VIII.] THE CORTEX AND BARK OF TREES, 205 



be easy to understand broadly what occurs as it 

 thickens with age. 



In the first place, it is clear that the continuous 

 sheet of cork {Co) must first be distended, and finally 

 ruptured, by the increasing pressure exerted from 

 within : it is true, this layer is very elastic and exten- 

 sible, and impervious to water or nearly so — in fact it 

 is a thin layer or skin, with properties like those of a 

 bottle cork— but even it must give way as the cylinder 

 goes on expanding, and it cracks and peels off. This 

 would expose the delicate tissues below, if it were not 

 for the fact that another layer of cork has by this time 

 begun to form below the one which is ruptured : a 

 cork-forming layer arises along the line ^, and busily 

 produces another sheet of this protective tissue in a 

 plane more or less parallel with the one which is 

 becoming cracked. This new cork-forming tissue 

 behaves as before : the outer cells become cork, the 

 inner ones add to the gi'een succulent parenchyma- 

 cells {pa). As years go on, and this layer in its turn 

 splits and peels, others are formed further inwards ; 

 and if it is remembered that a layer of cork is 

 particularly impervious to water and air, it is easy to 

 understand that each successive sheet of cork cuts off 

 all the tissues on its exterior from participation in the 

 life processes of the plant, and they therefore die : 



