EXOGENOUS STRUCTURE. 123 



206. The limits of each year's growth in diameter in exogenous 

 wood are apparent in the cross-section in the form of concentric lay- 

 ers, from two causes, either separate or combined ; viz., the greater 

 abundance of ducts in the earlier part of each annual increment, 

 and the smaller size of the woody fibres in the latest growth of the 

 season, which is destitute of ducts, and forms a finer-grained bor- 

 der to the ring. This is well shown in the cross-section of bass- 

 wood, where the ducts compose the greater part of the wood at the 

 inner edge of each layer, and very gradually diminish in number 

 towards the outer edge, which is marked by a thin stratum of rni- 

 nute, laterally flattened wood-cells ; probably a portion of the 

 cambium-layer that took no further growth. This fine exterior 

 border alone marks the layers in white-pine wood, where there 

 are no ducts or other vessels interspersed, and in such wood as 

 that of the Sugar Maple, where the ducts are somewhat equably 

 distributed through the whole breadth of the layer. In oak and 

 chestnut wood, the layers are most strikingly marked, by the ac- 

 cumulation of all the large dotted ducts, here of extremely great 

 size and 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 exterior solid part of the successive layers. 



207. The annual layers are most distinct in trees of temperate 

 climates like ours, where there is a prolonged period of total re- 

 pose, from the winter's cold, followed by a vigorous resumption of 

 vegetation in spring. In tropical trees they are rarely so well de- 

 fined ; but even in these climes 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. 



208. In many woody climbing or twining stems, such as those 

 of Clematis, Aristolochia Sipho, and Menispermum Canadense, the 

 annual layers are obscurely, if at all, 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 one of our Trumpet-creepers (the Bignonia capreolata) 

 the annual rings, after the first four or five, are interrupted in four 



