OF THE CELLULAR INTEGUMENT. 37 



" that reproductive power. The old layers remain, are 

 " pushed outward by the new ones, and form at length 

 " the rugged dry dead covering of the old trunks of 

 " trees." 



When we come to consider the curious functions of 

 leaves, we shall find this part to be of the very first im- 

 portance. In it the principal changes operated upon 

 the juices of plants by light and air, and the consequent 

 elaboration of all their peculiar secretions, take place. 



