246 ELEMENTARY STUDIES IN BOTANY 



The most common " modifications " of the stem are those 

 which arise when it is an underground structure, and thickened 

 .subterranean stems are not only common among plants, 



FIG. 213. Plants of passion-flower showing axillary (and therefore branch) tendrils. 



but also are often of great use to man. Since the stem is 

 primarily a leaf-bearing structure, it continues to bear 

 leaves when underground, but often these leaves are either 

 reduced in size so as to be mere rudiments, or are used in 



