146 



STEMS 



Probably the oldest classification of plants is the classi- 

 fication into herbs, shrubs, and trees. This classification is 



based upon quali- 

 ties which pertain 

 to the stems. 

 Herbs are smaller 

 and weaker than 

 shrubs and trees, 

 the wood or xylem 

 in them is not 

 largely developed, 

 and they die 

 down, at least to 

 the ground, when 

 winter comes. 

 Shrubs are inter- 

 in size 

 between herbs and 

 trees, and their woody stems are usually arranged in a 

 cluster of approximately equal shoots which arise at or near 

 the ground, thus forming what are called bushes. Trees 

 have tall, woody, perennial stems, and usually one main 

 stem, called the trunk or bole. 



This classification into herbs, shrubs, and trees is often 

 convenient in general descriptions of vegetation, but it is 

 not satisfactory for other purposes, for the reason that 

 there are so many gradations between trees and shrubs 

 and between shrubs and herbs. 



Fig. 49. — Cross section of a corn stem showing the way 



in which the vascular bundles are scattered through mediate 

 the pith. 



A . Erect Stems. — Stems are usually erect. Under the 

 ordinary conditions of plant growth an erect stem is the 

 best device for properly relating the leaves to light. Erect 



