7f 
 
 Average dailu, mean temperature. deq.F" 
 
 65 
 
 OL-. 
 
 « 
 
 67 
 
 67 
 
 c5 
 
 64 
 
 62 
 
 
 
 
 
 65 
 
 Average duiKt relative cvaporatioo 
 index. 
 
 HG 
 
 119 
 
 69 
 
 9a 
 
 97 
 
 tii 
 
 64 
 
 63 
 
 
 
 
 
 94 
 
 Averuqc da.l^ relative sunshine 
 intensity. 
 
 i2£ 
 
 IIS 
 
 lOfe 
 
 MO 
 
 I09 
 
 06 
 
 97 
 
 82 
 
 
 
 
 
 lO.j 
 
 Average daiU relative increment in 1 
 stem heiqnt. 
 
 6b 
 
 7a 
 
 99 
 
 69 
 
 72 
 
 ei 
 
 63 
 
 J7 
 
 
 
 
 
 71 
 
 Avcra^G da'rl^ i-eJative, increment m | 
 leaf ar^a. 
 
 4l 
 
 <o7 
 
 91 
 
 75 
 
 eo 
 
 9o 
 
 72 
 
 32 
 
 
 
 
 
 71 
 
 A veracjc d ailv, relative incremcntin 1 
 drv. wc'iqbt. 
 
 o"l 
 
 97 
 
 io3 
 
 as 
 
 86 
 
 S3 
 
 T6 
 
 46 
 
 
 
 
 
 79 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 CMEW5VILLC 
 
 1-week periods* 
 EXPOSED STATION. 
 
 riAv 
 
 19 
 JUME 
 
 16 
 
 jurtc 
 jume 
 
 JO 
 
 JUME 
 
 16 
 JULY 
 
 JUKE 
 
 JULY 
 
 26 
 
 JULY 
 
 19 
 
 AUG 
 1 1 
 
 JULY 
 
 26 
 
 AUG. 
 25 
 
 Auq 
 1 1 
 
 StFT 
 
 a 
 
 AUG 
 
 25 
 5EP1 
 
 22 
 
 S7iPl 
 
 8 
 
 OCT. 
 
 7 
 
 SEPT 
 
 22 
 
 OCT 
 
 OCX 
 
 1 
 NOV 
 
 3 
 
 
 AV. 
 
 Cu l+ure nu mbcr 
 
 1 
 
 E. 
 
 3 
 
 9 
 
 5 
 
 6 
 
 T 
 
 e 
 
 9 
 
 to 
 
 It 
 
 
 
 Lenq+h of qrowinq period, da\is. 
 
 26 
 
 28 
 
 ze 
 
 £8 
 
 28 
 
 as 
 
 2a 
 
 ze 
 
 29 
 
 28 
 
 27 
 
 
 
 Number of plants. 
 
 6 
 
 fo 
 
 4 
 
 & 
 
 s 
 
 4 
 
 3 
 
 J 
 
 6 
 
 61 
 
 6 
 
 
 
 Remainder summation index 
 
 as 1 ) 
 
 SIS 
 
 925 
 
 976 
 
 973 
 
 96T 
 
 953 
 
 738 
 
 650 
 
 629 
 
 4 90 
 
 
 ©23 
 
 Averaqc dail^ relative pn^sioloqiaal 1 
 Tern perature index. 
 
 99 
 
 113 
 
 KG 
 
 131 
 
 131 
 
 1Z-9 
 
 129 
 
 7& 
 
 3"0 
 
 52 
 
 37 
 
 
 c)G 
 
 Averaqe daiKj meon "temperatLire,deq. ff 
 
 TO 
 
 It 
 
 72 
 
 tl 
 
 79 
 
 79 
 
 73 
 
 &e> 
 
 61 
 
 62 
 
 SB 
 
 
 69 
 
 Averaqe. da d\j relative evaporation 
 i iid^x. 
 
 H3 
 
 ••'. 
 
 91 
 
 qz 
 
 98 
 
 96 
 
 93 
 
 so 
 
 76 
 
 69 
 
 71 
 
 
 90 
 
 Averaqe da'il^ relative sunshine 
 
 1 25 
 
 12.3 
 
 „. 
 
 1 19 
 
 ioa 
 
 96 
 
 B3 
 
 86 
 
 89 
 
 59 
 
 56 
 
 
 9J 
 
 Averaqe dculsi relative increment in 1 
 stem heiqnt. 
 
 1-S 
 
 103 
 
 122 
 
 ^1 
 
 91 
 
 64 
 
 &4 
 
 SO 
 
 41 
 
 4 1 
 
 31 
 
 
 IS 
 
 Averaqe dailv. relative increment in 1 
 
 73 
 
 91 
 
 132 
 
 ©3 
 
 89 
 
 94 
 
 99 
 
 37 
 
 44 
 
 45 
 
 12 
 
 
 79 
 
 Averaqe daik re-latwe increment in 1 
 O r ^ v»eiqlit 
 
 IS 
 
 IO£ 
 
 142 
 
 BT 
 
 92 
 
 S6 
 
 &e> 
 
 62 
 
 51 
 
 38 
 
 24 
 
 
 76 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 MONROVIA 
 
 A «ceh periods, 
 EXFOSED STATiOrt 
 
 MAY 
 
 18 
 
 >uitE 
 
 i5 
 
 JUNE 
 2S 
 
 JUNE 
 IS 
 
 JULY 
 13 
 
 juris 
 
 29 
 
 2T 
 
 JULY 
 IS 
 
 AU<J 
 IO 
 
 JULY 
 ■27 
 
 AUG 
 2*) 
 
 AUG. 
 
 to 
 
 SEPT 
 
 7 
 
 AU<5 
 24 
 
 3EPT 
 21 
 
 3EFT 
 
 7 
 OCT. 
 
 a 
 
 3EPT 
 
 21 
 OCt 
 
 19 
 
 OCT 
 
 a 
 rxov 
 
 2. 
 
 
 AV. 
 
 Culture number 
 
 1 
 
 e 
 
 3 
 
 9 
 
 J 
 
 S 
 
 T 
 
 e 
 
 9 
 
 IO 
 
 " 
 
 
 
 Lenqfb of qrowinq • period, daij-s 
 
 ee 
 
 ee 
 
 ee 
 
 ee 
 
 2© 
 
 He 
 
 28 
 
 £8 
 
 31 
 
 28 
 
 25 
 
 
 
 Mumbtir o^ plants. 
 
 3 
 
 3 
 
 & 
 
 9 
 
 G 
 
 3 
 
 & 
 
 3 
 
 6 
 
 6 
 
 & 
 
 
 
 Remainder summation index. 
 
 i7& 
 
 932 
 
 932 
 
 9SO 
 
 999 
 
 979 
 
 9<H 
 
 an 
 
 770 
 
 646 
 
 963 
 
 
 851 
 
 Averaqe daily relative pbn&ioloqical 
 temperature index 
 
 IOS 
 
 117 
 
 "S 
 
 134 
 
 137 
 
 131 
 
 12. J 
 
 82 
 
 S£> 
 
 56 
 
 53 
 
 
 IOZ 
 
 Averaqe dailv^ rnGon temperature,deqF 
 
 - 
 
 72. 
 
 72 
 
 79 
 
 IS 
 
 14 
 
 73 
 
 66 
 
 &Z 
 
 ©3 
 
 SS 
 
 
 6-^ 
 
 Averaqe dail^ relative evaporation 
 
 indc* 
 
 - 
 
 1-99 
 
 12.3 
 
 105 
 
 ..? 
 
 1 IG 
 
 90 
 
 82 
 
 89 
 
 79 
 
 81 
 
 
 lOl 
 
 Averaqe dail^ relative sunshine 
 inte.nsit\j 
 
 (37 
 
 M3 
 
 103 
 
 106 
 
 iob 
 
 I02 
 
 OO 
 
 94 
 
 do 
 
 S9 
 
 5fl 
 
 
 96 
 
 Averaqe daii^ reiaTive increment In 
 stem heiqht 
 
 63 
 
 78 
 
 8 4 
 
 84 
 
 7a 
 
 91 
 
 84 
 
 Sb 
 
 49 
 
 41 
 
 zd 
 
 
 66 
 
 Avcroqe dallu relative increment in 
 leuf orea 
 
 71 
 
 iO0 
 
 68 
 
 99 
 
 71 
 
 jOfl 
 
 7J 
 
 TO 
 
 39 
 
 43 
 
 12 
 
 
 71 
 
 Avcroqe da.lu relative increment in 
 drq vvQiqnt, 
 
 70 
 
 i i*l 
 
 lOfi 
 
 99 
 
 a9 
 
 I&5" 
 
 81 
 
 87 
 
 41 
 
 48 
 
 24 
 
 
 
 79 
 
366 
 
 F. Merrill Hildebeandt 
 
 TABLE V 
 Four-week data for exposed stations, College, Baltimore and Darlington 
 
 COLLEGE 
 
 4-weck ' periods. 
 
 EXPOSBO STATlO^ 
 
 Culture number. 
 
 Length of qrowinq period,daxjs. 
 dumber of- plants. 
 
 Remainder SLimmaHon Indc^. 
 
 tZ^r^Ut?^. p-s»-.oi«..-cui- 
 
 Averaqc doiU) mcuu feinperohire, Jcq. K 
 
 fl «™3 c d °' K > rclot.ve evaporation" 
 
 sSS^ai'M ^'^'"c increment- i,, 
 
 I^Jt- a S?ca dt "^ rolu "^ 'Mcrcniontio 
 
 ^Wa^,^ S re|uh<g , ncremaBt , ,„ 
 
 BALTIMORE 
 
 Culture lumber 
 
 Lenqti? of qro^inq period, da s5 
 Number of plants. 
 
 Remainder summation indax. 
 
 /Wage dall, meon-hamjaralura.deq F 
 
 "mdeT da ''^ re "*>"»«porot,o»- 
 
 aqe da.lx. relative 
 ^n3i7\j. 
 
 "^^h?.?fi(. """*"* "—ept ,„" 
 
 "^^q^TKj relaW incr5; ^ 5;n _. 
 
 ^Sr^S!^^ 
 
 JULI 
 
 17 
 AUG 
 
 AUG 
 SEPT. 
 
 AUG. 
 £7 
 SEPT. 
 
 DARLIMGTOM 
 
 1-ujeek periods 
 Expq 5ep station. 
 
 Culture number 
 
 Lenq+h of qrow.nq period, dcn Js . 
 
 Number of plants. 
 
 Remainder summation index. 
 
 _^raqad QI |. i mean temperature.^! 
 
 *irtd£ e da, 't rslo1 "~ e ""PoraTioiT" 
 
 
Climatic Conditions of Maryland 
 
 367 
 
 TABLE VI 
 Four-week data for exposed stations, Coleman, Easton and Princess Anne 
 
 COLEMAN 
 
 4- week periods. 
 EXPOSED STATION. 
 
 13 
 
 ii 
 
 1AH 
 
 JUNE 
 
 -■I 
 
 IUL.V . 
 
 <> 
 
 UMI 
 
 24 
 ULY. 
 
 JUkf JUUVAUQ AUG 3EFr. 
 
 8 Z2 S 19 £ 
 AUG AUG, SEPT SEFT 3EPT 
 S 19 z. IG 30 
 
 DEPT 
 
 IG 
 OCT. 
 
 .111 
 
 ZO 
 OCT. 
 2CJ 
 
 AV. 
 
 Culture number 
 
 i 
 
 C 
 
 3 
 
 1 
 
 5 
 
 S 
 
 7 
 
 e 
 
 9 
 
 IO 
 
 II 
 
 
 
 Lenqt" °{ qrowinq period, cla\js. 
 
 =9 
 
 zi 
 
 .-' ( 
 
 £6 
 
 ze 
 
 2S 
 
 ee 
 
 28 
 
 ze 
 
 27 
 
 26 
 
 
 
 lumber of plants- 
 
 G 
 
 & 
 
 1 
 
 1 
 
 G 
 
 G 
 
 ^ 
 
 G 
 
 s 
 
 4 
 
 37 
 
 
 
 Remainder sutnmah'on index. 
 
 OIO 
 
 B99 
 
 ■>2l 
 
 04s 
 
 OT7 
 
 OSS 
 
 050 
 
 qio 
 
 709 
 
 ' 
 
 " 
 
 
 939 
 
 Average daily relative. piT\|sIo]oqical 
 temperature index. 
 
 9T 
 
 II& 
 
 121 
 
 ISO 
 
 isa 
 
 ISE 
 
 133 
 
 to 
 
 85 
 
 - 
 
 " 
 
 
 IZS 
 
 73 
 
 Averaqe daily mean tcmperciture.,dcq. f: 
 
 G9 
 
 io 
 
 II 
 
 17 
 
 Ttl 
 
 77 
 
 77 
 
 72. 
 
 67 
 
 - 
 
 - 
 
 
 Averaqe dai Im relative eva poraticm 
 index. 
 
 i-io 
 
 145 
 
 iaq 
 
 132. 
 
 119 
 
 i a r 
 
 ioe 
 
 I 14 
 
 134 
 
 116 
 
 I S 
 
 
 I2B 
 
 Averaqe daily relative sunshine, 
 
 in+cHSit-j. 
 
 iSi 
 
 113 
 
 93 
 
 IOI 
 
 iiJ 
 
 I2I 
 
 102 
 
 - 
 
 - 
 
 " 
 
 - 
 
 
 I2.I 
 
 Averaqe dail-y relative increment in 
 stem lieiqiit; 
 
 6') 
 
 Bl 
 
 94 
 
 lOG 
 
 IOQ 
 
 1 09 
 
 B4 
 
 72 
 
 S3 
 
 SO 
 
 53 
 
 
 ao 
 
 MveAjc\c daiVy relative increment in 
 leaf area. 
 
 116 
 
 ISO 
 
 1ST 
 
 111 
 
 I3S 
 
 1-4 5 
 
 ioe 
 
 89 
 
 71 
 
 - 
 
 MT 
 
 
 I IT 
 
 Averaqe daily relative increment in 
 dry, w » qiit 
 
 IZG 
 
 1 53 
 
 159 
 
 143 
 
 I34 
 
 H6 
 
 83 
 
 113 
 
 oT 
 
 75 13 
 
 
 (13 
 
 
 
 
 
 
 
 
 
 
 
 £A5TOrS 
 
 1-u;eeV\ periods. 
 
 EiPObCU JTATlOM. 
 
 S 
 
 a 
 
 JUNE 
 22 
 
 jura 
 a 
 
 juru 
 
 JULY 
 ZO 
 
 JUL.Y 
 to 
 
 AUG, 
 
 3 
 
 ZO 
 
 AUG. 
 17 
 
 AUS. 
 
 AUG. 
 
 31 
 
 AUG 
 17 
 
 AUG 
 11 
 
 AUG. SCF7 
 31 11 
 
 sEPt ocr. 
 26 II 
 
 5EP1 
 
 ^a 
 
 OCT. 
 ZG 
 
 OCT. 
 
 nov. 
 
 AV. 
 
 Culture number 
 
 i 
 
 2 
 
 3 
 
 4 
 
 3 
 
 £> 
 
 7 
 
 8 
 
 9 
 
 IO 
 
 II 
 
 1 2. 
 
 
 Lenqth oj-qrowioq period, daqs 
 
 31 
 
 28 
 
 ze 
 
 26 
 
 ze 
 
 36 
 
 28 
 
 ze 
 
 SO 
 
 Z7 
 
 2e 
 
 26 
 
 
 Mumber- of plante- 
 
 & 
 
 4 
 
 ^ 
 
 G 
 
 1 
 
 3" 
 
 37 
 
 5 
 
 s 
 
 G 
 
 & 
 
 ■3 
 
 
 Remainder summation, index. 
 
 e»9 
 
 901 
 
 941 
 
 |OZ1 
 
 lOi7 
 
 100a 
 
 1032 
 
 9H 
 
 T75 
 
 707 
 
 G73 
 
 ■499 
 
 SM 
 
 Averaqe dailvj relative pbijaio/oqical 
 "temperature, index.. 
 
 go 
 
 „, 
 
 12.1 
 
 113 
 
 111 
 
 139 
 
 1137 
 
 II7 
 
 82 
 
 70 
 
 3"G 
 
 39 
 
 i03 
 
 Averaqe daiN mean "te.mpcrature,dcq. r~ 
 
 tfe 
 
 Tl 
 
 73 
 
 7G 
 
 76 
 
 73" 
 
 To 
 
 72 
 
 G7 
 
 G3- 
 
 G3 
 
 3"G 
 
 70 
 
 Averaqe doilv relative evaporation 
 index. 
 
 115 
 
 132 
 
 ME 
 
 89 
 
 no 
 
 131 
 
 I2I 
 
 I 12 
 
 IOS 
 
 96 
 
 S3", 
 
 93 
 
 iOR 
 
 A vc r-a qc ciu'i l ^ rel at i\e su nshi ne 
 in+ensit^. 
 
 isq 
 
 IS7 
 
 131 
 
 \Z2. 
 
 I IG 
 
 113 
 
 IOS 
 
 ge 
 
 ■9G 
 
 eo 
 
 sa 
 
 GO 
 
 I03 
 
 Averaqe daily relative increment in 
 s,tcm heiqlir. 
 
 it 
 
 72 
 
 91 
 
 I03 
 
 103 
 
 1 13 
 
 I OJ 
 
 ei 
 
 J6 
 
 JO 
 
 3"G 
 
 30 
 
 75 
 
 Averaqe daily relative increment" in 
 leaf area. 
 
 T4 
 
 T^, 
 
 ,1, 
 
 qo 
 
 IOI 
 
 121 
 
 I2I 
 
 93" 
 
 GG 
 
 U.-1 
 
 3d 
 
 8 
 
 82 
 
 Average daily relative increment in 
 drq weiqht. 
 
 SO 
 
 SI 
 
 119 
 
 9i 
 
 IIS 
 
 ■ 2.1 
 
 i 21 
 
 ei 
 
 72 
 
 1a 
 
 32 
 
 22 
 
 83 
 
 
 
 
 PRIMCE55 /\nriE 
 
 1-ujeek periods. 
 
 E.X POSSD 5r^TIO/S 
 
 i-iav 
 
 juru 
 o 
 
 MAY 
 Z3 
 
 JUNE 
 
 JUU 
 7 
 
 dimf 
 23 
 
 7 
 
 AUG 
 
 ia 
 
 sei'r 
 
 ia 
 
 5EP 
 
 IS 
 
 SET 
 
 SEP 
 29 
 
 S£ P 
 IS" 
 
 ocr 
 ia 
 
 5Ef 
 29 
 
 ocr 
 27 
 
 
 AV. 
 
 Culture numben 
 
 1 
 
 z 
 
 3 
 
 *l 
 
 5 
 
 t? 
 
 7 
 
 e 
 
 9 
 
 IO 
 
 II 
 
 
 
 Lenqth of qrow w?q period, days. 
 
 zs 
 
 ze 
 
 £9 
 
 ze 
 
 za 
 
 aa 
 
 za 
 
 £S 
 
 aa 
 
 27 
 
 ze. 
 
 
 
 Number of plants. 
 
 1 g 
 
 5 
 
 .5 
 
 » 
 
 G 
 
 fa 
 
 3 
 
 G 
 
 & 
 
 S 
 
 & 
 
 
 
 FlCiiiainclcr summation index.. |7&3 
 
 909 
 
 997 
 
 IOO" 
 
 997 
 
 99fa 
 
 1 033 
 
 =?OT 
 
 730 
 
 678 
 
 G43 
 
 
 sao 
 
 Averaqe da\Kj relative phijsioloqicul 
 temperature "index. 
 
 £< 
 
 I02 
 
 117 
 
 is a 
 
 I3C 
 
 136 
 
 113 
 
 I IO 
 
 78 
 
 GG 
 
 52 
 
 
 106 
 
 Averaqe daily mean temperature, deq.F 
 
 61 
 
 73 
 
 IS 
 
 7J 
 
 73 
 
 "5 
 
 7G 
 
 72 
 
 fc>6 
 
 G1 
 
 G2 
 
 
 7 i 
 
 Averaqe daily relative evaporation 1 
 
 indev 
 
 126 
 
 - 
 
 - 
 
 - 
 
 73 
 
 92 
 
 90 
 
 7S 
 
 7e 
 
 74 
 
 55 
 
 
 83 
 
 Averaqe daily relative sunshine 
 intensity 
 
 U.7. 
 
 ,„ 
 
 94 
 
 07 
 
 9Z- 
 
 BO 
 
 ei 
 
 S3 
 
 ei 
 
 73- 
 
 S3~ 
 
 
 67 
 
 Averaqe daily relative increment in 
 stern helq lit. 
 
 o 
 
 ee» 
 
 103 
 
 IO". 
 
 i3a 
 
 iaj 
 
 1 36 
 
 ri4 
 
 &3 
 
 3b 
 
 1^ 
 
 
 92 
 
 Averaqe daiJtj relative increment in 
 leaf **r&a. 
 
 tz.3 
 
 \S5 
 
 no 
 
 - 
 
 1 27 
 
 ™ 
 
 ZC*. 
 
 IOE 
 
 ->■■• 
 
 72 
 
 36 
 
 
 II7 
 
 Averaqe daily relative increment "m 
 dry weiqljt: 
 
 !._'■._ 
 
 JOC 
 
 137 
 
 i If 
 
 I37 
 
 133 
 
 I7I 
 
 1 ? 
 
 a > 
 
 
 
 e.7 
 
 38 
 
 
 HG 
 
368 
 
 F. Merrill Hildebrandt 
 
 TABLE VII 
 
 Data for covered stations, Oakland and Baltimore 
 
 OAKLAMD 
 
 5-week periods. 
 
 COveRED ^>T^-riQr*4. 
 
 MAY 
 22 
 
 JUNE 
 
 4 
 
 JUNE 
 
 4 
 
 JUNE 
 
 ia 
 
 JUflE 
 16 
 
 JULY 
 
 2 
 
 july 
 
 z 
 
 JULY 
 
 15 
 
 IS 
 
 JULY 
 30 
 
 JULY 
 30 
 
 AUG. 
 13 
 
 AUQ. 
 1.5 
 
 AUG- 
 26 
 
 AUQ. 
 
 SO 
 
 sept 
 
 5EPT 
 
 3EPT 
 24 
 
 
 
 
 AV 
 
 Culture number 
 
 1 
 
 z. 
 
 Z> 
 
 •q 
 
 3 
 
 £> 
 
 7 
 
 & 
 
 9 
 
 
 
 
 
 Lenqth of qrowinq period, daus. 
 
 13 
 
 ia 
 
 14 
 
 \2> 
 
 |3 
 
 1-4 
 
 13 
 
 iG 
 
 i3 
 
 
 
 
 
 dumber of plants. 
 
 s 
 
 <3 
 
 fc 
 
 3 
 
 e? 
 
 - 
 
 3 
 
 3 
 
 3 
 
 
 
 
 
 Ayeraqe daiKj relative evaporation 
 index. 
 
 nz 
 
 147 
 
 133 
 
 log 
 
 Hi 
 
 112, 
 
 i ia 
 
 85 
 
 93 
 
 
 
 
 120 
 
 Averaqe daiKj relative increment 
 in stem heicjlffc 
 
 so 
 
 112 
 
 DO 
 
 12.1 
 
 ICO 
 
 - 
 
 S4 
 
 TO 
 
 5& 
 
 
 
 
 92 
 
 Averaqc daiKj relative irrcrement 
 iii leaf -product: 
 
 IS 
 
 85 
 
 12.7 
 
 96 
 
 134 
 
 - 
 
 B9 
 
 <34 
 
 9 
 
 
 
 
 T8 
 
 OAKLAND 
 
 4-week periods. 
 
 COVeR&D STATION. 
 
 MAY 
 
 JUNE 
 IS 
 
 JUNE 
 4 
 
 JULY 
 2 
 
 JUNE 
 10 
 
 JULY 
 15 
 
 JULY 
 
 2 
 
 JULY 
 30 
 
 JULY 
 
 t5 
 AUG. 
 
 13 
 
 JULY 
 30 
 
 AUG.. 
 2fo 
 
 AUG 
 13 
 
 5EPr 
 
 n 
 
 AUG, 
 2fi 
 
 SEPT. 
 Zf\ 
 
 
 
 
 
 AV. 
 
 Culture number 
 
 1 
 
 2 
 
 3 
 
 4 
 
 5 
 
 6 
 
 7 
 
 8 
 
 
 
 
 
 
 Lenqth of qrowincj period, da^s. 
 
 27 
 
 28 
 
 Z.T 
 
 ZG 
 
 29 
 
 27 
 
 sq 
 
 2.9 
 
 
 
 
 
 
 dumber of plants. 
 
 S 
 
 6 
 
 e 
 
 5 
 
 6 
 
 - 
 
 3 
 
 -5 
 
 
 
 
 
 
 Ayeraqe daiKi relative evaporation 
 i a d ex . 
 
 l&O 
 
 190 
 
 izj 
 
 1 13 
 
 \ 15 
 
 * 
 HZ 
 
 99 
 
 79 
 
 
 
 
 
 in 
 
 Averaqe daily relative increment in 
 stent taeiqhlr. 
 
 56 
 
 Si 
 
 ee 
 
 ioe> 
 
 °n 
 
 - 
 
 03 
 
 so' 
 
 
 
 
 
 61 
 
 Averaqe daiKj relative increment \n 
 leaf area. 
 
 - 
 
 107 
 
 145 
 
 155 
 
 1 IS 
 
 - 
 
 - 
 
 66. 
 
 
 
 
 
 l 16 
 
 Ayeraqe dail^j relative increment 
 in drq weiqhh 
 
 57 
 
 9T 
 
 12.1 
 
 123 
 
 ... 
 
 - 
 
 80 
 
 59 
 
 
 
 
 
 93 
 
 BALTIMORE. 
 
 j-wecK penoas. 
 
 CQVERED STATION. 
 
 
 
 JUNE 
 IO 
 
 JUNE 
 £5 
 
 JUNE 
 ZS 
 
 JULY 
 
 . 9 
 
 JULY 
 9 
 
 JULY 
 23 
 
 JULY 
 23 
 
 AUQ. 
 & 
 
 AUG 
 
 6 
 
 Aua 
 20 
 
 AUG- 
 20 
 
 SEPT 
 
 3 
 
 SEPT 
 
 3 
 SEPT 
 
 19 
 
 SEPT 
 19 
 
 OCT. 
 
 OCT. 
 
 OCT. 
 
 14 
 
 
 AV. 
 
 C u 1 1 u i^e number: 
 
 
 
 3 
 
 4. 
 
 3 
 
 & 
 
 7 
 
 e 
 
 9 
 
 \o 
 
 1 1 
 
 
 
 Lenqrh of qrowinq period, days. 
 
 
 
 15 
 
 H 
 
 11 
 
 14 
 
 14 
 
 14 
 
 16 
 
 12. 
 
 13 
 
 
 
 Number of plants. 
 
 
 
 & 
 
 6 
 
 4 
 
 6 
 
 J 
 
 J3 
 
 3 
 
 .5 
 
 •A 
 
 
 
 Averaqe clailvj relative evaporation 
 i ndex . 
 
 
 
 U9 
 
 9-5 
 
 112. 
 
 log 
 
 I l 7 
 
 ST 
 
 SI 
 
 TT 
 
 73 
 
 
 97 
 
 Averaqe dail^ relative incremenif 
 In stem he'iq n/n 
 
 
 
 157 
 
 i&o 
 
 326 
 
 233 
 
 197 
 
 191 
 
 36> 
 
 70 
 
 155 
 
 
 172 
 
 Averaqe daiKj relative increment" in 
 leaf - product. 
 
 
 
 1 1 1 
 
 150 
 
 eoz 
 
 W7 
 
 ieo 
 
 IS3 
 
 51 
 
 21 
 
 ■43 
 
 
 125 
 
 BALTIMORE 
 
 4-weck periods. 
 COVERED 5TATIOJ-H. 
 
 
 MAY 
 29 
 
 JUNE 
 25 
 
 JUNE 
 IO 
 
 JULY 
 9 
 
 JUNE 
 25 
 
 JULY 
 23 
 
 JULY 
 
 9 
 
 AUG. 
 
 <3 
 
 JULY 
 23 
 
 AUG". 
 20 
 
 AUG 
 
 3EPT 
 
 3 
 
 AUG; 
 
 zo 
 
 5Epr 
 19 
 
 SEPT 
 
 3 
 OCT, 
 
 1 
 
 SEPT 
 
 19 
 OCT 
 
 14 
 
 OCT. 
 
 OCT 
 3» 
 
 
 A.V. 
 
 Culture number 
 
 
 z 
 
 3 
 
 4 
 
 3 
 
 6 
 
 7 
 
 8 
 
 9 
 
 IO 
 
 H 
 
 
 
 Lenqth of qrowinq period, day3. 
 
 
 2.7 
 
 29 
 
 Z6 
 
 2S 
 
 2S 
 
 2© 
 
 30 
 
 z& 
 
 25 
 
 30 
 
 
 
 Number of plants. 
 
 
 S 
 
 b 
 
 & 
 
 A 
 
 & 
 
 3 
 
 3 
 
 3 
 
 3 
 
 ■4. 
 
 
 
 Averaqe dai Ki relative evaporation 
 index. 
 
 
 119 
 
 IOT. 
 
 107 
 
 11) 
 
 t 13 
 
 102 
 
 84 
 
 79 
 
 73 
 
 70 
 
 
 97 
 
 Ayeroqe daiKj relative increment 
 in s re m hefq ht- 
 
 
 156 
 
 I2S 
 
 134 
 
 231 
 
 IG9 
 
 153 
 
 131 
 
 72 
 
 I03 
 
 78 
 
 
 I3& 
 
 A ve raqe d a 1 Kj re 1 a ti ve inc re m <s.\-& 
 in leaf area. 
 
 
 " 
 
 -- 
 
 >&6 
 
 23Q 
 
 137 
 
 I'M 
 
 l-S"i 
 
 ri 
 
 IOI 
 
 - 
 
 
 154 
 
 Averaqe daily relative increment 
 m dry weiqrir. 
 
 
 (34 
 
 5*1 
 
 1 IO 
 
 163 
 
 103 
 
 135; 
 
 >14 
 
 6^( 
 
 SO 
 
 3i 
 
 
 9S 
 
Climatic .Conditions of Maryland 
 
 369 
 
 TABLE VIII 
 
 Data for covered station at Easton and for forest station at Baltimore 
 
 EASTorn 
 
 2-weeK periods. 
 COVERED 3TATIOM, 
 
 
 MAY 
 JUNE 
 
 a 
 
 JUNE 
 
 a 
 
 JUNE 
 22 
 
 JUNE 
 22 
 
 JULY 
 6 
 
 JULY 
 
 JULY 
 20 
 
 JULY 
 ZO 
 
 AUG 
 3 
 
 AUG 
 3 
 
 AUG 
 
 n 
 
 AUG 
 
 17 
 
 AUG 
 
 31 
 
 AUG 
 
 31 
 5EPT 
 
 11 
 
 SEPI 
 
 11 
 
 SEPT 
 
 es 
 
 SLIT 
 
 za 
 
 OCT 
 1 I 
 
 
 AV. 
 
 Culture number 
 
 
 z 
 
 3 
 
 4 
 
 J> 
 
 & 
 
 7 
 
 & 
 
 9 
 
 to 
 
 1 1 
 
 
 
 Lenqtlz op qrovv'mq period, da^s. 
 
 
 11 
 
 14 
 
 11 
 
 H 
 
 14 
 
 11 
 
 14 
 
 14 
 
 14 
 
 14 
 
 
 
 Number of plants. 
 
 
 A 
 
 e 
 
 ■1 
 
 3 
 
 (3 
 
 & 
 
 J 
 
 6 
 
 fa 
 
 e> 
 
 
 
 Ayeraqe dailvj relative evaporation 
 index. 
 
 
 1 66 
 
 135 
 
 SO 
 
 I I2 
 
 i-m 
 
 is9 
 
 I3i 
 
 131 
 
 104 
 
 IO0 
 
 
 12.4 
 
 Averaqe daily relative iiicrenicnhn 
 stern heiqkr. 
 
 
 i 29 
 
 i ie 
 
 1" 
 
 n t 
 
 |77 
 
 177 
 
 \&S 
 
 <?o 
 
 9C> 
 
 " 
 
 
 145 
 
 Averaqe daiKi relative increment 
 ia leaf- product 
 
 
 IZ.Z 
 
 .13 
 
 eq 
 
 ISO 
 
 ies 
 
 ZIO 
 
 ZZ5 
 
 13.5 
 
 55 
 
 - 
 
 
 (4Z. 
 
 LtASTOfH 
 
 4- week periods. 
 
 COVERED 5TAriON, 
 
 
 MAY 
 
 JUNE 
 Z-Z 
 
 JUHE 
 
 a 
 
 JULY 
 
 JUNE 
 22 
 
 JULY 
 SO 
 
 JULY 
 
 £. 
 
 AUG- 
 
 3 
 
 JULY 
 
 ZO 
 
 AUG. 
 17 
 
 AUG. 
 
 3 
 
 AUG 
 31 
 
 AUG 
 17 
 
 5EPT 
 14 
 
 AUG 
 51 
 
 SEPT 
 2S 
 
 SEFT 
 
 14 
 OCT. 
 
 1 1 
 
 SEPT 
 
 ze 
 
 OCT 
 £6 
 
 OCT 
 
 MOV. 
 fa 
 
 AV. 
 
 Cu 1 tui'C number 
 
 
 Z 
 
 3 
 
 4 
 
 5 
 
 6 
 
 T 
 
 e 
 
 9 
 
 IO 
 
 U 
 
 12. 
 
 
 Lenqth o[ qrowi'nq pen ocl* daws. 
 
 
 z.e 
 
 ze 
 
 as 
 
 ze 
 
 ZS 
 
 28 
 
 28 
 
 2& 
 
 £7 
 
 ZS 
 
 26 
 
 
 Number of plants 
 
 
 4 
 
 6 
 
 & 
 
 - 
 
 6 
 
 ° 
 
 •S - 
 
 6 
 
 fa 
 
 ~ 
 
 - 
 
 
 Averaqe daiivj relative evaporation 
 index. 
 
 
 iSl 
 
 loe 
 
 qG 
 
 • 
 
 137 
 
 130 
 
 I3t 
 
 US 
 
 «OG 
 
 icjA 
 
 n& 
 
 I2.0 
 
 Averaqe daiivj relative increment 
 in stem he'iq^t. 
 
 
 103 
 
 12a 
 
 12-5 
 
 - 
 
 134 
 
 IS& 
 
 125 
 
 75 
 
 ei 
 
 - 
 
 - 
 
 H6 
 
 Averaqe daiKj relative, increment 
 in lea f ar~G.a. 
 
 
 1 17 
 
 i9' 
 
 90 
 
 - 
 
 138 
 
 202 
 
 1<S9 
 
 104 
 
 es 
 
 - 
 
 - 
 
 137 
 
 Averaqe ctaiKj relative increment 
 "hi dv~y weight". 
 
 
 108 
 
 144 
 
 ee 
 
 - 
 
 107 
 
 HO 
 
 1^1 
 
 tos 
 
 5(5 
 
 - 
 
 — 
 
 109 
 
 BALTIMORE. 
 
 2-week periods. 
 FOREST 5TAT fOM. 
 
 
 
 
 JUhE 
 
 JULY 
 
 9 
 
 JULY 
 9 
 
 JULY 
 £3 
 
 JULY 
 Z3 
 
 AUG 
 & 
 
 AUG 
 6> 
 
 AUG 
 20 
 
 AUG 
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 SEPI 
 3 
 
 5EFT 
 3 
 
 SEPT 
 19 
 
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 19 
 OCT. 
 
 OCT 
 
 OCT. 
 14 
 
 
 AV. 
 
 C u It li re n li m be r 
 
 
 
 
 4 
 
 5 
 
 fa 
 
 7 
 
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 9 
 
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 Lenqtb of q^owinq period , da\js. 
 
 
 
 
 iQ 
 
 14 
 
 14 
 
 14 
 
 14 
 
 l& 
 
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 13 
 
 
 
 number of plants 
 
 
 
 
 e> 
 
 ■5" 
 
 " 
 
 ^ 
 
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 ■4 
 
 6 
 
 3 
 
 
 
 Ayei-aqe dailvj relative evaporation 
 index. 
 
 
 
 
 - 
 
 75 
 
 * 
 
 82 
 
 67 
 
 72 
 
 G4 
 
 57 
 
 52 
 
 
 G7 
 
 Averaqe dail\j relative increment 
 in srem neiqlit. 
 
 
 
 
 z&i 
 
 458 
 
 - 
 
 396 
 
 Z©4 
 
 171 
 
 1Z.I 
 
 ZOS 
 
 
 Z7l 
 
 Averaqe dailu relative increment in 
 lea f - p rod uor. 
 
 
 
 
 94 
 
 to 4 
 
 - 
 
 136 
 
 59 
 
 13 
 
 & 
 
 at 
 
 
 ea 
 
 BALTIMORE. 
 
 4-week periods. 
 
 FOR.£ST- 5TATION 
 
 
 
 
 june 
 
 21 
 
 JULY 
 23 
 
 JULY 
 9 
 
 AUG. 
 & 
 
 JULY 
 23 
 
 AUG. 
 20 
 
 AUG 
 
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 3 
 
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 3 
 
 OCT 
 
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 SEPT. 
 
 OCT. 
 14 
 
 oar. 
 1 
 
 OCX 
 
 31 
 
 
 AV. 
 
 Culture number 
 
 
 
 
 4 
 
 5 
 
 6 
 
 7 
 
 & 
 
 9 
 
 10 
 
 It 
 
 
 
 Lenqth of qrowinq period, days. 
 
 
 
 
 32. 
 
 2S 
 
 ZO 
 
 ze 
 
 30 
 
 ze 
 
 Z.5 
 
 30 
 
 
 
 rtuimber o\ plants. 
 
 
 
 
 G 
 
 3 
 
 - 
 
 4 
 
 5 
 
 5 
 
 e 
 
 3 
 
 
 
 Averaqe daikj relative evaporation 
 index.- 
 
 
 
 
 - 
 
 79 
 
 *- 
 ■75 
 
 70 
 
 &S 
 
 Gl 
 
 55 
 
 55 
 
 
 66 
 
 Averaqe da<"l\j relative increment in 
 stem heiqnt. 
 
 
 
 
 300 
 
 444 
 
 - 
 
 310 
 
 Z13 
 
 200 
 
 zoo 
 
 12.5 
 
 
 256 
 
 Averaqe daibj relative increment 
 in leaf area. 
 
 
 
 
 104 
 
 104 
 
 - 
 
 9-5 
 
 73 
 
 40 
 
 36 
 
 aa 
 
 
 69 
 
 Averaqe dailu relative increment 
 in di~\j wciqnt. 
 
 
 
 
 .52 
 
 G2 
 
 - 
 
 4G 
 
 37 
 
 30 
 
 33 
 
 2.7 
 
 
 41 
 
370 F. Merrill Hildebrandt 
 
 The 2-week tables for the exposed stations (tables I— III) show in line 5 
 the remainder-summation temperature index for each culture period, this 
 being obtained by subtracting 39° from each daily mean and then summing 
 the remainders for the period. Line 6 gives the average daily relative 
 physiological index for each period. Line 7 gives the average daily mean 
 temperatures for each period, line 8 the average daily relative evaporation 
 index, and line 9, the average daily relative sunshine-intensity value Line 
 10 shows the values of the average daily relative increment of stem height, 
 and line 11 the values of the average daily relative increment of leaf-product. 
 The two-week tables for the covered stations correspond to the two-week 
 tables for the exposed stations, except that no temperature or sunshine data 
 are here available and the tables thus contain only the relative evaporation 
 indices and the two plant values. This is also true for the Baltimore forest 
 station. 
 
 The 4-week tables correspond, line for line, with the 2-week ones, except 
 that the 4-week tables show the average daily relative increment of leaf 
 area (instead of the average daily relative increment of leaf-product) and a 
 line is added to the 4-week tables giving the average daily relative increment 
 of dry weight. Each 4-week value of the relative daily physiological temper- 
 ature index, the relative daily evaporation index, and the relative daily 
 sunshine intensity, was obtained by averaging the relative values of these 
 climatic factors for the 2-week periods in question. The 4-week value 
 of the remainder-summation index for each period was obtained by adding 
 the values of this index for the two 2-week periods that make up the 4-week 
 period under consideration. The average daily mean temperature for the 
 longer periods was obtained by taking the mean of the two average daily 
 means for the two 2-week periods involved. 
 
 It will be noted that the plant values are uniformly given at the bottom 
 of the table, with a double rule separating them from what precedes. 
 
 Figures 2-6 present graphically certain of the data given in tables I-VIII. 
 Graphs for plant values are denoted by black lines and those for climatic 
 values are shown in red. In all of these graphs the ordinates represent 
 magnitudes of the plant and climatic relative values and the abscissas repre- 
 sent the time of the year. The ordinate scale is given at the left of each 
 set of graphs, for convenience of reference, and the dates of the beginnings 
 of successive culture periods are shown on the base line. Thus, for the first 
 2-week period at Oakland, the ordinates show the average daily relative 
 values of the plant and climatic measurements for the 2-week period begin- 
 ning May 23. The 100-line of the ordinate scale represents the average 
 seasonal value for all stations (as previously noted) , this being the unit used 
 in expressing the corresponding relative values. Full black lines (appearing 
 only on 4-week graphs) represent dry weight. Dash black lines represent 
 height. Dotted black lines (only on 4-week graphs) represent leaf area. 
 
Climatic Conditions of Maryland 371 
 
 Dash-and-dot lines (only on 2-week graphs) represent leaf-product. Full 
 red lines represent temperature. Dash red lines represent evaporation. 
 Dotted red lines represent light. 
 
 The results obtained will now be brought forward, with some discussion, 
 which is to be read with reference to tables I-VIII and figures 2-6. 
 
 Results for Stations in the Open 
 
 The data for the stations in the open will be considered as of two main 
 groups, the 2-iveek data and the 4-week data. 
 
 THE 2-WEEK VALUES 
 
 The 2-week -plant data for stations in the open (see figs. 2 and 3, black lines) 
 
 As has been stated, the plant measurements here in question were taken 
 about two weeks after planting and included stem height and leaflet dimen- 
 sions. From these have been derived (1) the relative mean daily rate of 
 increase in stem height per plant and (2) the relative mean daily rate of 
 increase in total leaf-product per plant, both for each 2-week period. 
 
 Therefore, one of these 2-week plant values represents the stem-producing 
 power of the plant and the other stands for its leaf-producing power, under 
 the given set of external conditions acting during that period. Since the 
 plants are taken to be alike at the start, (seeds) these two derived plant values 
 should be the same for all individuals if all were subjected to the same effect- 
 ive environmental conditions throughout the period, and when the various 
 plants are exposed to different environments the values just mentioned 
 become criteria by which the effectiveness of one environment may be com- 
 pared with that of another, with reference, of course, to the particular set of 
 internal conditions represented by the plants at the beginning of the tests. 
 The two plant values just mentioned may thus be regarded as relative meas- 
 ures of the effectiveness or efficiency of the environmental complex for the 
 2-week period considered, as it acted to produce stem elongation and leaf- 
 product increase, upon the soy-bean plants employed in this investigation. 
 For convenience, the following discussion will refer to the graphs (figs. 2 and 
 3) rather than to the tables, but tables and graphs both present the same 
 data in every case. This discussion will be given under two headings: (1) 
 Correlations between the two plant graphs and (2) Trend of the plant 
 values and their seasonal averages for the various stations. 
 
 Correlations between the two 2-week plant graphs. — It is readily seen that 
 the two graphs showing relative rates of increase in stem height and in leaf- 
 product agree in their general direction of slope from period to period, through- 
 out the season and for all stations. In many cases the two plant graphs not 
 only slope in the same general direction (upward or downward) but their 
 
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 Fig. 2. Graphs of 2-week data for exposed stations, as named. 
 
 Black: Height, ; Leaf product, — ■ ■ • — 
 
 Red: Temperature index, ; Evaporation index, ; Sunlight 
 
 index, 
 
 372 
 

 
 
 
 
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 Fig. 3. Graphs of 2-week data for exposed stations, as named (continued). 
 (Lines as in fig. 2.) — Graphs of 4-week data for exposed stations, as named. 
 
 Black: Dry weight, ; Height, ; Leaf area; 
 
 Red, as in fig. 2. 
 
 373 
 
374 F. Merrill Hildebrandt 
 
 corresponding angles of slope are nearly the same and their corresponding 
 ordinates are about equal, so that they nearly coincide for considerable por- 
 tions of their length. In other words, there appearsto have been a pronounced 
 general agrement between the effectiveness of the environment to produce 
 stem elongation and its effectiveness to increase the magnitude of the leaf- 
 product, as shown by these cultures. If this agreement were perfect it 
 would mean, of course, that the environment exerted the same influence 
 upon the process of leaf-surface increase (as measured by leaf-product) and 
 upon the process of stem elongation, and either of these two criteria would 
 be a measure of the other. But the coincidence of the two graphs is not by 
 any means perfect and it becomes a matter of interest to study their differ- 
 ences, as shown by the corresponding relative values of their ordinates. 
 
 Inspection of the graphs shows that, leaving those for Oakland out of 
 account, the index for stem height increase is frequently greater than the 
 other plant index for the early and late portions of the frostless season, and 
 that this relation generally is reversed for the middle portion. In other 
 terms, the graph for stem elongation generally lies below the other graph 
 for the middle of the season and above it for the beginning and end of the 
 season. In still other words, the seasonal maxima of leaf-product values are 
 generally relatively higher than those of stem elongation, while the seasonal 
 minima of the former are lower than those of the latter. It may be stated, 
 as an approximation, that when these two plant values are both about 100 
 (as the data are presented in this paper) the leaf-product value is generally 
 the higher of the two, while when both are below 100 the elongation value is 
 usually the higher. In the case of Oakland, both values are comparatively 
 very low throughout the season and, while the index of stem elongation reaches 
 somewhat above- 100 for two periods, this index is never surpassed in magni- 
 tude by the index of leaf-product increase. 
 
 The generalization just stated indicating a relation between the rates of 
 two plant processes, seems to be a physiological one, dependent upon the 
 nature of the soy-bean plant and hence largely predetermined by the internal 
 conditions of the seed. Within the range of environmental conditions en- 
 countered in this study it appears that the taller and more leafy the plant 
 becomes in the first two weeks of growth, the lower is the value of the ratio 
 of final height to final foliar expanse. The two growth processes here consid- 
 ered are, therefore, clearly interrelated and neither one alone is to be regarded 
 as a criterion of plant growth in general. The average of these two indices 
 may be considered as a tentative index of the general growth of the plants 
 during the first two weeks from the seed. Inspection of the 2-week graphs 
 leads to the impression that this mean of the two values offers perhaps the 
 most promising way to obtain from them a single index of plant growth. The 
 two are always so nearly parallel throughout the season (nearly coinciding 
 for many periods, as has been stated) that the charts have not been further 
 
Climatic Conditions of Maryland 375 
 
 complicated by introducing the graph for the average, but the form of this 
 graph is readily appreciated from the two graphs that are given. 
 
 The general relation between the two plant values that has just been 
 emphasized does not always hold, and the more detailed discussion of the 
 plant graphs for individual stations, given in the next following paragraph, 
 is of value in showing the main exceptions. 
 
 For Oakland the height value lies above that for leaf-product throughout 
 the season. The two graphs have the same general direction of slope except 
 for the period beginning July 16. For Chewsville the height graph exhibits 
 the same general direction of slope as does the leaf-product graph, from 
 period to period, throughout the season, with the former well above the latter 
 for the last four periods (beginning August 25, September 8, September 22 
 and October 27). For the periods beginning June 16 and June 30 the latter 
 relation also holds, although the index values are large, especially in the case 
 of the first of these two periods. For Monrovia the two plant graphs follow 
 each other very closely throughout the entire season. The graphs for Col- 
 lege for the periods beginning July 3, July 17 and July 31, illustrated the tend- 
 ency of the height values to decrease relatively to those of leaf-product when 
 both values are large. For the periods beginning September 10, September 
 25 and October 10 for this station, both values are small and, as would be 
 expected, stem height is relatively greater than leaf product. For the period 
 beginning June 19 the expected relation between the two graphs does not ob- 
 tain. For Baltimore the periods beginning June 10, July 23 and August 20 
 are exceptions to the proposition that the height graph should he below the 
 graph of leaf -product when both plant values are large. The generalization 
 is true, however, for the remaining periods of the Baltimore season. For 
 Darlington the two plant graphs agree closely in value throughout the season. 
 For Coleman stem height and leaf-product show the expected relation. For 
 Easton the generalization holds, with three exceptions: the height index is 
 lower than the other for the period beginning May 8, although both indices 
 have low values, and this relation is reversed for the periods beginning June 
 22 and July 20, in spite of the fact that both values are large in these cases. 
 For Princess Anne the graphs show values of the height index higher than 
 those of the other index for the periods beginning June 23, July 7 and July 21, 
 although both indices are large for all three periods. Otherwise these graphs 
 agree with the generalization. 
 
 The fact that the generalization given above" holds in the great majority 
 of the cases here studied renders the exceptions of special interest. Assum- 
 ing that the seeds were all alike at the beginning of all cultures and that no 
 disturbing influence was introduced by soil conditions, it may be supposed 
 that the periods characterized by exceptions to this generalization should also 
 be characterized by some sort of corresponding peculiarities in the aerial 
 environmental complexes. Now, a study of the charts for the exposed sta- 
 
376 ■ F. Merrill Hildebrandt 
 
 tions brings out the following fact: most of the 2-week periods for which 
 both plant values are large and yet the index of stem elongation is greater 
 than that of leaf-product increase, are characterized by low indices of sun- 
 shine intensity. This suggests that the plants of these cultures experienced 
 an acceleration in their rates of stem elongation due to low light intensity, in 
 short that they exhibited some of the effects of incipient etiolation. They 
 seem to show a somewhat increased rate of stem elongation and a somewhat 
 decreased rate of leaf expansion, as compared with plants receiving more 
 radiation. This interpretation is not to be regarded as at all well established, 
 but it is at least a suggestion of one way in which the external condition of 
 light intensity and duration may be registered in such plants as were here 
 employed. 
 
 Trends of the 2-week plant values and their seasonal ranges for the several 
 stations. The following consideration of the seasonal marches of the 2-week 
 plant values for the various stations will be limited in extent, since most of 
 the facts and deductions that seem to be of importance in this connection 
 can be better brought out later. Attention will here be called only to two 
 characteristics of the plant. graphs: (1) They begin with values of about 100 
 rise to high midsummer values and then fall to low values at the end of the 
 season. (2) Differences in the magnitude of the midsummer maxima con- 
 stitute the chief differences between the graphs for the various stations. 
 
 Oakland shows lower values for stem height and leaf-product than does 
 any other station, due largely to the low temperatures prevailing at this 
 station throughout the season. The data of this study indicate that the 
 climate of Oakland, so far as it affects the plants, is very unlike that of any of 
 the other stations employed. Both plant graphs for this station show the 
 typical low values at the beginning and end of the season, however, with 
 midsummer maxima of 124 for stem height and 89 for leaf-product. Chews- 
 ville shows typical graphs, the highest value reached by stem height being 
 143 while the leaf-product maximum is 139. The end of the season at this 
 station is characterized by very low leaf-product values. The graphs for 
 Monrovia are also typical, with low values of both indices for the period begin- 
 ning May 18 and low values at the end of the season, after midsummer 
 maxima of 121 (stem height) and 138 (leaf-product). An explanation of the 
 low values shown for the Monrovia periods beginning September 7 and 
 October 8 may he in the fact that a minimum temperature only several degrees 
 above freezing was reached during each of these periods. For both Chews- 
 ville and Monrovia the plant values are, for the most part, lower than 100, 
 with relatively low midsummer maxima of 132 and 203, for stem height and 
 leaf-product, respectively. The Baltimore plant graphs begin with high 
 values and reach maxima of 183 and 233, for stem height and leaf-product 
 respectively. For Darlington the main distinguishing features are the very 
 high maxima of 228 for stem height and 295 for leaf-product, for the period 
 
Climatic Conditions of Maryland 377 
 
 beginning July 10, and the relatively high values shown by the graphs for the 
 beginning of the season. The midsummer maxima for Coleman are 157 
 (stem height) and 211 (leaf product). The plant graphs for Easton show 
 relatively low values of the midsummer maxima, 146 being the highest value 
 reached for stem height and 163 for leaf-product. Also, the plant values 
 are low for the beginning of the season for this station. The midsummer 
 maximum for stem height for Princess Anne is 197 and the corresponding 
 maximum for leaf-product is 163. 
 
 The plant graphs, as may be seen from the above outline of their main 
 features, fall into three groups: (1) The Oakland graphs, which show values 
 of the leaf-product index below 100 for all periods and similar low values of 
 the stem-height index for all periods except those beginning June 14 and 
 August 14, while the maxima of these graphs are relatively low. (2) The 
 graphs for Chewsville, Monrovia, Princess Anne and Easton, showing higher 
 midsummer values of the plant growth-rates than do the Oakland graphs, 
 the maxima being about one and one-half times the seasonal average. (3) 
 The graphs for College, Baltimore, Darlington and Coleman which are distin- 
 guished by high relative values of their maxima. This classification serves to 
 summarize such characteristics of the graphs as are of present interest. 
 
 The 2-week climatic data for stations in the open (see figs. 2 and 3, red lines) 
 
 The 2-week climatic data consist of the average daily relative values of 
 the indices for temperature, evaporation, and light for each of a series of con- 
 secutive periods extending through practically the entire growing season, 
 each period being about 14 days long. These values therefore furnish a con- 
 tinuous record of the growing season at each station. The 4-week periods, 
 however, overlap, each one including the last two weeks of the preceding and 
 the first two weeks of the following 4-week period, so that the cUmatic aver- 
 ages based on the 4-week data form a smoother curve than do the 2-week 
 values in every case, small variations in the conditions being to a great extent 
 obscured by, averaging the overlapping periods. This series of 2-week values 
 therefore exhibits the march of the climatic conditions at each of the vari- 
 ous stations in somewhat greater detail than do the corresponding series of 
 4-week values. The former will therefore be made the basis for a somewhat 
 detailed and comparative discussion of the climatic conditions at the various 
 stations, temperature receiving attention first and light and evaporation being 
 afterwards considered together. In each case, the general characteristics 
 (common to most or all of the stations) of the seasonal march of the condition 
 considered will be brought out, after which attention will be given to peculi- 
 arities of the values for individual stations. 
 
378 F. Merrill Hildebrandt 
 
 The 2-week temperature data 
 
 The most obvious general characteristic of the physiological temperature 
 index is that its value is high for mid-summer and low for the beginning and 
 end of the season, for all stations. Graphs of similar form are obtained when 
 daily means and remainder summations are correspondingly plotted, but the 
 midsummer rise is much more pronounced in the graph of physiological index 
 values (here employed) than in either of the others. The second general 
 characteristic of all the graphs of the physiological index of temperature is 
 that each graph possesses two maxima, both of which have about the same 
 magnitude. The first occurs for the last two weeks of July and the second 
 for the last two weeks of August, this statement being true for all the stations 
 considered except Oakland, for which station they both occur relatively 
 early in the season, in the last two weeks in June and July respectively. A 
 third feature which is common to most (though not all) of these graphs is that 
 the upward slope is more gradual before the occurrence of the high midsummer 
 maxima than is the downward slope after their occurrence. A generali?ed 
 temperature-efficiency graph, representing averages of the corresponding 
 values for all of the stations, is not symmetrical about the ordinate for its 
 highest midsummer value; it slopes upward less rapidly than downward. 
 A fourth general characteristic of these graphs lies in the fact that the final 
 low index values of the frostless season are not very different for the various 
 stations. The following consideration of the graphs for some of the individ- 
 ual stations will serve to bring out the points mentioned above and will give 
 opportunity to note exceptions to the general statements just made. 
 
 With regard to the forms and other characteristics of the 2-week tempera- 
 ture-efficiency graphs, the nine stations studied may be placed in five groups: 
 (1) Chewsville and Monrovia, (2) Baltimore, Darlington and Coleman, (3) 
 Easton and Princess Anne, (4) College, and (5) Oakland. These five groups 
 are discussed in order below. It will be noted that groups 1, 2 and 3 are 
 composed of stations that are located near each other, and this probably 
 accounts for the grouping. 
 
 . Chewsville and Monrovia. The graph of physiological temperature indices 
 for Chewsville shows all the characteristics mentioned as general throughout 
 the series of stations. It rises gradually during the first three periods, (per- 
 iod beginning May 19 to period beginning June 16), then drops slightly during 
 the fourth period (beginning June 30) after which it rises for the period begin- 
 ning. July 14 to a primary maximum of 149. The value for the 6th period 
 (beginning July 28) is relatively low (112), after which a secondary maximum 
 (145) occurs for the period beginning August 11. The index value in question 
 then decreases rapidly during the next two periods attaining a magnitude of 
 48 for the 9th period (beginning September 8) and remaining low until the 
 end of the frostless season. Monrovia has the same sort of graph as Chews- 
 
Climatic Conditions of Maryland 379 
 
 eille, the maxima coming in the periods beginning July 13 and August 10. 
 The minimum relative value of the temperature index is 53 for the period 
 beginning September 21. 
 
 Baltimore, Darlington and Coleman. For Baltimore, the physiological tem- 
 perature values increase gradually to a primary maximum of 162 for the 
 period beginning July 29. The secondary maximum occurs in the period 
 beginning August 6, after which there is a relatively rapid decline of the 
 index values, to 62 for the period beginning September 3. The Darling- 
 ton graph has its first maximum in the first two weeks of July and its second 
 in the 2-week period beginning August 7, and then falls off rapidly to a mini- 
 mum of 46 for the first period in September. The graph for Coleman shows 
 a gradual rise, two maxima for the periods beginning July 8 and August 5, 
 and a rapid fall. The temperature record is incomplete at this station and 
 the low values for the end of the season are not available. 
 
 Easton and Princess Anne. For Easton there is a gradual rise to a maxi- 
 mum of 154, for the first period in July, the second maximum coming in the 
 period beginning August 17. The curve then falls to a minimum of 48, for 
 the last period of the season. The Princess Anne curve shows the two typi- 
 cal maxima in the periods beginning July 9 and August 18, with a minimum 
 of 43 for the last period of the season. 
 
 College. The College graph of physiological indices is unusual in showing 
 a marked rise for the period beginning June 19, thus giving the graph three 
 maxima (129, 148 and 143, for the periods beginning June 19, July 17 and 
 August .14, respectively). The graph descends rapidly to a value of 50, for 
 the period beginning September 25. 
 
 Oakland. The temperature-index values for Oakland are all relatively 
 low, being always considerably less than the seasonal average for all periods 
 and stations. This graph shows two maxima, one for the latter half of June 
 and the other for the latter half of July. Each of these maxima occurs about 
 a month earlier than do the corresponding ones for the other stations here 
 studied. The Oakland graph is also unlike those for the other stations in 
 that its downward slope is more gradual. Its final relative value is 43, for 
 the 2-week period beginning September 12, which was the last full period for 
 this station before the occurrence of a killing frost. The most outstanding 
 characteristics of the Oakland season, in respect to this temperature-effi- 
 ciency graph, as compared with the seasons at the other stations, are: (1) 
 general low values of the physiological temperature-index. (2) short duration, 
 owing to the occurrence of late spring and early fall frosts, and (3) early 
 occurrence of the maxima. These marked differences between the Oakland 
 graph and those for the other stations here dealt with are no doubt largety 
 due to the relatively high altitude of Oakland as compared with the others 
 as has been mentioned by McLean in his comparative study of the Easton 
 and Oakland seasons based on these same data. 
 
380 F. Merrill Hildebrandt 
 
 The generalized graph. Leaving the graph for Oakland out of account, 
 those for the other stations may be described as a single generalized graph, 
 in the following general terms. Beginning with a relative index-value of 
 about 80 (for the first part of May) the graph rises to a maximum (about 
 150) for the first part of July, falls slightly and rises again to a second maxi- 
 mum of about the same value as the first, for the first part of August, and 
 finally falls to a minimum value of about 50 for the last period of the frost- 
 less season. That the initial values are not lower is no doubt due to the 
 fact that the cultures were not started until somewhat after the beginning 
 of the frostless season. (See McLean's paper, already cited.) This gener- 
 alization of the temperature values for the various stations is not, of course, 
 to be considered otherwise than as a statement of what occurred in the par- 
 ticular season during which this investigation was carried out. 
 
 Light and the evaporating power of the air, 2-week data 
 
 The 2-week graphs of the index values for light and atmospheric evaporat- 
 ing power will be treated together since che seasonal marches of these two 
 climatic conditions generally exhibit the same main characteristics. Three 
 points may be noted in regard to them. (1) Both graphs have, in general, 
 a downward slope from the beginning to the end of the season. (2) In the 
 majority of cases they agree with each other in direction of slope, from period 
 to period, throughout the season. (3) They agree in having a primary maxi- ' 
 mum with a very high value, for an early period of the season and one or 
 more secondary maxima with lower values, for periods that occur later. The 
 secondary maxima of the graphs for light and evaporation sometimes (but 
 not always) coincide, as to time of occurrence, with a corresponding maximum 
 of the graph for temperature efficiency. The following consideration of the 
 individual station graphs for the two conditions may serve to bring out these 
 points. 
 
 For Oakland, the primary maximum in the graph of atmospheric evapo- 
 rating power (153) occurs in the first period (beginning May 23). The value 
 of the evaporation index then decreases steadily to a relative magnitude of 
 79, for the first two weeks in July, after which it increases to (104), which 
 corresponds in time of occurrence (period beginning July 6) to the secondary 
 maximum of the graph of temperature efficiency for this station. After 
 passing through this high value the evaporation graph descends again, to 
 the low values 57 and 69 for the last two periods (beginning August 27 and 
 September 12). The sunshine-intensity index for Oakland varies from an 
 initial value of 122 to a final value of 81, with maxima for the periods begin- 
 ning July 16 and August 14. Inspection of these two graphs for Oakland 
 shows that the direction of slope is the same, from period to period, for the 
 greater part of the season. 
 
Climatic Conditions of Maryland 381 
 
 For Chewsville, the two graphs agree in direction of slope throughout the 
 entire season, except between the periods beginning August 25 and September 
 8. Both are approximately parallel to the temperature-efficiency graph for 
 this station, from the period beginning July 14 to the period beginning August 
 25 and both have a downward slope, in general, from the beginning to the 
 end of the season. Moreover, they agree in direction of slope from the 
 period beginning June 15 to that beginning October 8. 
 
 For College, the evaporation maximum for the period beginning July 27 
 corresponds to a secondary minimum in temperature efficiency. The graph 
 of the evaporating power of the air for College has a primary maximum for 
 the second period (beginning May 28) and a well-marked secondary maximum 
 for the period beginning July 22. No sunshine data are available for this 
 station. 
 
 For Baltimore, the two graphs in question agree in direction of slope up to 
 the period beginning July 9 after which evaporation passes through a second- 
 ary maximum which corresponds, in a very rough way, to the double maxi- 
 mum of temperature efficiency. 
 
 The Darlington light and evaporation graphs show the general character- 
 istics mentioned at the beginning of this discussion, for the greater part of 
 the season. The atmometric values for this station are relatively very low, 
 all but two of them being less than the seasonal average for all periods and 
 stations. 
 
 For Coleman, the sunshine record is incomplete, but the two graphs gen- 
 erally agree in direction of slope, so far as comparison is possible, excepting 
 between the periods beginning July 17 and July 31. 
 
 For Easton and Princess Anne, the graphs are typical. For the latter 
 station, evaporation data are lacking for the periods beginning June 8 and 
 June 23. 
 
 The comparatively close agreement between the graphs for sunshine and 
 evaporation, for all the stations employed in this study, together with the 
 fact that evaporation exhibits no well-defined relation to temperature effi- 
 ciency, appears to indicate that the rate at which water evaporated from the 
 white cylindrical cups employed as atmometers in this investigation was de- 
 termined to a considerable extent by the amount of radiant energy absorbed 
 by the cups, and that air temperature played a secondary part in the deter- 
 mination of this rate. The fact that the physiological temperature index is 
 here used for expressing temperature values does not militate against this 
 conclusion, since, as has been previously stated other methods of expressing 
 the temperature values give graphs which slope for the most part, in the same 
 direction as does the graph of physiological temperature indices. A large 
 effect of sunshine on evaporation, the sunshine intensity being measured by 
 a black-bulb sunshine recorder, has been found by Briggs and Shantz. 18 
 
 18 Briggs, L. J., and Shantz, H. L. Hourly transpiration rate on clear days as determined by cyclic 
 environmental factors. Jour. Agric. Res. 5: 583-650. 1916. 
 
382 F. Merrill Hildebrandt 
 
 These authors were able to calculate approximately the amount of evaporation 
 from a shallow blackened tank using a formula which involved sunshine 
 intensity and the saturation deficit of the air, sunshine intensity having a 
 preponderating influence. They also state that while the cups and the tank 
 respond in different ways to the daily cycle of changes in the evaporating 
 power of the air, a certain average ratio exists between the evaporation from 
 the tank and that from the cups. It is therefore to be expected from their 
 work that the rate of evaporation from Livingston porous cups is largely 
 influenced by sunshine intensity, and that air temperature exerts a secondary 
 influence on evaporation as measured by these instruments. It must be 
 remembered, also, that the evaporation measurements of this study were 
 made in the plant enclosures, while air temperature was measured by ther- 
 mometers located in a shelter about 1.5 meters (5 feet) above the ground and 
 often 4 or 5 meters (15 feet) from the plant enclosures. This may account 
 in some measure for the apparent absence of any marked effect of air temper- 
 ature on the evaporating power of the air as measured by porous-cup atmome- 
 ters. As Livingston has remarked, the porous cups are exposed in some- 
 what the same way as are plant leaves, and the foliage of McLean's plants 
 was freely exposed to sunshine, as were his atmometers also. Air tempera- 
 ture is always obtained from shaded instruments. 
 
 Variability of temperature and evaporation values 
 
 It may be noted that the temperature-efficiency values for the stations 
 here considered, exclusive of Oakland, are much more nearly alike for any 
 given 2-week period than are the sunshine and evaporation values. The 
 values of these three climatic indices for the first 2 weeks of June and for the 
 first 2 weeks of August, for the eight stations, are given in table IX. Since 
 the dates of observation were not the same for all stations, these values have 
 been approximated from the graphs, but they may be considered as suffi- 
 ciently accurate to illustrate the manner in which the data at hand support 
 the conclusion just stated. 
 
 If the highest value given for each of the three indices and for each of the 
 two periods be divided by its lowest value, the ratios presented in the next 
 to the last line of the table are obtained. Each ratio represents the magni- 
 tude of the range of variation of the climatic index that it represents, for the 
 eight stations in question. The average value for these periods is given in 
 the last fine. It thus appears that the variation of the temperature-efficiency 
 index due to difference in location of the stations is markedly less than is the 
 corresponding variation in the index of sunshine or that of evaporation. This 
 relation holds generally throughout the season. In short, the temperature- 
 efficiency values exhibit a smaller degree of geographical or local variation 
 than is exhibited by the index for sunshine or for the evaporating power of 
 the air. 
 
Climatic Conditions of Maryland 
 
 383 
 
 TABLE IX 
 Values of the three climatic indices for the first % weeks in June and the first % weeks in 
 August, vyith ratio of highest to lowest value for each index, for all stations excepting 
 Oakland. 
 
 STATION 
 
 EVAPORATION 
 
 SUNSHINE 
 
 temperature 
 
 efficiency 
 
 (physiological 
 
 index} 
 
 
 1st 2 
 
 weeks of 
 
 June 
 
 1st 2 
 weeks of 
 August 
 
 1st 2 
 
 weeks of 
 June 
 
 1st 2 
 weeks of 
 August 
 
 1st 2 
 
 weeks of 
 
 June 
 
 1st 2 
 weeks of 
 August 
 
 Chewsville 
 
 115 
 146 
 156 
 109 
 115 
 145 
 132 
 125 
 
 1.5 
 
 90 
 115 
 147 
 110 
 
 7S 
 135 
 135 
 
 95 
 
 1.9 
 
 130 
 122 
 
 92 
 115 
 145 
 165 
 110 
 
 1.8 
 
 95 
 103 
 
 77 
 112 
 120 
 115 
 
 75 
 
 1.6 
 
 105 
 110 
 103 
 102 
 98 
 115 
 112 
 102 
 
 1.2 
 
 120 
 
 Monrovia 
 
 125 
 
 College 
 
 133 
 
 Baltimore 
 
 152 
 
 Darlington 
 
 125 
 
 
 152 
 
 Easton 
 
 140 
 
 Princess Anne 
 
 135 
 
 Ratio of highest to lowest value 
 
 1.3 
 
 
 
 Average for the 2 periods 
 
 1 
 
 70 
 
 1 
 
 70 
 
 1.25 
 
 
 
 
 Correlation of the 2-week plant and the climatic values 
 
 During the course of this study a number of attempts were made to corre- 
 late the climatic measurements with those representing the growth rates 
 of the plants, but these were unsuccessful and no scheme applicable in a 
 quantitative way to this problem has yet been formulated. For example, 
 one of the simpler correlation schemes to be tried was based on the assump- 
 tion that the growth of the plants was directly proportional to the index 
 values for temperature and light and inversely proportional to those for 
 evaporation. Stated as an equation, this assumption takes the form: 
 
 G = 
 
 KTL 
 
 E ' 
 
 in which G represents the. plant growth rate and T, L, and E represent the 
 indices of temperature, light and evaporation, respectively, while K is a 
 constant of proportionality. Values were obtained for the right-hand mem- 
 ber of this equation for the successive 2- week periods for all stations and these 
 values were compared with the corresponding growth-rate indices derived 
 from the plant measurements. No close correspondence was generally to be 
 detected. The equation is given as an illustration of the kind of methods by 
 which the discovery of correlations between the plants and their climatic 
 environment was attempted. Many combinations of the three climatic 
 
384 F. Merrill Hildebrandt 
 
 conditions were made and compared with the plant growth rates but, as 
 noted above, without satisfactory results. 
 
 It seems probable that the difficulty experienced by every student who has 
 thus far attempted this sort of correlation may arise partly from the fact 
 that the environmental conditions have not been measured in the right 
 way, and partly from the use of inadequate methods for the integration of 
 the quantitative data that are obtained. It is hardly to be expected that 
 either of the growth criteria here used should be as simple a function of the 
 climatic conditions as the formula given above might suggest. Just as soon 
 as facilities become available for actual experiments in this field, — experi- 
 ments in which all the influential conditions may be controlled and analytic- 
 ally understood, — the problem here brought forward prematurely maybe 
 seriously attacked. Until such experiments may be begun, all discussion 
 regarding the relations between plant growth rates and environmental con- 
 ditions must remain vague and unsatisfactory. 
 
 There is no doubt that the distribution of high and low values of any one 
 of the climatic conditions, during the growth period of the plants, is an impor- 
 tant factor in determining the degree of their development. To take an ex- 
 treme case as an example, a few days with a very low sunshine intensity 
 would have no direct influence on plants not yet above ground, but such an 
 occurrence would exert a very marked influence on plants with a considerable 
 leaf area. Obviously, two periods showing similar average values of any 
 climatic condition may have a widely differing distribution of high and low 
 values of this condition. In the present study, while the distribution of 
 high and low temperature and fight values is known, the corresponding 
 stages of the development of the plants are not, and it is thus practically 
 impossible to take account of this distribution factor. The difficulty of 
 correlating growth rates and climatic conditions is further increased by the 
 fact that, in measuring dry weight, stem height, etc., we are not measuring 
 single processes in the plants, but rather the combined effects of a number of 
 processes taken together. 
 
 It is of interest to call attention at this point to certain features of the 
 growth of the soy-bean plants of this study whose causes can only be sur- 
 mised. These features may be of significance, however, since they show a 
 departure from what may be termed the "normal" for plant behavior. In 
 the first place, although all of the temperature graphs show two maxima, the 
 plants, except in the case of Oakland, failed to respond to the second tem- 
 perature maximum by a correspondingly high rate of growth. For Chews- 
 ville, as an example, in the period beginning August 11, we have a low value 
 for the leaf-product with a high temperature index and the other conditions 
 at about the seasonal average. For Monrovia in the period beginning August 
 10, with a high temperature value and with sunshine intensity at about the 
 seasonal average, the plants show a relatively low value of the leaf-product. 
 
Climatic Conditions of Maryland 385 
 
 This may be contrasted with the period beginning June 15 for this same sta- 
 tion, which, with a leaf-product about the same as that of the first-mentioned 
 period, seems to show less favorable growing conditions — namely, a much 
 lower relative temperature index, a very high evaporation rate and a sun- 
 shine value only a little higher than the corresponding value for the period 
 beginning August 10. For College, the periods beginning July 17, July 31 
 and August 14, with about the same values for temperature and evaporation, 
 show magnitudes of 152, 204 and 150 respectively for the leaf-product. This 
 variation may possibly be related to differences in the value of sunshine inten- 
 sity for these periods, but sunshine data are lacking for this station. For 
 Baltimore, the periods beginning July 23, August 6 and August 20 show large 
 differences in the leaf-product with comparatively slight differences in the 
 climatic conditions. Evaporation was slightly less rapid for the period 
 beginning July 23 than for the period beginning August 8, and considerably 
 less for the period beginning August 20, but this seems to have occurred with- 
 out the expected effect on the plants. For Coleman the plant graph slopes 
 upward to a value of over 200 for the period beginning July 8, while for the 
 period beginning August 5, which has climatic conditions apparently as favor- 
 able, the relative value of the leaf -product is only 138. For Easton the 
 leaf-product is lower than would be expected for the period beginning August 
 17, and for Princess Anne the plant values for the period beginning August 8 
 are much lower than for the period beginning July 7, which had approxi- 
 mately the same climatic conditions as the first-mentioned period. 
 
 A second feature of the plant graphs, and one that cannot be correlated 
 with the climatic data, is that the rate of stem elongation reaches its highest 
 value for the season before the occurrence of the maximum leaf-product, for 
 all stations except Darlington and Coleman. For Darlington, the highest 
 value for stem height and leaf-product both occur for the period beginning 
 July 10 and for Coleman the maximum value for stem height occurs for the 
 period beginning July 22, while the leaf-product reaches its highest value for 
 the season at this station in the preceding period. For the remaining sta- 
 tions, the highest value for stem height occurs two weeks or a month earlier 
 than does the highest value for leaf-product. 
 
 THE 4-WEEK VALUES 
 
 The 4-week plant and climatic data derived from the exposed stations are 
 presented in the tables and graphs already explained, and the following con- 
 sideration of these values will refer to the graphs, as in the case of the 2-week 
 values. 
 
386 F. Merrill Hildebrandt 
 
 The 4-week plant data for stations in the open (see fig. 4, black lines) 
 
 For the 4-week data, the rate of stem elongation may be compared with 
 the rate of leaf expansion as determined from actual measurements of leaf 
 area. This comparison shows, the same general relations as appeared to exist 
 between stem height and leaf-product for the 2-week growth periods. Owing 
 to the fact that the 4-week plants were grown for a longer time, however, the 
 4-week data show fewer cases with the rate of stem elongation greater than 
 the rate of leaf expansion. In most cases the rate of stem elongation is con- 
 siderably smaller than the rate of leaf expansion. This illustrates the tend- 
 ency of the soy-beans to show a low rate of height growth relative to the 
 rate of leaf expansion when both rates are large. 
 
 For Oakland, the stem-height graph is above the leaf area graph for the 
 first three periods of the season and below it for the other five periods. For 
 Chewsville the three plant graphs follow each other very closely and the 
 differences in their relative positions are probably due, for the most part, to 
 individual variations in the plants of the separate cultures. The Monrovia 
 graphs also support the assumption that stem height shows a well-defined 
 tendency to remain below leaf area during the first part of the season. For 
 College, the stem-height graph is below that for leaf area for the entire season, 
 except for the two periods beginning June 19 and September 25. The Bal- 
 timore graphs show stem-height values higher than the corresponding leaf- 
 area values for the periods beginning May 14, May 29, June 10 and August 
 20, due possibly to low light intensities. The Darlington cultures show very 
 high values of both growth rates, with stem height below leaf area for the 
 entire season. For Coleman, the stem-height graph remains below the leaf- 
 area graph for all the periods except the last, in which case it rises very slightly 
 above the leaf-area graph. For Easton, the two growth rates are about 
 alike, showing nearly the same relative values for each culture period. 
 For Princess Anne, the stem-height and leaf-area graphs show a departure 
 from the usual behavior during the first three periods of the season. For 
 these periods, the leaf-area values are relatively large and those for stem 
 height are relatively low, for some reason not apparent from the climatic 
 conditions. 
 
 A very striking relation is shown between the 4-week values for leaf area 
 and for dry weight. For most of the cultures these two kinds of growth 
 rates have practically the same relative numerical values for any given period. 
 The Oakland graphs show this for all periods except the one beginning June 
 5, for which dry weight is markedly larger than leaf area. For Chewsville, 
 the general relation just mentioned shows very well throughout the season. 
 For Monrovia, the leaf-area value shows a rather large deviation from the dry- 
 weight value for the periods beginning June 16, June 30 and Aug. 25, but 
 otherwise the two growth rates correspond in their relative values during the 
 
Climatic Conditions of Maryland 
 
 387 
 
 >* to ■< ca J o L * ~K , " l !£ h ' 
 
 • 
 
 
 
 
 
 
 
 
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 rJOTOrt 
 
 ' 
 
 
 
 „/ 
 
 
 
 
 \ 4-veehptuoto 
 
 
 
 
 
 
 ( 
 
 
 \ Exposed sttrtiwi 
 
 
 / 
 
 1 
 
 \ 
 
 
 / 
 
 \.. 
 
 
 
 
 / 
 
 
 i \/ 
 
 
 
 \ 
 
 
 
 \ 
 
 
 
 i / 
 
 /\ 
 
 if 
 
 
 
 \\ 
 
 \ \ 
 
 s 
 
 \ 
 
 
 
 J- 
 
 / 
 
 '/ 
 
 
 
 
 
 
 V 
 
 1 
 
 
 
 
 ,. 
 
 
 
 
 
 
 
 
 
 
 
 \\ 
 
 
 '• 
 
 
 
 
 
 
 
 
 
 
 
 » 
 
 
 '■ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 rv\i hai June ji>f<i jui-i jiii ajji; au- au* sepi 5 
 
 
 Fig. 4. Graphs of 4-week data for exposed stations as named (continued). 
 (Lines as in 4- week graphs of fig. 3.) 
 
388 F. Merrill Hildebrandt 
 
 entire season. The College graphs show close agreement, with diy weight 
 above leaf area during the first part of the season. For Baltimore, the rela- 
 tive leaf-area value differs considerably from the value for relative dry 
 weight for the periods beginning August 6 and October 1, but the remaining 
 periods show close agreement. The two Darlington graphs show close agree- 
 ment for all periods. For Coleman, dry weight and leaf area agree well for 
 all periods, except those beginning August 5 and August 19. For Easton, 
 no large differences between these two rates occur for any of the cultures. 
 For Princess Anne, the period beginning August 4 is the only one showing a 
 difference of considerable magnitude between the relative leaf-area value and 
 the dry-weight value. 
 
 This property or characteristic of soy-bean renders possible the use of the 
 leaf area of the plant as an index of the dry weight of the tops, and appears 
 to render soy-bean particularly promising as a standard plant for climatic 
 investigations, as has been pointed out in a previous paper,' 9 from which the 
 following paragraph is taken. 
 
 "If the method proposed by Livingston and McLean (1916), of employing 
 the growth rates of standard plants as indices for the comparison of different 
 climates as these influence plant growth in general, is to be of value, it is of 
 course necessary that suitable plant characteristics be chosen for measure- 
 ment in determining the growth rates, and it is desirable that the measure- 
 ments be such as may be made from time to time without injury to the 
 plants. The most generally accepted criterion of plant growth, dry weight 
 of tops, can be obtained but once for any individual plant, since the plant is 
 destroyed during the determination. Also, the accurate determination of 
 leaf area is very difficult unless the plants are destroyed. On the other hand, 
 as McLean has emphasized, leaf dimensions may be obtained repeatedly 
 during the development of the plant, without serious danger of inflicting 
 injury. It may therefore be of considerable importance if leaf area, and even 
 dry weight can be satisfactorily estimated for soy-bean by the employment 
 of the leaf-product as an index." 
 
 Dry weight and actual leaf area were both determined only for the 4-week 
 periods, the plants being then destroyed, but the lengths and breadths of 
 all leaflets were obtained for both the 2-week and the 4-week periods. Con- 
 sequently, to study the correlation between total leaf area and total ieaf- 
 product per plant, only the 4-week data are available and these are the 
 ones here considered. 
 
 Since soy-bean leaflets are approximately elliptical in form and since the 
 area of an ellipse is proportional to the product of its axes, the leaflet-product 
 (length times breadth) of any leaflet should be nearly proportional to the 
 area of that leaflet. Whether this relation may hold during the growth of 
 
 19 Hildebrandt, F. M. Leaf product as an index of growth in soy-bean. Johns Hopkins Univ. Circ, 
 March, 1917. P. 202-205. 
 
Climatic Conditions of Maryland 389 
 
 the leaflet under different sets of climatic conditions depends upon how nearly 
 the elliptical form is retained. The sum of the individual leaflet-products of 
 any plant, which is the total leaf-product for that plant, should be approxi- 
 mately proportional to the total leaf area of the plant, if the relation given 
 above holds. In the discussion that follows it will be shown that such an 
 approximate proportionality does exist in the case of the 4-week soy-bean 
 plants. 
 
 In order to find out whether the actual areas of the leaves in these cultures 
 were proportional to the corresponding leaf-products, the ratio of the two 
 quantities was worked out for a number of the stations. It was found that 
 the leaf-product divided by the leaf area gives a number that varies only 
 slightly from the value 1.28. In other words, if we measure the two diame- 
 ters of the leaflets of a 4-week soy-bean plant, multiply these two numbers 
 for each leaflet, and add the products, a number is obtained which, when 
 divided by 1.28, closely approximates the actual leaf area of that plant. 
 Instead of using the sum of the products of length and breadth as an index 
 of the area per plant we may use the sum of the squares of the lengths of the 
 leaflets or the sum of the squares of the breadths of the leaflets. The numbers 
 thus secured do not, however, bear as nearly constant a ratio to the actual 
 leaf area as does the total leaf-product, and hence neither is as satisfactory 
 an index of the area as is the leaf-product itself. 
 
 One of the most interesting properties of the 4-week soy bean plant is that 
 the dry weight of stem and leaves is approximately proportional to the total 
 leaf area. Having, therefore, a means by which the leaf area may be con- 
 veniently estimated, it is possible to calculate the dry weight of the plant 
 approximately, by multiplying the leaf-area by the proper constant. The 
 proportionality between the weight of the plant and its leaf area is not quite 
 so constant as that between leaf area and leaf-product, but in the great 
 majority of cases the variation in the ratio of dry weight to leaf area, from 
 a constant value, is less than 10 per cent. The relations given hold over 
 a very wide range of climatic conditions and for plants varying in height 
 from 2 or 3 cm. to 18 or 20 cm. Since none of the plants in these experi- 
 ments were grown to maturity, it is impossible to say whether this relation 
 holds up to that time. 
 
 From the foregoing facts it may be concluded that the dry weight and 
 leaf area of soy-beans 4 weeks old from the seed can be determined approxi- 
 mately from their leaflet dimensions. Soy-bean should therefore be very 
 suitable for use as a standard plant for the measurement of climate in the 
 manner suggested by Livingston and McLean, since the rate of its growth 
 can be approximately determined from easily obtained leaf measurements. 
 Also, the properties of soy-bean given above should make it a useful plant 
 for any piece of physiological research in which it is desired to know approxi- 
 mately the dry weight of the plant used, at various stages of its development. 
 
390 F. Merrill Hildebrandt 
 
 The 4-week climatic data for stations in the open (see fig. 4> fed lines) 
 
 It will be remembered that the cultures were started every two weeks and 
 that each grew for a period of four weeks. The 4-week periods thus over- 
 lap, and attention has been called to the fact that averages of the climatic 
 factors for these over-lapping periods form a smoother graph than averages 
 for the 2-week periods. The 4-week graphs, therefore, show the general 
 seasonal march of the index values for various stations better than do the 
 2-week ones, while the latter show the details of the seasonal march better 
 than the former. This fact will be brought out by a brief reference to the 
 graphs at this point. 
 
 The values of the physiological temperature indices for the 4-week periods 
 show the seasonal marches of this condition for the various stations, from 
 low values in May to high midsummer values, and then to low values again 
 in the last part of the season. The graphs for all of the stations except 
 Oakland show a steeper slope after the midsummer maximum has been 
 passed than for the periods during which the temperature was rising to this 
 maximum. The two maxima that were present in most of the 2-week graphs 
 are eliminated in the 4-week averages and the graphs of temperature values 
 show instead a period of about 6 weeks during which this condition remains 
 approximately constant. 
 
 The 4-week evaporation and light data show the general characteristics of 
 the seasonal marches of these conditions previously noted as exhibited by the 
 2-week data. It will be seen, in the first place, that both graphs exhibit a 
 downward slope from the beginning to the end of the season; and, in the 
 second place, that both graphs show, in addition to their high primary maxi- 
 mum in the early part of the season, one or more secondary maxima later. 
 In some cases the secondary maxima of the evaporation graphs coincide, as to 
 time of occurrence, with temperature maxima. Both of these general char- 
 acteristics shown in the 4-week graphs of evaporation and light are shown by 
 the 2-week graphs but since small variations are ehminated by averaging 
 the over-lapping periods, there are fewer secondary maxima in the 4-week 
 graphs. In the case of evaporation, there is usually one secondary maximum 
 occurring in or near the 4-week period including the last 2 weeks of July and 
 the first 2 weeks of August. In the case of all stations this is one of the 
 three 4-week periods showing high temperature values. The 4-week climatic 
 graphs need not be discussed further here. The method by which the 4-week 
 data were derived from the 2-week data amounts to the same thing as smooth- 
 ing the 2-week graphs and only the more pronounced characteristics of the 
 graphs remain after averaging. Interest in the 4-week climatic data thus 
 lies mainly in their relation to the plant growth rates. 
 
Climatic Conditions of Maryland 391 
 
 Results for the Three Covered Stations 
 introductory 
 
 All of the data discussed up to this point were obtained for the open, with 
 no covering other than a screen of wire netting of large mesh, to protect the 
 plants from injury. At three of the stations, Oakland, Baltimore and Easton, 
 as has been noted, a series of cultures was also grown under glazed cold-frame 
 sash, supported three feet above the ground, these cultures being designated 
 as the Oakland, Baltimore and Easton covered stations. The behavior of 
 the plants grown under glass was very different from the behavior of those 
 grown in the open, and the results for the covered stations will be considered 
 in this section. 
 
 The covered cultures were placed near the exposed cultures at each of the 
 three places mentioned, so that the climatic conditions for the two would be 
 practically the same, except as modified by the glass. 
 
 THE PLANT DATA, COVERED STATIONS 
 
 (See figs. 5 and 6, black lines) 
 
 The effect of the glass cover was shown by the plants in two ways: (1) 
 growth was always greater for the covered stations than for the exposed, 
 and (2) the plants of the covered stations showed a marked difference in 
 manner of growth from the plants of the exposed stations. The greater 
 growth of the covered plants was shown in some cases by one, in some cases 
 by two, or even by all three of the growth measurements taken. Not only 
 did the plants show greater growth, but the maxima in the graphs of the 
 various growth measurements for the covered plants do not usually occur at 
 the same times as do the maxima in the corresponding graphs for the plants 
 grown in the open. The principal effect of the covering on the way in which 
 the plants grew is shown by a disturbance of the relation between dry weight 
 and leaf area. In previous discussion of this relation for the exposed plants 
 it was noted that the relative dry-weight and leaf-area values are approxi- 
 mately the same for the 4-week plants. In the case of the covered stations, 
 on the other hand, every culture shows relative leaf area as higher (usually 
 very much higher) than relative dry weight. Stem height for the covered 
 cultures usually shows high .values as compared to the corresponding exposed 
 cultures. The tendency noted in previous discussion for this growth rate to 
 fall off relatively, as the plants become larger, seems to be only slightly in 
 evidence here. The following consideration of the covered cultures in detail 
 will bring out these features. It should be noted that the culture periods for 
 the covered stations each agree in length, to within a day or two, with those 
 for the corresponding exposed stations. Such slight differences as exist in 
 
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 \ 
 
 \ 
 
 -■ 
 
 
 
 ' 
 
 
 
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 \ 
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 ao - 
 
 -- 
 
 - 
 
 ., 
 
 
 
 
 
 eo 
 
 
 
 
 
 
 — 
 
 
 Fig. 5. Graphs of 2-week and 4-week data for covered and forest 
 stations, as named. 
 
 Black, as in figs. 1 and 3. 
 Red, Evaporation index. 
 
 392 
 
Climatic Conditions of Maryland 
 
 393 
 
 the lengths of the culture periods do not in any degree account for the differ- 
 ences in the plant measurements nor interfere with the general comparisons 
 here made. In comparing growth for the exposed and covered cultures, no 
 attempt will be made to account in detail for the differences between the 
 two sets of plants in terms of climatic conditions, since the climatic influencs 
 acting on the covered plants are not even so well known as in the case of the 
 exposed stations, and it has already become clear that a really satisfactory 
 interpretation of growth rates by means of such climatic measurements as 
 are here employed is nearly hopeless at present. After the peculiarities of 
 the covered plants have been pointed out, however, some suggestions as to 
 the probable causes of these peculiarities will be brought forward. 
 
 I EK5T0M 
 
 -4-week, pcnod 
 + COV&R-ED STATION, 
 
 zzo 
 
 2iO 
 
 ZOO 
 
 i*)o 
 
 ■80 
 
 no 
 
 160 
 ISO 
 
 130 
 l£Q 
 
 no 
 ioo 
 9o 
 30 
 TO 
 feO 
 30 
 
 f 
 
 I 
 
 4. 
 
 V 
 
 \ 
 1 
 
 \ 
 
 u 
 
 1 
 
 e*5Tcm 
 
 ' Z-weeK. period 
 
 COVERED STAHOW. 
 
 «v 
 
 Fig. 6. Graphs of 2-week and 4-week data for Easton covered station. 
 (Lines as in fig. 5.) 
 
 The Oakland covered station. — The covered and exposed cultures for Oak- 
 land differ less than do the corresponding sets for Baltimore and Easton, but 
 they show the general features outlined above. The plants of the 2-week 
 covered cultures for Oakland exhibit a much higher value of the leaf-product 
 than do the corresponding exposed cultures, for the periods beginning June 
 18, July 2, and July 15, and the stem-height value is greater for the covered 
 station than for the exposed station, for the periods beginning June 4, July 2 
 and July 15. The highest value of leaf-product occurs for the period begin- 
 ning July 15 for the covered, and in the period beginning July 16 for the 
 
394 F. Merrill Hildebrandt 
 
 exposed 2-week plants. Each set of cultures show two seasonal maxima in 
 the plant graphs, but these are much higher in the case of the covered plants 
 than in the case of the exposed. In the 4-week graphs for the covered plants, 
 leaf area is higher than dry weight for the whole season, while the exposed- 
 station graph for leaf area is well below that for dry weight, from the period 
 beginning May 23 to the period beginning July 16, inclusive. The maximum 
 for all the growth measurements of the 4-week exposed plants occurs for the 
 period beginning June 19, while the maximum for the covered station occurs 
 for the period beginning July 2. Also, the graphs for the 4-week plants all 
 exhibit higher values than do the graphs for the 2-week plants for most of the 
 culture periods of the season. This is especially true of leaf area. Covering 
 the plants with glass seems to have produced a relatively high rate of leaf 
 expansion, in spite of the fact that the evaporation value is somewhat higher 
 for the covered than for the exposed plants. 
 
 The Baltimore covered station. — The 2-week plant data for the covered sta- 
 tion at Baltimore are plotted to a scale one-half as great as the scale used in 
 plotting the exposed plant values, on account of the high values of stem height 
 and leaf-product shown by the covered culture beginning July 9. The values 
 of both leaf-product and stem height for this station are both uniformly 
 above the corresponding values for the exposed station. Also, the tendency 
 of the covered plants to elongate relatively more rapidly than did the exposed 
 plants is shown by the stem-height values for Baltimore covered station for 
 both the 2- and 4-week periods. It is interesting to note that the covered 
 plants do not show specially high values of the plant growth rates for the 
 period beginning August 6, as do the exposed plants. The 4-week graphs for 
 the covered plants show very well the tendency of leaf area to reach values 
 relatively higher than those for dry weight, the leaf-area graph being well 
 above the dry-weight graph for the entire season. 
 
 The Easton covered station. — For Easton the covered plants, as compared 
 with the exposed, show the general tendencies noted above. The 2-week 
 growth rates of the covered plants, especially for stem height, are higher than 
 the corresponding rates of the exposed plants. It will be observed that the 
 maximum growth for the season, in both the covered and exposed 2-week 
 cultures occurs for the period beginning August 3. The 4-week plants of 
 the covered cultures show leaf area relatively higher than dry weight. The 
 values for the culture period beginning June 22 are relatively low for the 
 covered as well as for the exposed cultures. 
 
 THE CLIMATIC CONDITIONS, COVERED STATIONS 
 
 (See figs. 5 and 6, red lines) 
 
 Of the three climatic factors generally dealt with in this study, evaporation 
 alone was measured for the covered stations, so that the climatic data are 
 much less satisfactory in this case than in the case of the exposed stations. 
 
Climatic Conditions of Maryland 395 
 
 It is safe to suppose that the climatic conditions under the glass differed from 
 those for the corresponding exposed stations in certain definite ways, period 
 by period. The rate of evaporation for the covered stations was considerably 
 greater than for the exposed as will be seen by comparing the values given in 
 the tables. We may be certain, also, that some of the incident light was 
 absorbed by the glass and that the light intensity under the cover was thus 
 less than the intensity of the light falling on the exposed plants. Also, we 
 may be reasonably sure that the air temperature under the glass was some- 
 what higher than that outside, especially on quiet days when circulation of 
 air was slight, and there was little tendency toward equalization of air tem- 
 peratures. In considering the behavior of the covered cultures as related to 
 climatic conditions, it may be mentioned that evaporation is known to have 
 been more intense and light intensity lower for these than for the corre- 
 sponding exposed stations and periods, while air temperature was probably 
 higher for the covered than for the corresponding exposed stations. 
 
 The differences between the behavior of the plants under glass and that of 
 the plants in the open seems to be primarily attributable to differences in 
 light conditions for the two sets of cultures. The more rapid stem elongation 
 occurring under glass is exactly what would be expected if the air temperature 
 was higher and the light intensity was lower than in the case of the corre- 
 sponding exposed cultures. The fact that leaf area is relatively high for the 
 covered plants, as compared with their final dry weight, may possibly be 
 related to a smaller amount of dry matter produced by photosynthesis per 
 unit of leaf area in the covered cultures. Such a difference might be expected 
 if the light energy available for photosynthesis were cut down by interposing 
 between the plant and the light source a screen that absorbed a part of the 
 light. 
 
 Whatever may be the true explanation of the behavior of these plants 
 under glass (and the true explanation will surely be much more complicated 
 than is here suggested), the facts indicate very clearly that the growth of 
 the plants under glass was quite different from the corresponding growth 
 in the open. This point must be important in physiological experiments con- 
 ducted in greenhouses. 
 
 Results foe the Baltimore Forest Station 
 
 (-See fig. 5) 
 
 The Baltimore Forest Station was located about 150 yards from the 
 exposed and covered stations at that place. Evaporation was the only 
 climatic feature measured for this station. The sunshine intensity was of 
 course very low, due to the shading and screening effect of the leaves of the 
 trees above the experimental plants. Air temperature was also probably 
 
396 F. Merrill Hildebrandt 
 
 considerably lower than that experienced by the exposed and covered plants. 
 The modification of growth habit in the case of the forest plants is very 
 striking, as can be seen by an inspection of the plant graph for this station. 
 The soy-beans were short erect growers in the open, and were erect with long 
 stems under the glass of the covered station, but were runners in the forest. 
 This effect on stem growth, which obviously cannot be explained as an effect 
 of temperature alone in the case of these cultures, is relatively very great, 
 the highest 2-week value for stem elongation being over four and a half 
 times as great as the seasonal average for all periods and stations, and the 
 highest 4-week value was a little less than four and a half times the seasonal 
 average. As compared with plants grown in the open, the 4-week forest 
 plants also show the same reversal in the relative positions of the leaf-area 
 and dry-weight graphs as was shown by the covered plants. The leaf-area 
 graph is above the dry-weight graph for the entire season in the forest. 
 These cultures are thus more like the covered ones than they are like the 
 exposed ones. This may possibly be accounted for by supposing that the 
 similarity in the behavior of the plants in the covered and forest stations at 
 Baltimore was related to a corresponding similarity in the light conditions for 
 these two sets of cultures, but the problem is doubtless very complex. 
 
 THE PLANT DATA AS MEASURES OF THE CLIMATIC 
 
 EFFICIENCY FOR GROWTH OF THE 
 
 STANDARD PLANTS 
 
 INTRODUCTORY 
 
 As has been stated, the investigation of which this study is a part was 
 planned with the idea of obtaining some quantitative measures of the cli- 
 matic complex for each of the various stations, in terms of plant activity. 
 Since the soil used was the same, since its moisture content was kept high 
 enough to support good growth at all times, for all stations and for all peri- 
 ods, and since seeds of the same lot were used in all cases, it is supposed that 
 the differences in the growth rates for the various periods and stations must 
 have been due to effective environmental differences other than those of soil 
 conditions. On account of the auto-irrigation of the cultures, precipitation 
 was practically without direct influence upon the cultures of the exposed and 
 forest stations, and it was of course quite without direct influence upon the 
 cultures of the covered stations. The influential environmental conditions 
 that differed from station to station and from period to period in these tests 
 were those usually considered as climatic, with the omission of precipitation. 
 The plant data, as set forth in the tables and graphs, may therefore be re- 
 garded as approximate measures of the integrated non-precipitation condi- 
 tions of the several climatic complexes under which the plants grew. These 
 
Climatic Conditions of Maryland 397 
 
 measures of course refer specifically to this particular variety of soy-bean 
 plant and to the particular set of soil conditions that was common to all cul- 
 tures. With another soil, or with another kind of plant, the plant values 
 would of course have been more or less different from those here recorded. 
 It remains to be found out whether or not soy-bean is a suitable standard 
 plant for use in this sort of climatic integration when the needs of agriculture, 
 forestry and general ecology are primarily considered. From what has been 
 said in the preceding sections it appears, however, that soy-bean is at least 
 especially well suited to preliminary and pioneer studies like the present one. 20 
 
 From this point of view, each of the graphs of the plant values (shown by 
 the black lines in figures 2-6) may be regarded as a representation of the sea- 
 sonal march of the non-precipitation portion of the climatic environment 
 for the particular station in question, the graphs for the exposed stations rep- 
 resenting the "natural" conditions, while those for the covered and forest 
 stations refer to the more or less modified climates experienced by these 
 cultures. Some of the more outstanding features of these plant graphs have 
 been mentioned in the preceding sections of this paper, and other features 
 will become evident from a careful study of the graphs themselves, or of the 
 tables from whose data the graphs were constructed. Much more might be 
 said in this connection than has been said, but the newness of the present 
 point of view, together wth the obvious complexity of the numerical results 
 here presented, make it undesirable to attempt a careful study of these data 
 at the present time. The tables and graphs of this paper render the numer- 
 ical values available for future study, when this aspect of climatology and 
 ecology shall have begun to attract more general and appreciative attention 
 than it now enjoys. 
 
 It should be emphasized that the plants have automatically weighted and 
 integrated all the fluctuating and differing conditions for the several culture 
 or exposure periods, and that the final summation is given in terms of the 
 amount of growth produced in 2 weeks or 4 weeks from the seed. Dividing 
 this final summation by the number of days in the corresponding period 
 gives the average plant producing power of the non-precipitation part of the 
 climatic complex for the given period and station. 
 
 It has been noted that these plant values generally show a seasonal march 
 for each station, the growth index being relatively low for periods near the 
 beginning and end of the season, and relatively high for midsummer periods, 
 and it has been suggested that temperature may be considered as the main 
 controlling condition in the bringing about of these seasonal marches, various 
 modifications being superimposed upon the temperature influence by other 
 climatic conditions such as the intensity, duration and seasonal distribution 
 of light, and the intensity and seasonal distribution of evaporation. 
 
 20 A study somewhat similar to this one, using wheat, pea and Drome-grass as standard plants, was carried 
 out by Sampson assisted by the author. See: Sampson, A. W. Climate and plant growth in certain vege- 
 tative associations. U. S. Dept. Agric. Bull. 700. 72 p., 37 fig. Govt. Printing Office: Washington, 1918. 
 
398 
 
 F. Merrill Hildebrandt 
 
 SEASONAL AVERAGES OF MEAN DAILY INTENSITY VALUES FOR THE SEVERAL 
 
 STATIONS 
 
 Aside from the characteristics of the seasonal marches of the climatic con- 
 ditions in question (which are best seen in the seasonal graphs themselves, 
 figs. 2-6), it is of interest to average all the corresponding plant-index values 
 for the season for each station, thus obtaining a seasonal average or mean 
 daily plant -producing power, as a single index for each growth criterion for 
 each station. This has been done for all the stations, for the 2-week and for 
 the 4-week periods and for each growth criterion, and the resulting seasonal 
 
 TABLE X 
 
 Relative seasonal daily means for the several stations, by each of the five growth criteria. 
 The letter H denotes high values; M, intermediate values; and L, low values. (The 
 covered station and the forest station are included for completeness.) 
 
 STATION NAME 
 
 Oakland 
 
 Chewsville 
 
 Monrovia 
 
 College 
 
 Baltimore 
 
 Darlington 
 
 Coleman 
 
 Easton 
 
 Princess Anne. 
 
 Oakland, covered. . 
 Baltimore, covered. 
 Easton, covered 
 
 Baltimore, forest. 
 
 NUMBER 
 
 2-WEEK 
 
 2-WEEK 
 
 4-WEEK 
 
 4-WEEK 
 
 OF 
 
 STEM 
 
 LEAP 
 
 STEM 
 
 LEAF 
 
 DAYS 
 
 HEIGHT 
 
 PRODUCT 
 
 HEIGHT 
 
 AREA 
 
 125 
 
 L86 
 
 L77 
 
 L71 
 
 L71 
 
 154 
 
 L87 
 
 L84 
 
 L75 
 
 L74 
 
 154 
 
 L79 
 
 L80 
 
 L66 
 
 L71 
 
 154 
 
 M95 
 
 M101 
 
 M80 
 
 mho 
 
 153 
 
 H125 
 
 H119 
 
 H104 
 
 Ml 15 
 
 154 
 
 H113 
 
 H118 
 
 H106 
 
 H148 
 
 168 
 
 M96 
 
 M107 
 
 M80 
 
 M117 
 
 171 
 
 M95 
 
 M105 
 
 M75 
 
 L82 
 
 169 
 
 M106 
 
 M96 
 
 M92 
 
 M117 
 
 — 
 
 M92 
 
 L78 
 
 M81 
 
 M118 
 
 — 
 
 H172 
 
 H125 
 
 H136 
 
 H154 
 
 — 
 
 H145 
 
 H142 
 
 HI 16 
 
 H137 
 
 — 
 
 HIP271 
 
 LL a 62 
 
 HH^SG 
 
 L69 
 
 4-WEEK 
 
 DRY 
 WEIGHT 
 
 L79 
 
 L78 
 
 L79 
 
 Ml 19 
 
 M105 
 
 H144 
 
 Ml 13 
 
 L83 
 
 M116 
 
 M93 
 
 M95 
 
 M109 
 
 LL a 41 
 
 a The doubling of a letter indicates an extreme condition; HH means very high, etc. 
 
 means are shown in the last column of each of the data tables (tables I— 
 VIII), where the corresponding seasonal averages for the climatic data are 
 also given. It is to be remembered that the values are all relative, each one 
 being stated in terms of the corresponding average for all stations and all 
 periods, this unit being considered as 100. 
 
 The seasonal averages for the various exposed stations are brought together 
 in table X and are shown graphically in figures 7 and 8, the former figure 
 dealing with the 2-week and the latter with the 4-week plant data. The 
 abscissas of these graphs are not quantitative; the vertical lines are equally 
 spaced and each one represents one of the exposed stations. The stations are 
 arranged in the order of their geographical locations, as far as this is possible 
 in a linear series. The ordinates of these graphs represent the seasonal 
 means. 
 
Climatic Conditions of Maryland 
 
 399 
 
 
400 F. Merrill Hildebrandt 
 
 The 2-week seasonal averages of the two growth measurements taken for 
 the nine exposed stations, represented graphically in figure 7, show that the 
 plant-producing power of the climatic complex is about the same whether it 
 is measured by stem height or leaf-product. The range of variation for stem 
 elongation is from 79 (Monrovia) to 125 (Baltimore). In terms of this 
 growth measurement, the average intensity of the Monrovia climatic com- 
 plex is 63 per cent of that of the corresponding Baltimore complex. Similarly, 
 the leaf-product mean varies from a minimum of 77 (Oakland) to a maxi- 
 mum of 119 (Baltimore); as measured by leaf -product, the mean intensity 
 of the Oakland climate is 65 per cent as efficient as the corresponding mean 
 for the Baltimore climate. Precipitation is of course left out of account here, 
 as in the other considerations of this paper. The nine stations fall into 
 three groups, according to these mean values: Oakland, Chewsville and 
 Monrovia have low relative values, Baltimore and Darlington have high 
 values, and College', Coleman, Easton and Princess Anne have intermediate 
 and similar values. (See the letters L, H and M in table X.) 
 
 Turning to the 4-week seasonal averages, as shown in figure 8, it is seen 
 that the graph for stem height agrees very well with the two 2- week graphs 
 just considered. It is also seen that the 4-week graphs for leaf area and dry 
 weight agree in a satisfactory manner. According to these two graphs, the 
 nine stations fall into the following three groups: Oakland, Chewsville, Mon- 
 rovia and Easton constitute the groups with low values, Darlington is alone 
 in the group with high values, and College, Baltimore, Coleman and Princess 
 Anne make up the group with intermediate and similar values. (See the 
 letters of table X.) 
 
 It is to be remembered that the two series of data (2-week and 4-week) 
 refer to the same total time interval. The plants of one series were register- 
 ing the same climatic conditions as those of the other series; indeed, they 
 were the same plants, for the 4-week measurements were obtained from the 
 same plants as those from which the corresponding 2-week measurements 
 had been secured. The fact that the seasonal averages of the leaf area 
 values (4-week) do not show the same grouping of the stations as do the leaf- 
 product values (2-week) is to be referred to the fact that the plant alters its 
 internal conditions with growth and age. A soy-bean plant exposed two 
 weeks is an entirely different instrument (as far as measuring environmental 
 efficiency is concerned) from the same plant exposed 4 weeks. For this 
 reason it seems desirable that such studies as the present one should be car- 
 ried out with as short periods of exposure of the standard plants as is feasible. 
 It is somewhat as though the instrument wore out and altered its character- 
 istics with too long exposure. Since it is obviously impracticable to obtain 
 a large number of plants that are approximately alike, excepting as seeds, it 
 seems desirable to begin each observation with new seed (as was done in 
 this investigation), and to take the final readings before the internal condi- 
 
Climatic Conditions of Maryland 
 
 401 
 
402 • F. Merrill Hildebrandt 
 
 tions of the plants have been too seriously altered through age and the 
 approach toward maturity. At the same time, the standard plants must of 
 course be allowed to grow long enough so as to be influenced by the fluctuat- 
 ing environmental conditions and long enough to give easily-obtained meas- 
 urements. McLean (1917) has given some attention to the difference between 
 the behavior of the soy-bean plant during the first and second two weeks of 
 its growth from the seed, under the same set of climatic conditions and 
 fluctuations, pointing out that the plant becomes more sensitive to evapora- 
 tion conditions as it grows older (since its leaf surface becomes larger). In 
 the use of standard plants as indicators of climatic efficiency the length of 
 time chosen for the exposure period is clearly very important. It may be 
 added that future studies may bring out certain advantages for a 3-week or 
 4-week exposure of soy-bean plants, as compared with a 2-week exposure, 
 but — as has been pointed out elsewhere in this paper — details will be more 
 apparent when the periods are relatively short, and the principles upon which 
 this sort of work is based are more nearly fulfilled with short periods. 
 
 To summarize this discussion, the nine exposed stations arrange themselves 
 in three groups by every one of the five criteria, the grouping is identical by 
 three of the criteria (2-week stem height, 2-week leaf-product and 4-week 
 stem height), it is identical by the two remaining criteria (4-week leaf area 
 and 4-week dry weight), but is it somewhat different by these two separate 
 ■ series of criteria. The differences are : that the second series of criteria 
 place Baltimore in the intermediate instead of in the high group, and Easton 
 in the low instead of in the intermediate group. 
 
 It is a striking fact that all five growth criteria agree in placing Oakland, 
 Chewsville and Monrovia in the group for low mean daily values, in giving 
 Darlington high values, and in giving College, Coleman and Princess Anne 
 intermediate values. Only for Baltimore and Easton, among the exposed 
 stations, are there discrepancies. 
 
 If the five seasonal values are averaged for each exposed station, the result 
 places Oakland (77), Chewsville (80) and Monrovia (75) in the group fol- 
 low averages, gives intermediate values for College (101), Coleman (103), 
 Easton (88) and Princess Anne (105), and gives high values for Baltimore 
 (114) and Darlington (126). These average values are shown, in the third 
 column of table XI. 
 
 The average data for the covered and forest stations, also shown in table 
 X, emphasize the influence of the glass covers and of the forest shade, etc. 
 
 It is perhaps important to emphasize that the criterion of stem elongation 
 gives the same grouping of the exposed stations by the 4-week as by the 
 2-week values. The ratio of the 2-week seasonal mean to the corresponding 
 4-week mean is shown for each exposed station below. 
 
Climatic Conditions of Maryland 
 
 403 
 
 Oakland 1.21 
 
 Chewsville 1.16 
 
 Monrovia 1 . IS 
 
 College 1.19 
 
 Baltimore 1 .20 
 
 Darlington 1 .07 
 
 Coleman 1.20 
 
 Easton 1.27 
 
 Princess Anne 1.15 
 
 The average of these ratios is 1.18. If, therefore, the stem height of the 
 soy-bean, grown as a standard plant, be used as a measure of the climatic 
 complex, and the measurement be expressed as relative average daily incre- 
 ments, as in this study, the 2-week readings may be approximately reduced 
 to 4-week readings (considering these as the standard) by dividing each 
 2-week reading by the constant 1.18. 
 
 THE TOTAL SEASONAL EFFICIENCIES FOR THE SEVERAL STATIONS 
 
 The efficiency of an environmental complex, or its power to produce growth 
 in a standard plant, is to be considered as the product of two factors, intensity 
 and duration. If the seasonal averages of the mean daily rates of growth, 
 
 TABLE XI 
 Relative generalized climatic-efficiency products for the several stations. 
 
 STATION NAME 
 
 Oakland 
 
 Chewsville 
 
 Monrovia 
 
 College 
 
 Baltimore 
 
 Darlington. . . 
 
 Coleman 
 
 Easton 
 
 Princess Anne 
 
 
 RELATIVE 
 
 
 NORMAL LENGTH OF 
 
 GENERALIZED SEA- 
 
 CLIMATIC 
 
 GROWING SEASON 
 
 SONAL AVERAGE 
 
 EFFICIENCY PROD- 
 
 (A) 
 
 OF DAILY MEAN 
 INTENSITY (B) 
 
 UCT (ab) 
 
 days 
 
 
 
 117 
 
 77 
 
 9009 
 
 156 
 
 80 
 
 12480 
 
 — 
 
 75 
 
 — 
 
 167 
 
 101 
 
 16867 
 
 223 
 
 114 
 
 25422 
 
 188 
 
 126 
 
 23688 
 
 205 
 
 103 
 
 21115 
 
 201 
 
 88 
 
 17688 
 
 181 
 
 105 
 
 19005 
 
 to which attention has thus far been confined, be taken as the intensuy fac- 
 tors for the respective stations, for the season of 1914, and if the length of 
 the entire growing season for each station be taken as the (duration factor, 
 or the length of time through which the corresponding intensity is consid- 
 ered as effective, then the product of the length of the season and the corre- 
 sponding intensity factor should give a value that may approximately repre- 
 sent the relative efficiency of the climatic complex for the station and year 
 in question, by the given plant criterion. Precipitation is of course neglected, 
 as it was not involved in this study. 
 
 The seasonal averages of the daily means for the growth rates, as used in 
 this study (table X), may be taken to represent the relative values of the 
 
404 F. Merrill Hildebrandt 
 
 climatic intensities dealt with, but the lengths of the growing seasons are 
 only approximated by the total lengths of the test periods. Rather than to 
 employ these lengths it will perhaps be better to use the mean (normal) 
 lengths of the growing seasons for the several stations here considered. 
 These may be obtained from Fassig's paper on this subject, 21 and they are 
 shown in table XI, along with the corresponding generalized climatic efficiency 
 products, obtained by multiplying Fassig's mean length of the growing season 
 by the corresponding average climatic intensity (including all five plant cri- 
 teria) as developed in the preceding section of this paper. It is to be empha- 
 sized that the intensity factors are all for the summer of 1914 and that the 
 duration factors are normal, or at least closely approximate normal values. 
 
 From table XI it appears that the lowest efficiency product is for Oakland, 
 as would be expected, while the highest is for Baltimore. The Baltimore 
 value is nearly thrice as great as is the value for Oaldand. If we regard 
 values above 20,000 as high and those between 10,000 and 20,000 as inter- 
 mediate, the stations may be grouped as follows: — 
 
 Low values: Oakland. 
 
 Intermediate values: Chewsville, College, Easton and Princess Anne. 
 
 High values: Baltimore, Darlington and Coleman. 
 These efficiency products may be taken to represent, more or less approxi- 
 mately, the relative values of the climatic conditions at the various stations, 
 to produce plant growth when irrigation is resorted to, so that drought periods 
 are avoided as far as soil moisture is concerned. While there is no reason for 
 thinking that these values (obtained from 2-week and 4-week periods and soy- 
 bean plants, with the particular soil used in this study) may give really 
 quantitative information on these climates as related to plant growth in 
 general, still the product indices here derived are perhaps more reliable than 
 any other series of numerical values that might be readily obtained, and they 
 illustrate a new method by which a beginning may be made aiming toward 
 the quantitative comparison of climatic complexes. 
 
 One of the aims of ecological climatology should be to evaluate climates 
 in somewhat the same manner as water-power, mineral deposits, and other 
 geographically restricted sources of power for the accomplishment of human 
 purposes, may be evaluated. The importance of this aim is very great for 
 agriculture and productive forestry, and it is not less important for the 
 fundamental principles of ecology. The above discussion presents one of 
 the first serious attempts to compare the plant-producing powers of several 
 climates by means of numerical indices. 
 
 21 Fassig, O. L. The period of safe plant growth in Maryland and Delaware. Monthly Weather Rev. 42: 
 152-158. 1914. 
 
Climatic Conditions of Maryland 405 
 
 GENERAL CONCLUSION 
 
 The results and suggestions attained by the study here reported leave the 
 problem of agricultural or ecological climatology still very far from solved, 
 but the purpose of this investigation has been achieved if some of the more 
 fundamental considerations that must be taken into account in this sort of 
 inquiry have been emphasized. The main points brought out are summar- 
 ized in the Abstract at the beginning of this paper and do not require repeti- 
 tion here. It is clear that this aspect of climatological science required other 
 measures and other methods of treatment than those thus far developed by 
 meteorological climatologists, and that much physiological knowledge must 
 be built into the structure of the new science. It appears that the use of 
 standard plants, in some such way as the soy-bean plants were used in this 
 investigation, and the avoiding of the immense complications due to soil 
 conditions when the same soil is not employed in all cases, will lead to progress 
 in this exceedingly difficult but both fundamentally and practically important 
 field of human advancement. If the relations that hold between climatic 
 conditions and plant growth are to be really understood it will be necessary 
 for the climatological student to interest himself in plant physiology in no 
 merely superficial way, and it will be necessary for much of the science of 
 climatology, as it is now represented in the literature, to be very lightly 
 stressed. The point that seems in need of emphasis is that this new aspect 
 of climatology (or of ecology) will have to deal with climatic conditions as 
 they affect plants; it will not need to give main attention to climatic fluctua- 
 tions and differences per se, nor to the meteorological, physical and astronomi- 
 cal reasons for their occurrence. 
 
VITA 
 
 The writer was bom December 26, 1888, at Baltimore, Maryland. He 
 entered the Baltimore Polytechnic Institute in 1903, being graduated in 
 1907. During the year 1908-1909 be taught in the public schools of Balti- 
 more. In 1909 he entered the Collegiate Department of the Johns Hopkins 
 University, receiving the degree of Bachelor of Arts in June, 1913. During 
 the years 1914-1917 he attended the Johns Hopkins University as a graduate 
 student in Plant Physiology, Physical Chemistry and Botany. He was 
 engaged in research for the Maryland State Weather Service during the 
 year 1915-1916, and carried on research for the U. S. Forest Service at the 
 Utah Experiment Station of the Forest Service during the summer of 1916. 
 
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