THE EXTERNAL CONDITIONS ON THE WILTING 
 COEFFICIENT OF PLANTS 
 
 BY 
 
 CHARLES ORLANDO PEAK 
 
 THESIS 
 
 For the 
 
 DEGREE OF BACHELOR OF ARTS 
 
 PLANT PHYSIOLOGY 
 
 COLLEGE OF LIBERAL ARTS 
 UNIVERSITY OF ILLINOIS 
 
 1921 
 

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 UNIVERSITY OF ILLINOIS 
 
 June 191?.^.. 
 
 THIS IS TO CERTIFY THAT THE THESIS PREPARED UNDER MY SUPERVISION BY 
 
 Char 1 e s . . 0 r ,1 an.do Peak 
 
 ENTITLED Thg,..Ext e;rnal. 
 
 Co e f f i c i e n t _ 0 f _ P 
 
 IS APPROVED BY ME AS FULFILLING THIS PART OF THE REQUIREMENTS FOR THE 
 DEGREE OF B.,... A,. 
 
 
 
 Instructor in Charge 
 
 Approved : 
 
 
 
 HEAD OF DEPARTMENT OF .BqMXIY, 
 
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Table of Contents. 
 
 Page. 
 
 I. Introduction. 1. 
 
 II. Methods. 7. 
 
 III. Results. 11. 
 
 IV. Conclusions. 12. 
 
 V. Literature Cited 14. 
 
Digitized by the Internet Archive 
 in 2016 
 
 https://archive.org/detaiis/externalconditioOOpeak 
 
- 1 - 
 
 I. INTRODUCTION. 
 
 In the complex factors which enter into the make 
 up of the environment of the plant the available soil water 
 is of great importance. The question has often arisen as to 
 just what portion of the soil moisture is available to the plant. 
 The quantity available varies with the nature and extent of the 
 root system, and the environmental factors surrounding the shoot. 
 The water of the soil cannot be removed completely by the plant. 
 As the available water is reduced through absorption by the roots 
 the physical attraction of the soil particle for the remaining 
 water becomes greater and greater until finally an equilibrium 
 is reached between the absorptive powers of the root and the 
 attractive force of the soil for the water. When this stage is 
 reached wilting because of lack of sufficient water to cover 
 that lost by evaporation ensues. 
 
 In the attempt to determine the wilting coefficient, 
 described later, of different plants, or of the same plant under 
 different conditions, numerous factors of the environment must 
 be taken into consideration. Most of the water taken up by the 
 plant through its roots is lost by transpiration. Thus we see 
 that conditions that increase the rate of transpiration call for 
 an increase in the absorption of water from the soil. The effioi 
 enoy of the roots of different species, or of different individu- 
 

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- 2 - 
 
 als of the same species growing under identical conditions can 
 be tested relative to their absorptive powers. The measure of 
 the ability to absorb water under trying conditions of soil and 
 of atmosphere is the point of wilting of the young shoots. 
 
 The non-available moisture left in the soil at the 
 time of wilting of the plant has been termed "the wilting coeffi- 
 cient". This method was first used by Sachs (l) in 1859 on 
 tobacco plants grown in various soils, and has since been used 
 and considerably extended by plant physiologists, physicists, 
 and ecologists. The work has been done entirely on plants grow- 
 ing in soils of definite moisture content and under normal at- 
 mospheric conditions. No attempt has been made to find the 
 effect that a change in the atmospheric conditions - wind, mois- 
 ture, etc. - have upon the wilting coefficient. In dealing with 
 the question of wilting in plants one is facing the rates or 
 proportion between two physiological processes, namely those of 
 transpiration and of absorption. These two processes are affected 
 a great deal by external conditions. As an example, on a hot 
 day a plant will wilt for a time because the absorption of water 
 by the roots in insufficient to cover that lost by transpiration. 
 
 In 1912 Briggs and Shantz (2) carried on extensive ex- 
 periments with 20 different kinds of soils and one hundred differ- 
 ent plants, including hydrophytes and xerophytes. They took a 
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 warrant further investigation. They concluded that for any 
 given texture of soil the wilting coefficient remains a constant, 
 regardless of the kind of plant or the conditions under which 
 the plant was grown. Instead of considering all the factors 
 which cause wilting, they have only considered one factor, and 
 that is the moisture in the soil. 
 
 V. H. Blackman (3) , working with plants under desert 
 conditions reached very different results from those of Briggs 
 and Shantz. Blackman found a wide variation under different 
 environmental conditions. Some of his readings on plants grown 
 under lath shelter, and in the open, varied as much as 40^ in 
 the moisture content of the soil at the permanent wilting stage. 
 
 A low rate of transpiration for a mesophyte, may be 
 very high for a zerophyte. Yet it has been found that mesophytic 
 and zerophytic plants transpiring at different rates and grow- 
 ing in soil of the same texture, will reduce the soil to the 
 same degree of dryness. Blackman gives as a possible reason for 
 this that the more actively transpiring plant has a larger ab- 
 sorbing surface with which to supply the plant with the necessary 
 water. 
 
 In the experiments carried on thus far in regard to the 
 wilting coefficient of plants, the investigators have been unable 
 to find any marked difference in regard to a difference in the 
 absorptive efficiency of roots of different plants. One would 
 

 
 
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-4- 
 
 expect to find in different plants differences in the poxer to 
 absorb water from the soil and thus reduce soil moisture. Black- 
 man (3) says in his article on "The Wilting Coefficient of the 
 Soil" - "Whether absorption is considered to be mainly a proto- 
 plasmic process or as mainly controlled by the osmotic pressure 
 of the cells and the condition of aggregation of the cell colloids, 
 there must be marked differences in such factors in different 
 plants which one would expect to affect the wilting coefficient." 
 That such differences have not been noted is due mainly to the 
 conditions under which the experiments have been carried out. 
 
 The soil moisture is reduced to a large extent before it becomes 
 the controlling factor in absorption. This would affect the 
 plant at a very late and critical stage. 
 
 For a thorough study of the wilting coefficient of 
 plants in nature, we must consider along the line of changing 
 environment. There may be as many wilting points as there are 
 factors in the environment. Caldwell (4) points out that wilt- 
 ing, brought about by high rate of transpiration of short duration 
 is less serious than wilting brought about by a long period of 
 slow transpiration. In the first case the root hairs are only 
 plasmolyzed, while in the second they are killed, and the recovery 
 of the plant takes place only after new root hairs are formed. 
 
 The relations of the wilting coefficient to the differ- 
 ent soils have been brought out very well by the work of V. H. 
 
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-5- 
 
 Elackman (3). The coefficient is related directly to the physi- 
 cal characteristics of the soil. Thus the smaller the particles 
 of soil the lower will be the rate of root absorption. The 
 conducting property of the soil must needs play an important 
 part, for even in a soil through which the roots are well distrib- 
 uted only a small portion of the soil is in actual contact with 
 the roots. 
 
 Crump -(5), in his paper on moorland soils, brought out 
 the relation of water content to the humus content of the soil. 
 
 He showed that the roots of a plant may occupy several different 
 kinds of soil layers, varying in moisture content. Thus he found 
 that the amount of water in the soil was directly related to the 
 amount of hurnus. This water content he called the coefficient of 
 humidity. The soil around the roots of a heather moor, though it 
 varied largely in water content, its coefficient of humidity varied 
 but very little and could be taken as characteristic of its asso- 
 ciation. The ground societies of nearly all the plants on the 
 moorland could be distinguished by means of this coefficient. \ 
 
 \ 
 
 He showed clearly that the wilting coefficient of Eriophorum 
 angustifolium and Colleraa vulgaris were closely related to the 
 humus content of the soil. Thus Crump’s coefficient of humidity 
 may be taken as the standard index in soils where humus is the 
 dominant constituent. 
 
 One of the phases of the problem of wilting coefficient 
 
 
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 is a study which shall lead to a formula that will apply to 
 show the relation between wiltings at various rates of transpira- 
 tion on the one hand and the water content and mechanical composi- 
 tion of the soil on the other. 
 
 It has been long maintained by plant physiologists, 
 ecologists and agriculturists, that plants differ in their power 
 to withdraw water from the soil. Until of late no definite ex- 
 periments have been carried on where the conditions have been 
 definitely under control. Hedgcock (6) has found that the 
 amount of moisture in the soil at the time of wilting is most in 
 the case of hydrophytes and decreases through xerophytes. Living- 
 ston (7), comparing the zerophytes - Allonia, Bouhavia and Eu- 
 phorbia with the mesophytes - Phaseolus, Helianthus and Vicia, - 
 that the desert forms show an adaptation to live in drier soils 
 than plants of a humid climate. 
 
 It has been found by SchroSder that the water content 
 of the plant is the same during the growing time regardless of 
 the conditions under which it was grown or the water content of 
 the soil. It has been thought that a relationship existed be- 
 tween the water content of the soil and that of the leaves. In 
 plants the water content may vary within considerable limits 
 without affecting in any marked way its life processes. The 
 water content of the soil so long as it affects the water content 
 of the plant within the limits of this critical range are without 
 
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-7- 
 
 material effect on the vegetal covering. A lowering of the 
 water content of the soil that will affect the water content of 
 the plant within the limits of its critical range, at once brings 
 about changes in growth and finally wilting in the plant. 
 
 II. METHODS. 
 
 The plants were grown in pure quartz sand obtained 
 from Ottawa, 111. It had a water holding capacity of 21.- grams 
 per 100 grams. The sand was used in glazed earthenware and glass 
 containers. The earthenware jars were 5 inches in diameter, by 
 3-^ inches deep; the glass Petri dishes were 7 inches in diameter 
 and 1 inch deep. 
 
 The containers were weighed and the weight recorded. 
 They were then filled with the quartz sand and again weighed, 
 and the ?/eights recorded. This sand was then poured out into a 
 pan and the amount of water added to give the desired per cent of 
 soil moisture. This was mixed thoroughly so as to insure an even 
 distribution of the water. The moist sand was now replaced in 
 the container and beans, sunflower, or diseased and disease free 
 corn, were planted. 
 
 The containers were weighed at twelve hour intervals 
 and the necessary water added to bring the weight back to normal. 
 This was kept up until the plants were put under the fan for the 
 
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- 8 - 
 
 purpose of testing the wilting coefficient. At first the plants 
 were grown in the green house hut due to the dry hot air, those 
 in the 10^ and 20^ moisture content wilted. This wilting would 
 inhibit the normal growth of the plant by reducing for a time the 
 normal turgor of the plant cells. To prevent this the plants 
 were grown in a trough 2-J- ft. x 4 ft. x 8 inches, and covered 
 with glass. The relative humidity in this chamber was between 
 30*^^ and 40'^, as compared with the 20^ and 25*^ in the laboratory. 
 
 The earthen crocks used at first proved unsatisfactory 
 because of the time necessary to bring the sand down to the wilt- 
 ing coefficient in the tests. To avoid this time factor, flat 
 granite pans 7 in. x 11 in. x 1 in. were used. After running 
 several series in the pans it was found that the pans were too 
 shallow, that is, they exposed too much surface for evaporation. 
 Preliminary experiments clearly demonstrated that the plants 
 grown in these pans showed a wide difference in the time and 
 degree of wilting an consequently made it impossible to determine 
 the residual moisture in the sand at the time of wilting of the 
 different individual plants. In the experiments that followed 
 single plants were grown in glass Soyka dishes 3 inches in dia- 
 meter and 1-J inches in depth. 
 
 The moisture content of the sand used in the different 
 series was 10^, 20^, 40'^, and 60'^ of its water holding capacity. 
 Beans, corn, and sunflowers planted in these dishes were allowed 
 
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 to grow from a week to ten days before they were placed at a 
 definite distance from an electric fan, and in the path of the 
 air current. When the plants were placed before the fan, water 
 was withheld. They were weighed every three hours and the rate 
 of loss in moisture was determined. When permanent wilting took 
 
 place, the plant was removed, and the sand was air ‘dried and 
 weighed. 
 
 In another series corn was grown under glass jars, 
 at a relative humidity of about 80f^ or 90fo, where the rate of 
 transpiration was low. The plants grew rapidly and were from 
 one to two days ahead of the corn grown otherwise. These seed- 
 lings, when placed in the air current from the fan showed a very 
 marked difference in the wilting coefficient. In place of the 
 plants wilting and falling over as in the first series, they dried 
 retaining the green color and not "firing" in any way. The plants 
 were permanently wilted in from 5 to 7 hours after being placed 
 before the fan. 
 
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 when placed in the air current would begin to "fire" at the tips 
 and this would gradually enlarge, and at the time of wilting ex- 
 tended half way dom the leaf. The stalks of the diseased com 
 would wilt and bend over at the sand, while in the disease free 
 corn they would wilt at the insertion of the first leaves. In 
 the sunflower the youngest plants ivould be the first to wilt. 
 
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 while in the bean the oldest plants would wilt first. In the 
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 as shown by the gradual shriveling. In the bean seedlings the 
 cotyledons showed a similar shrinkage due to shriveling. The 
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 were dried up, and then they rapidly wilted. The bean never 
 fell over after permanent wilting. 
 
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 difference was in sand of saturation. Here the root develop- 
 ment of the diseased corn was very weak. The length of the roots 
 in the disease free corn was from 1^ to 2 times that of the 
 diseased. In sand of 60^ saturation molds would form around the 
 shoot of the diseased corn and the mycelium would branch out 
 over the sand and grow up the stalk. 
 
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 difficulty was met with in keeping the water content of the sand 
 uniform throughout the dish. This was practically impossible to 
 do for the roots would have the bottom of the sand dry, while 
 evaporation from the surface would keep this below the moisture 
 content. The decrease I got in the ability of my corn to take 
 moisture from the soil was due to the physiological effect of 
 moisture upon the fungus which causes root rot of corn. 
 
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-11 
 
 III. RESULTS. 
 
 Table 1. 
 
 Average results obtained on series including 46 beans. 
 
 57 sunflowers, and 
 
 93 diseased 
 
 and disease free 
 
 (Root Rot) 
 
 Plant 
 
 
 Soil 
 
 Moisture 
 
 ' 
 
 
 lOfo 
 
 20fo 
 
 40^ 
 
 eofo 
 
 Disease free corn 
 
 .27 
 
 .309 
 
 .73 
 
 .45 
 
 Diseased corn 
 
 .894 
 
 1.08 
 
 .999 
 
 .495 
 
 
 lOfo 
 
 20fo 
 
 ZOfo 
 
 40fo 
 
 Bean 
 
 .27 
 
 1.46 - 
 
 6.37 
 
 10.31 - 
 
 Sunflower 
 
 .462 
 
 1.12 
 
 1.48 
 
 8.26 
 
 Table 2. 
 
 Results on bean and sunflower seedlings. 
 
 Number 
 
 Dry weight 
 (grams) 
 
 Plant 
 
 Original water 
 content 
 
 Percent of non- 
 avail able water 
 
 16 
 
 1052 
 
 S. F. 
 
 40f 
 
 10.6 
 
 18 
 
 1083 
 
 n 
 
 30f 
 
 2.06 
 
 6 
 
 1092 
 
 tt 
 
 40^ 
 
 11.7 
 
 3 
 
 1101 
 
 H 
 
 lOf. 
 
 .27 
 
 5 
 
 1007 
 
 M 
 
 30> 
 
 1.03 
 
 4 
 
 1061 
 
 (( 
 
 20% 
 
 1.03 
 
 10 
 
 1007 
 
 II 
 
 20% 
 
 .69 
 
 59 
 
 1073 
 
 Bean 
 
 20% 
 
 1.3 
 
 60 
 
 1048 
 
 It 
 
 30^ 
 
 1.3 
 
 68 
 
 1093 
 
 II 
 
 30fo 
 
 6.69 
 
 58 
 
 1134 
 
 II 
 
 20% 
 
 2.06 
 
 72 
 
 1070 
 
 t! 
 
 i0% 
 
 10.3 
 
 56 
 
 1120 
 
 II 
 
 10% 
 
 .27 
 
 55 
 
 1073 
 
 II 
 
 10% 
 
 .27 
 
 57 
 
 1065 
 
 II 
 
 10% 
 
 .27 - 
 
 67 
 
 1079 
 
 II 
 
 00% 
 
 6. 55 
 
 70 
 
 1079 
 
 II 
 
 Wfo 
 
 10.69 
 
 15 
 
 1126 
 
 It 
 
 00% 
 
 6.89 
 
 71 
 
 1047 
 
 II 
 
 40f. 
 
 8.6 
 
i 
 
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-13- 
 
 Table 3. (continued). 
 
 Number 
 
 Dry weight 
 
 Plant 
 
 Original water 
 
 Percent of non- 
 
 
 (grams) 
 
 
 content 
 
 available water 
 
 69 
 
 1038 
 
 Bean 
 
 30^0 
 
 5.8 
 
 61 
 
 1107 
 
 n 
 
 30fc 
 
 5.8 
 
 63 
 
 1090 
 
 ti 
 
 20i 
 
 6.5 
 
 66 
 
 1114 
 
 II 
 
 40‘fo 
 
 13.0 
 
 64 
 
 1067 
 
 II 
 
 40i 
 
 10.0 
 
 53 
 
 1116 
 
 II 
 
 30i 
 
 1.7 
 
 65 
 
 1041 
 
 ti 
 
 40i 
 
 10.3 
 
 63 
 
 1075 
 
 It 
 
 30i 
 
 6.3 
 
 53 
 
 1103 
 
 It 
 
 30% 
 
 1.03 
 
 IV. CONCLUSIONS. 
 
 Individual plants show a wide variation to drought 
 resistance. This is due in part to the more efficient absorptive 
 power of the roots of some which can overcome to a greater degree 
 than others the physical forces that hold the water of the soil. 
 
 In general it may be stated that under similar conditions differ- 
 ent plants have different wilting coefficients in the same texture 
 of soil. 
 
 In making wilting coefficient determinations the follow- 
 ing precautions should be observed: 
 
 1. The soil used should be of uniform texture. 
 
 3. The soil should be brought to a uniform water content before 
 being used. 
 
 3. The moisture lost by transpiration should be replaced at 
 frequent intervals. 
 

 
 
 
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4. All sudden fluctuations in temperature should be avoided. 
 
 5. The moisture determination should be made as soon as the 
 plant has reached the stage of permanent wilting. 
 
 There is a marked difference in the root development 
 of different plants. This difference may become more marked 
 under certain soil moistures. Thus in a soil of 10^, 20*^, 30^, 
 and 40^ moisture content the root systems of the bean and sun- 
 flower seedlings showed essentially the same development, but 
 the bean seedling had a lower wilting coefficient, than the sun- 
 flower seedling at 10^, while at 20/^, 30^, and 40'^ moisture con- 
 tent the sunflower had the lower wilting coefficient. 
 
 This shows conclusively that the wilting is dependent 
 on the absorptive power of the roots rather than on root develop- 
 ment. This was well brought out in the case \7here sunflower and 
 bean seedlings were grown in the same pot under identical condi- 
 tions. IIThere the plants were of uniform height and vigor the 
 bean seedlings under certain conditions, would wilt from one to 
 three hours before the sunflower. This may in part be due to the 
 difference in structure. In the case of the bean the cotyledons 
 would dry up before the leaves would show signs of wilting. 
 
 One may conclude that the difference in the wilting 
 coefficients of plants is due to a difference in the absorptive 
 powers of the roots rather than to differences in development, 
 which in a way may affect the wilting coefficient. 
 

 
 
 
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-14- 
 
 V. LITERATURE CITED. 
 
 1. Sachs. Tobacco plants in different soils. 1859. 
 
 2. Briggs, L. J. & Shantz, H. L, The wilting coefficient for 
 
 different plants and its indirect determination. 
 
 U. S. Dept, of Agr. Bur. of Plant Industry. Bull. 230. 1912 
 
 3. Blackman, V. H. The wilting coefficient of soils. 
 
 Jour, of Ecology. Vol. 2. 1914. 
 
 4. Caldwell, J. S. The relation of environmental conditions to 
 
 the phenomenon of permanent wilting in plants. 
 
 Physiological Researches. No. 1. 1913. 
 
 5. Crump, W. B. The coefficient of humidity. 
 
 New Phytologist 12:125. 1913. 
 
 6. Hedgcock, G. C. The relation of the water content of the 
 
 soil to certain plants. Bot. Survey of Nebr. Vol. 6. 
 
 7. Livingston, B. E. & Brown, W. H. Relation of the daily march 
 
 of transpiration to variations in the water content of 
 foliage leaves. Bot. Gaz. 52:309. 1912.