THE INlrtlE, ORGANIZATION OP TREES. 41 



leaves. The fluid contents of these cells are, therefore, as 

 manifold as that of the parenchyma cells. The bast-cells 

 of one plant contain quite a different fluid matter to that-.of 

 another plant. In one, the fluid is poisonous; in another, 

 nutritive : here it is a white, yellow, or orange-colored 

 milk sap : there, caoutchouc, chlorophyl, or resinous mat- 

 ters are present. 



Although the bast-cells form the innermost layer of bark, 

 in no case do they rest immediately on the wood. They 

 are always separated by a bed of cells more or less thick, 

 called the cambium layer. 



THE WOOD. 



The wood includes nearly the whole of that part of the 

 stem situated beneath the bark. It consists of a number 

 of ligneous circles, visible on the cross-section, which are 

 traversed by lines radiating from the centre to the circum- 

 ference of the stem, that is to say, from the medullary 

 canal to the bark, called the medullary rays. 



This disposition of the wood in circular beds or layers, 

 takes place in all the trees of countries where the season of 

 growth has only a limited duration, and is followed by a 

 period of cold and vegetable inactivity. Each year there is 

 formed in spring a new bed of wood, and at the same time 

 a fresh layer of bark. The age of a tree is, therefore, in 

 most cases, the same as the number of ligneous circles 

 which can be counted on the cross-section of its stem. In 

 the same manner the age of the branches may be computed. 



This fact must be mentioned with some restrictions ; for, 

 Jn certain circumstances, as for instance, when a warm 

 spring is succeeded by a wet, cold summer, vegetation re- 

 ceives a check, and a ring is formed prematurely; with the 

 return of more favorable weather, the tree again makes a 

 rapid growth, so that at the close of the vegetative season 

 two rings have been formed during the same year. The 

 rings become less distinct in Exogenous trees as we travel 

 South. On approaching warmer climates, where vegetation 

 continues almost without interruption throughout the whole 



