252 ELEMENTARY STUDIES IN BOTANY 



When a stem increases in diameter by the annual increase 

 of xylem, the epidermis usually does not keep pace with it 

 and is sloughed off. In any event, a cambium appears in 

 the cortex and produces cork cells that form a most efficient 

 protective jacket and that give character to what is called 

 "bark." 



Subterranean stems can be distinguished from roots by 

 the presence of nodes bearing leaves, or at least rudiments 

 of leaves. The subterranean habit is often associated with 

 food storage, the significance of which is that it enables the 

 plant to develop its aerial parts with great rapidity, and 

 thus to make the most of a short season. This is one of 

 the chief reasons why rootstocks, tubers, and bulbs are used 

 in the propagation of cultivated plants, new plants being 

 obtained much more speedily than by means of seeds. 



