EXOGENOUS STKUCTUKE. 123 



the two ai'e always organically connected by an extremely delicate 

 tissue of young and vitally active cells, just in the state in which 

 they multiply by division (33). The bai-k, indeed, is very readily 

 detached from the wood in spring, because the cambium-layer is then 

 gorged with sap ; but the separation is effected by the rending of 

 a delicate forming tissue. And if some of this apparant mucilage be 

 scraped off from the surface of the wood, and examined under a good 

 microscope, it will be seen to be a thin stratum of young wood-cells, 

 with the ends of medullary rays here and there, interspersed, and 

 appearing much as in Fig. 193, only the young wood-cells are mostly 

 shorter. The inner portion of the dambium-layer is therefore nas- 

 cent wood, and the outer is nascent bark. And it is by the growth 

 of the cambium-layer, renewed year after year, that the 



221. Annual Increase of the Wood of Exegenous plants is effected: 

 As the cells of this layer multiply, the greater number lengthen ver- 

 tically into prosenchyma or woody tissue ; while some are trans- 

 formed into ducts, and others, remaining as parenchyma, continue the 

 medullary rays or commence new ones. In this way a second layer 

 of wood is formed the second season, over the whole surface of the 

 former layer and between it and the bark, and continuous with the 

 woody layer of the new roots below and of the leafy shoots of the 

 season 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 exogenous stem, 

 therefore, exhibits the wood disposed in concentric rings between 

 the bark and the pith ; the oldest lying next the latter, and the 

 youngest occupying the circumference. Each layer being the pro- 

 duct 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.* 



* 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, fol- 

 lowed 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, oceun-ing, 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 tliosa of Clematis, Aristo- 



