Structure of Wood. 69 



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 yery 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, 

 the age of a tree can not 

 ascertained from them. 



248. In the soft-woods, there 

 is scarcely any difference be- 

 tween the lavers, and tlie sepa-^- section of Mahogany, showing indistinct 



' r 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 «.-— -=s5sr^j»:Ks»'Ti:- - 



afterwards, the record of the sea- fp#^^^iiil^?l?| 



sous for a long period may be de- fS^^^^^^^^^^^3itS^ 



termined, at least in effect, by the ^^^^^^^$^S| 



width of the rings of annual ^.^^^^^^^^■^M 



erowth. We sometimes find, at SS. Effect of Different Seasons upon the 

 " . . . Growth of Wood. 



recurring intervals, a narrow ring, 



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



