OF THE CELLULAR IXTEGUMENT. 19 



*' substance, covered on each side by the cuticle. The 

 " stems and branches of both annual and perennial 

 *' plants are invested with it ; but in woody parts it is 

 ** dried up and reproduced continually, such parts only 

 ** having that reproductive power. The old layers 

 " remain, are pushed outward by the new ones, and 

 " form at length the ruii^ed dry dead coverins; 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 

 importance. In it the principal changes operated upon 

 the juices of plants by light and air, and the conse- 

 quent elaboration of all their peculiar secretions, take 

 place. 



c ^ 



