THE INNER ORGANIZATION OP TREES. 41 



and often attain to a considerable length. It is in the bast 

 fibres that the sap descends after its elaboration in the 

 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 matters are 

 present. (See Plate I.) 



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 tra- 

 versed 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, in 

 certain circumstances, as for instance, when a warm spring 

 is succeeded by a wet, cold summer, vegetation receives 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 approach- 

 6 



