STEM FORMS AND USES 147 



the least possible extent of surface in proportion to the 

 substance contained in it. 



210. Weapons of Defense. Examples of these may be 

 seen in the thorns of the 

 honey locust, the hawthorn, 

 and old field plums. An 

 examination of the haw, 

 crab tree, plum, and pear 

 will show stems in all stages 

 of transformation from 

 short, stubby branches to 

 well-defined thorns. This 

 kind of thorn must not be 

 confounded with briers or 

 prickles like those of the 

 rose and smilax, which are 



mere appendages of the epi- 291. Thorn branches of Holocantha 



dermis, while thorn branches Emor >' i ' a P lant e rowin s in arid re s ions - 

 have their origin in the wood beneath. They usually come 

 from adventitious buds. 



211. Stems as Tendrils. Stems are also frequently 



met with under the 

 form of tendrils. As 

 normal buds and 

 branches never grow 

 except from the axils 

 of leaves, this kind of 

 tendril can always be 

 recognized by its posi- 

 tion. In the grape and 

 Virginia creeper, where 



the leaves on alternate 



8ideS f the Stem ' 



they represent terminal 

 flower buds which have been pushed aside by stronger 



