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NORTHERN HEMLOCK AND HARDWOOD 
 MANUFACTURERS ASSOCIATION 
 
 Bulletin 1 
 
 R. B. GOODMAN, President 
 O. T. SWAN, Secretary 
 
 In Co-Operation with the 
 
 Forest Service 
 U. S. Department of Agriculture 
 
 HENRY S. GRAVES, Forester 
 
 The Structure of Wood and Some 
 of its Properties and Uses 
 
 An Address Before the Members of the Northern Hemlock and Hardwood 
 
 Manufacturers Association, at the Forest Products 
 
 Laboratory April 28, 1915 
 
 BY 
 
 ELOISE GERRY 
 
 Microscopist, Forest Products Laboratory 
 Madison, Wis. 
 
 FORTKY 
 
 OF& AGrUCyi 
 
 OSHKOSH, WISCONSIN 
 JANUARY, 1916 
 
NORTHERN HEMLOCK AND HARDWOOD 
 MANUFACTURERS ASSOCIATION 
 
 Bulletin 1 
 
 R. B. GOODMAN, President 
 O. T. SWAN, Secretary 
 
 In Co-Operation with the 
 
 Forest Service 
 U. S. Department of Agriculture 
 
 HENRY S. GRAVES, Forester 
 
 The Structure of Wood and Some 
 of its Properties and Uses 
 
 An Address Before the Members of the Northern Hemlock and Hardwood 
 
 Manufacturers Association, at the Forest Products 
 
 Laboratory April 28, 1915 
 
 BY 
 
 ELOISE GERRY 
 
 i/ 
 Microscopist, Forest Products Laboratory 
 
 Madison, Wis. 
 
 p* out rrr *f* 0tVf toi 
 
 FORESTRY 
 
 s QfiAG 
 
 OSHKOSH, WISCONSIN 
 JANUARY, 1916 
 
ffio V" 
 
 The Structure of Wood and Some of 
 Its Properties and Uses* 
 
 By ELOISE GERRY 
 
 Microscopist, Forest Products Laboratory, Madison, Wisconsin 
 
 THE STRUCTURE OF WOOD. 
 
 A microscopical study of wood brings out the characteristics* 
 by which different kinds of woo 1 can be identified. It makes clear 
 many of the reasons for the variations in the properties, both 
 chemical and physical, and for the behavior of wood un:ler differ- 
 ent circumstances. 
 
 Variability of \V<><i<l. For building 1 purposes wood competes, 
 for instance, with tile, brick, steel, and other inorganic compounds* 
 of a relatively homogeneous character. Wood, which is produced! 
 by the growth of living cells, shows marked variations in its struc- 
 ture, due to the conditions, such as abundant or seanty moisture, 
 nutrition, or light which surround the individual plant during its 
 fcrijrition. The variations found among different pines, oaks, and 
 maples, are striking, but besides these variations within the genus 
 (e.g. pine, oak, maple) these woods vary so widely from one another 
 that each may be said to represent me of three separate groups into 
 which all woods may be divided. (ee Plates I, II, and III.) 
 
 and Ilearhrood. Besides the variations in minute 
 structure there are in every tree two regions whieh differ markedly 
 from each other. These are the sap and heart wood. 
 
 The sapwood is the layer of wood on the outer circumference of 
 the tree just under the bark. It varies in different trees from less 
 than one-half an inch in width, for example, in osage orange or about 
 an inch in most oaks, to five or more inches in width in certain 
 pines such as loblolly. The sapwood differs from the heartwood 
 because it contains living cells and the functional elements which 
 conduct the crude sap, that is the water and dissolved mineral salts, 
 from the roots to the leaves. In the leaves, in the presence of the 
 green coloring material, callH chlorophyll, and sunlight this crude 
 sap is converted into the elaborated sap which provides nourish- 
 ment for the young growing tissue. The elaborated sap is eon- 
 ducted down the tree from the leaves through the inner layers of 
 the bark. From here it is distributed to the growing cells, especially 
 those in the outer sapwood. 
 
 The heartwood which occurs around the pith at the center of the 
 tree, does not contain living cells. It is often conspicuous in contrast 
 to the light colored sapwood by reason of its darker color, as for 
 example in red cedar, black walnut, and ebony. It is usually more 
 durable than the sapwood. Every year as the inner sapwood cells 
 oease to function they add a little to the width of the heartwood 
 at its outer limit. Therefore, as the tree grows older the amount 
 
 - b f0 < \i l [ he ^ niembor ' s of th( ' Xrthern Hemlock and Hardwood Manufac- 
 on, at the Forest Products Laboratory April 28, 1915. 
 
of heartwood is constantly increased. The amount of sapwood re- 
 mains always about the same. The sapwood is continually recru-iteu 
 on us outer circumference by the addition of new ceils formed by the 
 cambium or growing layer which is located between the wood and 
 the bark. This is the layer of soft thin-walled cells which is broken 
 when in the spring the small boy makes whistles by slipping the 
 woody cylinder away from the bark on a willow shoot. 
 
 The Annual Rings. In temperate climates, like that of the 
 United States, the tree lays down a yearly increment of growth. 
 This is produced from the cambium and forms the outermost layer 
 of the sapwood. This addition of new material is called an annual 
 growth ring, ("ar" Plate I, II, and III.) The age of a tree can 
 be estimated with considerable accuracy by counting these rings. 
 
 Opting and Summer wood. The part of the annual ring which 
 is formed early in the season is called the springwood ("Sp" Plates 
 I, II, III, and IV) and that which grows later the summerwood .("S" 
 Plates I, II, III, and IV.) The trees in a temperate climate cease 
 growing during the winter months. Springwood cells are usually 
 larger in diameter and have thinner walls than those formed later in 
 Ili- year. The summerwood in red or Norway pine, for instance, is 
 the hard, somewhat orange-colored, conspicuous portion or the 
 "grain." (cf. Plate I, "S".) 
 
 $oft Woods and Hard Woods. The so-called softwoods and 
 hardwoods are not divided according to their actual hardness in 
 all cases. Therefore, these terms which are so commonly used are 
 not strictly accurate. For example, longleaf pine (Plate 1) is 
 harder than basswood or aspen (Plate II) but longleaf is regarded 
 as a soft wood and basswood a hard wood in the accepted classifi- 
 cation according to structure. Softwoods are variously known as 
 gymnosperms, conifers, and non-porous woods. They are the woods 
 from the needle-leaved trees which in most cases (larch and cypress 
 excepted) are "evergreens." Plate I illustrates woods of this class. 
 Hardwoods are known as angiosperms and porous woods, either 
 diffuse or ring porous, according to the arrangement of the pores in 
 the annual ring. These woods are produced by the broad-leaved 
 trees. On the whole the softwoods have a much simpler structure 
 than the hardwoods. 
 
 Fibers or Tracheids. The elements of the softwoods are chiefly 
 fiber-like cells which are called traclieids ("t" Plate I). These 
 fibers are approximately 1/10 to 1/5 inches long and have tapering 
 ends. They are practically closed cells. The sap passes from cell 
 to cell through thin places in the cell wall which are known as 
 bordered pits. The thin membranes of these have been found* in 
 SBome cases to contain very minute perforations which assist in the 
 passage of liquids through these fibers. On the whole, however, 
 the elements of the softwoods may be said to lack the porous type 
 of structure which assists the rapid transfer of larae quantities of 
 water, such as exists in the hardwood trees. For this reason they 
 are called non-porous woods. The fibers of the softwoods are placed 
 end to end in the tree. The ends, however, lap each other approxi- 
 
 *I. W. Bailey "Preservative Treatment of Wood." For Quar. Vol. XI, No. 1, 
 March, 1913. 
 
 3 
 
 477626 
 
inateiy one-third of the liber length and the fibers run parallel to 
 the longitudinal axis of the tree. The sap 111 passing up a comierous 
 tree zoo feet in height muse tnen traverse approximately 3t> f2,OUU 
 of the fibers. These elements serve two purposes, viz., to give the 
 required mechanical strength to hold the crown of leaves up into 
 the light and air, and also to conduct the crude sap 'from the roots 
 to the leaves. 
 
 Medullary Rays. Other important elements are the medullary 
 rays. (!See 4 'mr " in Plates 1, li, ill, and 1V.J These are made up 01 
 short brick-shaped cells. Tlie rays extend from the pitn to tiie bant 
 and run at right angles to the longitudinal axis of the tree. In any 
 cross sectional disc from a tree they are seen to extend irom the pitn 
 to the circumference like the spoKes in a wheel. The rays are much 
 more conspicuous in such hardwoods as oak or beech where they 
 produce the so-called "silver grain" ("mr" in Hate Iff) than they 
 are m any of the soft woods. The medullary rays serve to conduct 
 the elaborated csysmward from the inner bark to the growing por- 
 tions of the sapwood. 
 
 liesiti uucts. In certain softwoods, viz., Douglas hr, spruce, 
 larch and pine we lind canals here and there among the cells, ("re 
 Hate f.) These are the points in the tree where the resins or pitch, 
 which are produced by these species, are located. They are 01 con- 
 siderable assistance in aiding the penetrance of a wood with pre- 
 servatives. They extend both vertically and horizontally through 
 the tree. The horizontal canals are in the medullary rays and are 
 smaller in diameter than the vertical canals. The two systems fre- 
 quently connect with each other. Among the important soft woods 
 where they are normally absent are cedar, hemlock, balsam, cypress, 
 and redwood. The American hardwoods do not have resin ducts 
 but in addition to the fibers and medullary rays, which are present 
 in both hard and soft woods, they possess pores or vessels. 
 
 Pores or Vessel a. These elements are specialized channels which 
 serve to conduct the ascending stream of the crude sap. They may 
 be compared to the water pipes which extend from the basement to 
 the top of a tall building. They are small hollow tubular struc- 
 tures. (See "v" Plates ft, III, and IV.) 
 
 Diffuse Porous Woods, If the pores are scattered with consider- 
 able uniformity through the annual ring and there is relatively 
 little difference in size between those in the springwood and those 
 in the summerwood, the wood is said to be diffuse porous. Maple, 
 birch, willow, poplar, basswood, cherry, gum and tulip are examples 
 of woods belonging to this class. (See Plate II.) 
 
 Ring Porous Woo ds. When there is a considerable difference 
 in size between the pores in the spring and those in the summerwood 
 and the large pores are located in the springwood so that they 
 form conspicuous concentric rings, as seen in the cross section of 
 a log, the wood is said to be ring porous. Oak, ash, hickory, elm. 
 catalpa, locust, mulberry, and osage orange are examples of woods 
 belonging to this class. (See Plate III.) 
 
 In the hardwoods there is a division of labor among the ele- 
 ments. The fibers- ("f" Plates II, III, and IV), which jrive the im- 
 
 4 
 
ate I. A Non-Porous Wood. Cross Section of Longleaf Pine; AR, annual ring; SP, spring- 
 wood; S, dense summerwood; T, tracheids; MR, medullary rays; RC, resin canals. 
 
chanical strength, are often thick walled. They frequently have 
 thicker walls in proportion to the lumen or cavity enclosed than is 
 found in the softwoods. They also share little, often not at all, in 
 the conduction of the sap. The vessels, on the other hand, are ad- 
 mirably constructed to assist in the rapid transfer of considerable 
 quantities of water solutions, since they have open porous ends 
 which facilitate the passage of liquids with greater ease and rapidity 
 than the tracheid type of element, where the passage always takes 
 place through membranes, even though these may be thin and in 
 some cases have minute perforations. 
 
 Tyloses. In certain hardwoods, e. g. white oak, the large pores 
 or vessels are closed more or less completely with cell-like growths 
 called tyloses ("t" Plate IV). These in many cases render the 
 wood practically impermeable to liquids and gases. It is because 
 of the presence of tyloses that white oak is suitable for making 
 liquid-tight barrels (tight cooperage stock). Red oak lacks these 
 cells and cannot be used for liquid containers (it is classed as slack 
 cooperage stock). This lack of tyloses makes red oak (See "V' : 
 Plate III) an easy wood to impregnate with preservative solutions, 
 such as coal-tar-creosote. 
 
 Durable hardwoods as a rule have abundantly developed tyloses. 
 Other factors such as chemical composition, rate of growth, and 
 hardness, as well as the presence of tyloses, are however also signifi- 
 cant in determining durability. Tyloses are abundantly developed 
 in the following very durable woods: Black locust, catalpa, osage 
 orange, mulberry, white oak, post oak (Plate IV) and red heart 
 beech.* 
 
 In the species where tyloses are present in the heartwood, they 
 are found more or less fully developed in the sapwood also. In the 
 few outside annual rings where the sap is being conducted up the 
 tree no tyloses are normally present. Tyloses may be said to act as 
 a natural filler in wood. It is easier for a carpenter to finish white 
 oak than red oak, for the tyloses in the white oak provide a natural 
 filler to catch the stain and varnish. In the cases of penetrance 
 treatments and the water-logging of wood tyloses act as a filler or 
 stopper to check the entrance of liquids. In the same way they 
 provide a barrier to the entrance of wood-destroying fungi, the or- 
 ganisms causing rot. 
 
 THE PROPERTIES OF WOOD IN RELATION TO ITS USES. 
 
 Chemical Composition. In selecting species of wood for special 
 uses it is important to consider their chemical constitution. Aside 
 from the resins, tannins, and other special secretions peculiar to 
 the different species of wood, the bulk of the wood substance is 
 made up of compounds of carbon, hydrogen, and oxygen, which are 
 chiefly in the form of celluloses and lignin. A compound which has 
 a high cellulose content is well adapted to the production of a high 
 yield of chemical pulp. In preparing a chemical pulp the individual 
 fibers of the wood are separated by cooking the wood in a liquor 
 which dissolves out the greater part of its lignin content. This 
 
 *See Jour, of Agric. Research, Mar. 25, 1914. Vol. I, No. 6. "Tyloses: Their 
 Occurrence and Practical Significance in Some American Woods." 
 
Plate II. Cross Section of Poplar or Aspen. A Diffuse Porous Wood. AR. annual ring; SP, 
 springwood; S, summer-wood; F, fibers; MR, medullary rays; V, pores or vessels. 
 
leaves the separated fibers somewhat thinner and weaker but with 
 an almost pure cellulose composition, similar to that of cotton fibers. 
 The less lignin present the easier it is to separate the fibers by cook- 
 ing. The yield of pulp secured is correspondingly high if the wood 
 has a high per cent of celluloses in its composition. The cellulose 
 content of wood is also important in the production of ethyl or 
 grain alcohol. Spruce is an example of a wood much used for this 
 purpose. The celluloses present in the sawdust from such a wood 
 are converted into sugar by treatment with acids. The solution ob- 
 tained is then neutralized and the sugar is fermented with yeast, 
 just as in the case of the preparation of ethyl alcohol from corn or 
 other grains. It has been found that woods with a high lignin con- 
 tent give an especially good yield of wood or methyl alcohol in the 
 process of destructive distillation. Such material as tops, slabs and 
 edgings are used for this purpose. The products obtained from wood 
 in this way are methyl alcohol (methanol), acetic acid, and char- 
 coal. The species which have thus far been most successfully used 
 for the manufacture of these products are beech, birch, and maple. 
 
 Physical Properties. The structure and mechanical properties, 
 such as strength, hardness and toughness, determine the suitability 
 of wood for particular uses. The density or weight of the wood 
 bears a direct relation to its strength. This depends upon the kind 
 of elements present and the way in which they are arranged. 
 Poplar (Plate II) and willow have many large open pores and rela- 
 tively thin walled cells. Consequently, they are weak as compared 
 with oak (Plate III) or black locust which have fewer large pores 
 and many thick walled fibers ("f" Plate III and IV). It has been 
 found that the density of wood substance in different species of trees 
 may, for practical purposes, be considered as uniform with a value 
 of 1.54*, yet the wood substance is so arranged in the different 
 species that the unoccupied space in a block may often be from two- 
 fifths to four-fifths of its volume. The arrangement of the wood 
 elements under these circumstances may be compared with that of 
 the cells in a honeycomb. 
 
 The openness of the pores and the uniformity of the structure 
 are of great assistance in drying a wood. For this reason white 
 pine, spruce and birch are easier to dry than oak. Water is con- 
 tained in wood in two ways, as free water in the cell cavities and 
 as more or less closely combined water within the cell walls them- 
 selves. It is a relatively easy matter to dry out the water present 
 in the cell cavities but as soon as an attempt is made to remove the 
 water within the cell walls, shrinkage or warping begins to take 
 place. 
 
 Tradfl Prejudices. Trade prejudices which are maintained 
 with surprising persistence sometimes hinder and sometimes appar- 
 ently assist the utilization of a wood. 
 
 It has been found that many of the soft woods, such as tam- 
 arack, hemlock, balsam, and jack pine, may be used for making 
 ground wood pulp for newspaper and fiber board stock. Some of 
 these woods do not produce a news sheet that is as white as that 
 
 *See Journal of Agricultural Research, September 21, 1914, Vol. II, No. (>. Tin- 
 Density of Wood Substance and the Porosity of Wood by Frederick Dunhip. 
 
te III. Cross Section of Red Oak. A Ring Porous Wood. AR, annual ring; SP, springwood (la 
 open pores); S, summerwood; F, fibers; MR, wide medullary ray (silver grain); 
 
 V, pores or vessels 
 
obtained from spruce. The slight degree of difference in color 
 would not, however, interfere with the legibility of the printed 
 words and the average reader would probably not notic'e the diff- 
 erence, yet the prejudice remains. 
 
 It is a well known fact that the sale of small cream cheeses is 
 better when they are exhibited in white basswood boxes than when 
 in pasteboard or other containers. 
 
 Hickory is one of the most important American woods. It is 
 remarkable because it possesses to such a degree the combined 
 properties of hardness, weight, stiffness, and toughness. The 
 hickory wagon spokes and rims have made possible the American 
 type of spring wagon with its superior lightness and strength. 65 
 per cent of the hickory cut is made into vehicle stock and handles. 
 Approximately one-half of this is used for spokes for wagon and 
 automobile wheels. The hickory handle has helped to make the 
 American axe known all over the world. It has been found in the 
 intensive studies of this wood made by the Forest Service that the 
 red heartwood of hickory, which was once largely wasted, is fully as 
 valuable for handles, etc., as the white hickory. The trade preju- 
 dice in favor of the white hickory is being gradually overcome. 
 This means a closer utilization of this valuable wood. A significant 
 step in this line is the adoption by the United States Navy Depart- 
 ment of a hickory handle specification which admits red hickory on 
 the same terms as white. A structural feature which influences I bo 
 strength of hickory to a marked degree is the width of the annual 
 rings. Slow growth material of this species is frequently brash and 
 weak, due to the fact that when a ring is narrow the porous struc- 
 ture is developed at the expense of the fiberous portion of the ring 
 which gives the strength to the wood. 
 
 Gmin find Te.rture. The terms ''grain" and "texture" are 
 often confused and misunderstood in present usage. The distinc- 
 tive application of each has been well given as follows* : 
 
 "Grain is a general term used in reference to the arrange- 
 ment and direction of the wood elements and to the relative 
 width of the growth rings. To have specific meaning it is es- 
 sential that it be qualified. Coarse grain applies to woods of 
 rapid growth, that is, it denotes wide rings; fine grain to woods 
 of slow growth. Straight grain as applied to a tree occurs when 
 the wood elements are parallel to the axis of growth." (The 
 pine in Plate I has a finer grain than the oak in Plate III be- 
 cause it shows three complete annual rings to the one shown in 
 the oak.) 
 
 "Texture is a term which refers to the relative size, quality, 
 or fineness of the elements. Like grain it requires qualifying 
 adjectives to attain specific meaning. The most common attri- 
 butes of texture are fineness and coarseness; evenness and un- 
 cvenness. Coarse texture applies to woods with many large ele- 
 ments (See Plate III Oak) or the average size of which is large. 
 For example, chestnut and sequoia. In fine texture the oppo- 
 site condition prevails, as in red cedar and poplar. (Plate II.) 
 Even texture or uniform texture are terms used to describe 
 woods whose elements exhibit little variation in size, for ex- 
 
 10 
 
Plate IV. Radial Section (Quarter Sawed) White Oak. A Ring Porous Wood. T, tyloses 
 (completely closing pores or vessels); AR, annual ring; Sp, springwood; S, summer- 
 wood; F, fibers; MR, medullary ray; V, pores or vessels. 
 
 11 
 
ample, cedar or poplar. (Plate II.) Uneven texture applies to 
 the opposite condition such as is common in all prominently 
 ring porous woods (for example oak (Plate III), elm and ash), 
 and in other woods with decided differences between early and 
 late wood (for example long-leaf pine (Plate I) and Douglas 
 fir). The distinctions as above expressed will obviate the diffi- 
 culty resulting from the attempt to make the term "grain" too 
 comprehensive." 
 
 CONCLUSION. 
 
 Approximately 52,800,000,000 board feet of wood are cut annual- 
 ly in the United States. This is enough to make a board walk two 
 inches thick and five feet wide which would reach approximately 
 forty times around the earth at the equator. The annual cut of 
 wood is estimated to be three times the amount of the annual 
 growth of the forests. A better knowledge of wood is necessary 
 to insure a closer and more advantageous utilization of the trees 
 cut. This must be brought about by the widespread co-operation of 
 all those interested in forest products whether land owners, lumber- 
 men, or users of the hundred and one commodities, from gunpowder 
 to silk neckties, which are prepared from wood. A general knowl- 
 edge of the structure of wood is of great importance. A thorough 
 knowledge of its minute structure is of fundamental significance in 
 the solution of the problems which are continually arising" as the 
 work of better utilization progre^se. The "open sesame" to this 
 knowledge is found in the use of tw r o commonplace instruments, 
 the "jack-knife" and the "reading glass" or small hand lens. 
 Highly perfected modifications of these which greatly assist the 
 more careful and minute study of wood structure, are the microtome, 
 which, by means of a very sharp razor, cuts sections of wood one- 
 five-thousandths of an inch in thickness, and the microscope by 
 which these sections may be seen greatly magnified in size. 
 
 It is like opening a closed book to read the history of the life 
 processes of the tree from these thin transparent sections where it is 
 possible to look through a single layer of the elements or cells which 
 compose the wood. It often seems that the book is as difficult to 
 understand as if it were written in a foreign language, but by dint 
 of study and experiment much of the meaning and the relationships 
 of the structural elements has been made clear and this information 
 is fast being made available for all. This stivlv revels not only the 
 wonderful perfection and the beantv of the once living tissue so 
 useful to mankind after the tree's life is over, but it. also makes ar>- 
 parent the underlying reasons for the behavior of the wood sub- 
 stance. On the results of snch scientific and thorough stndv it. is 
 possible to lay the foundation for the conservation or. in other 
 words, the efficient practical utilization of our forest resources. 
 
 *Se S. J. Ro-H's "Economic Woods of tin- United Stjitos." .1. Wiley # Sons. 
 Publishers. 
 
 12 
 
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