THE STEM OF VASCULAR PLANTS. 167 



of layers, each of which presents a fibrous zone inter- 

 nally, and a cellular one externally ; and it is traversed 

 by medullary rays, similar to those of the woody body, 

 but less distinct. They do not differ from those of the 

 central system, and do not deserve to detain us here ; 

 but we will now study, separately, the cortical layers in 

 general, and the external cellular covering, which is 

 nothing but the cellular zone of the outer layer. 



§ 2. — Of the Cortical Layers. 



The bark is, as we have said, composed of layers 

 placed upon one another, as the woody ones, but in an 

 inverse direction. In the first year the stem is formed 

 of a woody zone and a cortical one, and each year it 

 forms one of each kind : the woody layer is applied 

 upon that which was formed the preceding year ; and the 

 cortical one under that which was developed before it. 

 Let us follow the consequences of this mode of develop- 

 ment. The newest, youngest, and most flexible cortical 

 layers, which represent in the bark the alburnum of the 

 wood, are found the most internal of the cortical body. 

 The name Liber has been given to them collectively ; 

 either because in several trees they detach themselves 

 from one another as the leaves of a book, or because, 

 formerly, this part of the bark of several trees was used 

 for making paper. 



The old cortical layers are thrown off towards the 

 outside, and they have retained the name of Cortical 

 Layers, properly so called. They represent in the bark 

 that which the wood is in the woody body ; but with 

 this great difference, — that the woody layers, placed over 

 one another in the order of their formation, remain 



