Structure of Wood. 



245. In many kinds of the " hard-woods," and especially in the 

 oak, ash, elm, etc., each annual layer is more spongy and porous on 

 the inner side, and harder on the outer side. The former is some- 

 times called the spring and the latter the autumnal growth. This 

 autumnal growth is, however, formed in the summer, and its dens- 

 ity and relative thickness appears to depend upon the character of 

 the season that follows after it has been deposited. If it remains 

 humid and cold, it will be less dense than if it be dry and warm. 

 The amount of growth for the year is usually determined by the 

 weather in the spring and early summer. 



246. In exceptional cases, such as an early and protracted drouth, 

 followed by heavy rains and a warm autumn, a second growth may 

 start ; the buds may expand into leaves, and blossoms may appear. 

 In such cases, it is possible for a double ring of growth to form, but 

 it will not be entirely distinct in every part. Such an autumn, if 

 it is followed by a cold winter, is very apt to prove fatal to trees, or 

 at least to check their growth for a time, if it does not destroy them. 



247. In tropical woods, the ^ 



annual layers are obscure, and lljZ&ff^., 

 the age of a tree can not be 



k'&Sf^M^f^i:";'/-' 



' *> .-V^- ' v-*-. 



ascertained from them. 



248. In the soft-woods, there S : v4j : Sp 

 is scarcely any difference be- 



txvppn tliplavpr<5 inrl flip ;pnn- 54 - Section of Mahogany, showing indistinct 

 ia\ ers, ai Layers of Growth in a Tropical Wood. 



ration between the growth of 



different years is sometimes difficult to find. 



249. Deciduous trees, when stripped of their leaves, as sometimes 

 occurs from insect ravages, will put forth a new crop, from the buds 

 intended for the next season. The formation of wood is thereby 

 greatly checked, and the foliation of the next season weakened. 

 The chance of blossoms for the coming year may also be ruined. 



250. In cross-sections made years 

 afterwards, the record of the sea- 

 sous for a long period may be de- 

 termined, at least in effect, by the 

 width of the rings of annual 



growth. We sometimes find, at 55. Effect of Different -Seasons upon the 



Growth of A\ ood. 



recurring intervals, a narrow ring, 



perhaps in every third year, that may have been caused by the loss 



