LIFE-HISTORY OF A TREE 71 



If light cannot be obtained for the lower limbs, the stems 

 climb upward to secure it and tall trees free of limbs near 

 the ground are the result. As there is a continual struggle 

 for light and food, it is the province of forestry so to reg- 

 ulate conditions that the best results will be obtained with- 

 out unnecessary expenditure of effort or exhaustion of 

 vitality of the tree, and at the same time to encourage the 

 struggle sufficiently to secure the desired end. In other 

 words, to so arrange for light, mineral food, and moisture 

 that the surviving trees will not be compelled to wage a 

 greater warfare in suppressing weaker ones than is abso- 

 lutely necessary to produce the requisite character of lum- 

 ber. This can be done by proper planting and thinning. 



Growth of Wood and Bark. As the substance required 

 for wood goes back from the leaves there goes with it that 

 which makes the bark, and while a layer of wood is de- 

 posited on the outside of the stem, branches, and roots 

 each year, we are considering only those trees which thus 

 make their growth, the exogens, for they are the only real 

 timber trees, there is likewise a thin layer of bark de- 

 posited, which, however, is separated from the wood by what 

 is botanically known as the Cambium Layer, a viscid secre- 

 tion that intervenes between the last formed layers of wood 

 and bark. This layer not only separates the wood and bark, 

 but at the end of the season's growth serves as a store- 

 house for food on which the buds and roots draw in the 

 beginning of the next year's growth, or until the leaves and 

 roots are developed enough to themselves obtain food from 

 the atmosphere and soil. The wood deposited is known, 

 when mature, as the Annual Rings. These are very dis- 

 tinct in some species, but quite obscure in others, and in 

 some tropical species not discernible in either wood or bark. 

 In most of our timber trees the wood first deposited is por- 

 ous filled with ducts and cells for the flow of sap. These 

 are conspicuous in some species, as in Ash, Oak, Chest- 

 nut, Elm, and some others. This cellular, first-deposited 

 accretion is denominated the Spring "Wood. As the sea- 



