r6 



HARDWOOD RECORD 



Tree Growth and the Vhysics of Wood. 



Composition and Functions of Bark and 

 Cambium. 



A flissection and analysis of the hoAy of 

 an exogenous tree discloses intricate and 

 varied parts — eaeli performing a distinct 

 function, and eacli iiaviug an important 

 worlt to do toward the construction and 

 jierfection of the whole. 



First comes the outer covering or bark, 

 which cloaks the tree completely from tip 

 of smallest root to end of topmost twig. 

 The body being thus protected by an im- 

 ])orvious sheath, evaporation which would 

 endanger the life of the delicate layer be- 

 low the bark is checked. Bark is always of 

 great aid in identifying trees, and often 

 ))resents the most or even the only decisive 

 feature. At the base of the trunk the bark 

 is frequently quite different from that 

 which is found ou the upper portion. In 

 birch, for instance, it is usually smooth and 

 even above, but rough and coarse at the 



STKLCTUKE OF K.KOfilONOUS TRUNK. 



bottom. The terms wrinkled, corky and 

 fissured, as applied to it arc self-explana- 

 tory; but the small pores or breathing 

 spaces arc not so well known to the casual 

 observer, and to them the botanical term 

 lenticels is applied. Their function is to 

 admit air to the cambium layer. Birch also 

 presents a good illustration of this feature, 

 tlie lenticels being exceedingly conspicuous 

 and forming the centers of long stripes 

 which increase in length as the tree becomes 

 greater in diameter. These lenticels are not 

 distinct canals or holes, susceptible to clog- 

 ging with dust or foreign particles, but con- 

 sist of porous tissue that filters the air as it 

 is taken into the cambium layer. The 

 twigs of many trees shod their smooth outer 

 covering or epidermis as their heavier bark 

 liegins to form and gradually becomes thick 

 and fissured. Although this process does 

 not completely obliterate the lenticels, it 

 tends to make them obscure. Some trees, 

 notably cherry, retain their glossy, gossa- 

 merlike cloalv on large branches, and even 

 on their trunks patches of it may often be 

 seen, where the tiny air passages appear as 

 horizontal slits, parallel to one another, 

 sometimes open, but more often filled with 

 tlie characteristic, cork-like tissue. 



The outer bark is composed of soft cel- 



lular tissue. Early in the life of the tree 

 this covering forms, and it is in substance 

 the same as our ordinary cork. In its green 

 or lining layer the cells are close and deli- 

 cate and contain a green pigment similar to 

 that found in the leaves. The bast or liber 

 comes next to it, and is commonly known 

 as the inner bark. In it the cells are closer 

 and finer than in the wood proper; they are 

 long, slender and tough, appearing more 

 like fibers, and imparting strength to the 

 tissue. In the outer or true bark, as com- 

 monly understood, the green lining grows 

 very little after the first year; and the 

 corky layer does not long continue to in- 

 crease itself, but becomes dormant. Its 

 exposure to storm and damage from other 

 outside sources tends to deaden it, and the 

 more rapid growth beneath strains it and 

 causes it to spread and split, often forming 

 great rifts. Those fissures are patched by 

 new corky layers, which in turn are sub- 

 jected to the same strain. Thus the outer 

 and older portions are continually decaying 

 and falling away. 



The liber or inner bark, as well as ihy 

 wood, makes an annual growth, but toward 

 the inside rather than outward. These tw'i 

 parts of the stem are the only ones whiili 

 form a new yearly addition. Between th' 

 liber and the wood itself is the delicate 

 cambium la\'er, composed of young and 

 tender life cells, liillions in number, which 

 forms a continuous undergarment or se])M 

 rating membrane, clothing the entire tree, 

 between the wood and bark of every twig 

 and root, as well as of the trunk and larger 

 branches. 



When spring comes the rich mucilaginous 

 sap circulates freely and supplies the cam- 

 bium with renewed vigor so that its cells 

 begin to increase, the outer ones attaching 

 themselves to and partaking of the nature 

 of the liber or under bark, while the inner 

 become identified with the wood rings. 

 Thus it is that year by year the life of the 

 stem is renewed. 



The idea that winter weather drives the 

 sap entirely out of a tree is not now in 

 accord with the theories held by the best 

 authorities. Undoubtedly there is a specific 

 period (winter) for rest, and during that 

 time trees do not grow, in either the south- 

 ern or northern states. All during this 

 stated time there is considerably less 

 moisture in the living wood and bark — due 

 to alterations in the nature of the cell con- 

 tents at the termination of the season's 

 growth — than at any other period of the 

 year. Although no comparative deductions 

 have been made of the contents in moisture 

 of northern and southern woods of the same 

 variety, there is nothing to bear out the 

 contention that there is a great difference 

 between the amount contained in a tree 

 cut in the far south during winter, and a 

 tree of comparable variety, age and density 



which has been felled in winter iu the ex- 

 treme north. 



The nourishment flowing freely through- 

 out the cambium cells bears starch an.l 

 oxygen. Thus the cell is fed and given the 

 power to divide and subdivide, •■which 

 process goes on at a rapid rate during the 

 growing season. In the course of this life 

 process the cells are not formed in hit-or- 

 miss fashion, but arise iu ' ' rhythmic succes- 

 sion" or in uniform groups. Often this 

 regularity cannot be traced because of their 

 abundance. A single cell divides into two 

 smaller ones, which soon become as large 

 and as prolific as the parent cell, being re- 

 stricted only by a diminishing suppty of 

 food I'.nd oxygen. 



After a season's growth, the cambium 

 layer proper still remains a membrane of 

 the last-formed, most fertile cells, while its 

 margins are a mass of cells whose walls 

 have become dense, and whose protoplasm 

 is dormant, belonging on the one side to the 



B.\ItK 01'- EIKCII. SIIDWINC I.K.NTICELS. 



bast or liber, and ou the other to the new 

 wood tissue, as explained above. 



Thus a cell which once belonged in the* 

 heart of the cambium layer, and contained 

 the active life principle, becomes a hollow 

 wood fiber — a part of the sapwood — through 

 which the secretion ascends to the twigs 

 and leaves. Graduallj- starchy and mineral 

 deposits form within it, it becomes compact 

 and no longer permits the sap to flow freely 

 through it, but forces it to seek other and 

 newer channels. Finally it grows hard and 

 dark colored; many layers of annual growth 

 interpose between it and the new forma- 

 tions, until at last it finds itself a part of 

 the core or heartwood of the tree; while its 

 fellow pores, originating at the same time 

 and from the same source, have many of 

 them been crowded in the opposite direc- 

 tion, and have long since lost their vitality 

 and moisture by contact with the outer air, 

 or have disappeared entirely from the sur- 

 face of the tree. 



One of the illustrations accompanyiug this 

 article shows the structure of a trunk, and 

 from right to left displays the epidermis, 

 corky layer, green lining, vessels, hard bast 

 cells, soft bast cells, sieve tubes, cambium 

 layer, dotted ducts, wood, medullary sheatli 

 and central pith. 



