INTERNAL STRUCTURE. 79 



between it and the bark ; and this is continuous with the wood}- 

 layer of the new roots below and of the leafy shoots of the sea- 

 son above. Each succeeding year another layer is added to the 

 wood in the same manner, coincident with the growth in length 

 by the development of the buds. A cross-section of an exoge- 

 nous stem, therefore, exhibits the wood disposed in concentric 

 rings between the bark and the pith ; the oldest lying next the 

 latter, and the }-oungest occupying the circumference. Each 

 layer being the product of a single year's growth, the age of an 

 exogenous tree may, in general, be correctly ascertained by 

 counting the rings in a cross-section of the trunk. 1 



144. Demarcation of the Annual Layers results from two or more 

 causes, separate or combined. In oak and chestnut wood, and 

 the like, the layers are strongly defined by reason of the accumu- 

 lation of the large dotted ducts (here of extreme size and in 

 great abundance) in the inner portion of each layer, where their 

 open mouths on the cross-section are conspicuous to the naked 

 eye, making a strong contrast between the inner porous and the 

 exterior solid part of the successive layers. In maple and beech 

 \vood, however, the ducts are smaller, and are dispersed through- 

 out the whole breadth of the layer; and in coniferous wood, viz. 

 that of Pine, Cypress, &c., there are no ducts at all, but only a 

 uniform woody tissue of a peculiar sort. In all these, the de- 

 marcation between two layers is owing to the greater fineness of 

 the wood-cells formed at the close of the season, viz. those at 

 the outer border of the layer, while the next layer begins, in its 



1 The annual layers are most distinct in trees of temperate climates like 

 ours, where there is a prolonged period of total repose, from the winter's 

 cold, followed by a vigorous resumption of vegetation in spring. In tropical 

 trees, they are rarely so well defined ; but even in these there is generally a 

 more or less marked annual suspension of vegetation, occurring, however, 

 in the dry and hotter, rather than in the cooler season. There are numerous 

 cases, moreover, in which the wood forms a uniform stratum, whatever be 

 the age of the trunk, as in the arborescent species of Cactus ; or where the 

 layers are few and by no means corresponding with the age of the trunk, as 

 in the Cycas. 



In many woody climbing or twining stems, such as those of Clematis, 

 Aristolochia Sipho, and Menispermum Canadense, the annual layers are 

 rather obscurely marked, while the medullary rays are unusually broad ; 

 and the wood, therefore, forms a series of separable wedges disposed in a 

 circle around the pith. In the stem of Bignonia capreolata, the annual rings, 

 after the first four or five, are interrupted in four places, and here as many 

 broad plates of cellular tissue, belonging properly to the bark, are inter- 

 posed, passing at right angles to each other from the circumference towards 

 the centre, so that the transverse section of the wood nearly resembles a 

 Maltese cross. But these are exceptional cases, which scarcely require 

 notice in a general view. 



