r6 



HARDWOOD RECORD 



Tree Growth and the Vhysics of Wood. 



Composition and Structure of Wood. 



Aiiropos oi' the study of wood pliysies, Hei- 

 hert Stone, the eminent English authority, 

 says : 



"It would be a difficult task to answer the 

 question why some plants possess the prop- 

 erty of forming wood, while others, nearly 

 related, do not. Why some plants run their 

 course in the brief period of a year, and per- 

 ish as soon as they have given birth to another 

 generation, while others persist and augment 

 their bulk year by year for centuries. There 

 is much food for reflection here. 



' ' Such ])lants as are endowed with the fac- 

 ulty of secreting a substance which resists 

 decomposition for a long time, and of forti- 

 fying their tissues with it, play a very dif- 

 ferent part in the world 's economy to that of 

 tlieir herbaceous relatives, which to-day are, 

 and to-morrow cast into the oven. They exist 

 long enough to acquire an individual history. 

 This history may not be written in human 

 records, but it has a record of another kind, 

 which may be read in the structure of the 

 tree itself.' ' 



Wood is maile viji largely of hydrogen, oxy- 

 gen and oarlion. When cjuite dry, about half 

 its weight is carbon — the other half oxygen 

 and hydrogen in about the same proportion 

 as they are found in water. It contains also 

 about one jiart in a hundred, by weight, of 

 earthy constituents, and nitrogen in the same 

 proportion. Trees draw their nourishment 

 from two sources — the air and the eai'th. A 

 stick thoroughly seasoned has lost the water 

 it contained. If it is burned, only ashes re- 

 main. These two elements, the water and the 

 ashes, were derived from the soil originally. 

 Those parts which were obtained from the 

 air return, during the burning process, to 

 their gaseous form, while most of those which 

 came from the earth assume once more their 

 tangible existence, although some are con- 

 verted by the heat into gases and also escape 

 into the air. 



If the dry stick be slowly smothered instead 

 of burned in the open, it will emerge from 

 the process as a piece of charcoal, with shape 

 and structure intact. It has become carbon, 

 with but one impurity — a trace of ashes. 

 The smothering to which it has been subject- 

 ed has caused what would otherwise have been 

 liberated as carbonic-acid gas to remain in 

 solid form, although flames have the power to 

 free it even in this form. 



Possibly the most familiar feature noted in 

 wood structure is the annual ring with which 

 the tree surrounds itself during each year of 

 its growth. This periodical addition is a 

 conical sheath which encircles the entire 

 plant, partaking of the nature of tue ring- 

 preceding it, l)ut distinct from it. Thus it is 

 that while a tree may be described as very 

 ancient, the statement of its great age is true 

 only of its innermost stem, the outer ano 

 upper parts being but recently acquired. With 

 the advent of in'w li\ing' portions year by 



year, certain inner layers lose their vitality, 

 jiartially or entirely, and become part of the 

 lieartwood of the tree. 



The Autumn and Spring zones, as exhibited 

 by the annual ring, are not always distinct, 

 although in trees which grow in temperate 

 climates they are often exceedingly clear, 

 owing to the difference in rapidity and vigor 

 of growth between wood formed in the early- 

 spring and that produced later in the sea- 

 son. Although the term ' ' annual rings ' ' has 

 sometimes been called into question, it is un- 

 doubtedly correct, for so long as a tree is 

 healthy and thrifty a ring is formed each 

 vear. It is a fact that two false rings may 

 appear during one growing season. But they 

 are generally so much thinner than the rings 

 on each side that they may be easily detect- 



(■I!()S8 SECTION OF CHESTNUT MAGNIFIED 

 TEN DIAMETERS. 



ed. Sometimes they do not entirely encircle 

 the tree, as does the true ring, provided the 

 tree is in good condition. When growth is 

 interrupted for a time, as is the case when 

 the foliage is destroyed by worms or when 

 the tree is subjected to a severe and pro- 

 longed ilrought — and then commences again 

 during the same season — a false ring is occa- 

 sionally formed. Insects inflict untold dam- 

 age upon forests every year, eating the sub- 

 stance of the leaves and buds. Caterpillars, 

 moths and beetles are among the most harm- 

 ful, as are also the borers which infest solid 

 wood, honeycombing it, and ruining it for 

 timber, or prying through the bark and sap- 

 I)ing the life of the cambium, which is so 

 vital to the tree's healthy existence. 



After an annual ring has been covered by 

 other younger layers, it becomes gradually 

 darker and harder. Its cell openings become 



clogged in such a manner that sap can no 

 longer circulate freely through them. From 

 the live wood, representing the active prin^ 

 ciples of the tree, it becomes the so-called 

 ' ' heartwood, ' ' which is dead, and therefore 

 not essential to the life of the organism, al- 

 though it forms part of the strong frame- 

 work which supports the living parts and 

 gives strength and substance to the tree. 

 Thus it is that hollow trees, if not subjected 

 to strain from weight or gale, may flourish 

 and even. bear perfect fruit. 



The living portion of the structure, ' ' sap- 

 wood, ' ' decays more easily than does heart- 

 wood, because it absorbs water freely and 

 therefore contains elements of food which are 

 prone to decompose. Because the sapwood is 

 the outer portion and the vehicle through 

 which nourishment passes, the life of a tree 

 is easily destroyed by girdling. Not all trees, 

 however, form distinct heartwood, and in such 

 varieties girdling is often survived for several 

 years. 



Every part of a piece of w'ood consisted 

 originally of cells, or tiny, closed cavities. 

 They are of various forms — some round, some 

 several-sided, others square or spindle-shaped, 

 aiul all bear some special relation to the life 

 nf the tree. Some of them conduct water 

 from root to crown, others merely strengthen 

 the structure. The wood of coniferous trees 

 — the pines and spruces — has but few varie- 

 ties of these cells, while the broadleaf trees, 

 or hardwoods, show a much more complicated 

 structure. With both, however, some of the 

 cells have thick walls and others thin. In 

 temperate climates, where a season of growth 

 is followed by one of rest, the cells of the 

 layer of new wood formed annually at the 

 inner surface of the cambium, are arranged 

 with great regularity. In the Spring a tree 

 makes thin-walled cells, through which the 

 abundant supply of water can rise rapiuly to 

 the new twigs and leaves. Later, when the 

 food store has become abundant, and the 

 demand for water is consequently less, the 

 cells formed are narrow and thick-walled. 

 Thus the W'Ood grown in summer is heavier, 

 stronger and darker than the spring wood 

 of many trees, notably the chestnut. 



The chief feature of the wood of conifers 

 and hardwoods is the system of rays, which 

 are always jjresent, and which are exceedingly 

 important in the classification of different 

 species of wood. These rays bind the wood 

 fibres together and are an important factor 

 in the production of ' ' figure. ' ' Most trees 

 exhibit but one kind of ray, although in the 

 oak two kinds are evident, large and small. 

 The large rays are composed of irregularly 

 arranged small cells, while the small ones 

 are made up of larger cells. As the circum- 

 ference of the annual rings becomes greater, 

 the original rays become wider apart, and 

 new ones arise frequently enough to preserve 

 the proportion of rays to mass of wood. 

 These new ones have nothing to do with the 



