AERIAL STEMS 149 



faces and remember them. Similarly we might notice 

 trees and remember them. 



Hollow stems have an advantage over soHd stems in 

 that they attain erectness and height at much less expense ; 

 that is to say, the amount of material which the plant 

 must manufacture in building them is much less than if 

 they were solid. To be sure, they are not always erect. 

 They sway in the wind. Perhaps you have noticed the 

 beauty of a field of grain or tall grass bowing as the breezes 

 blow over it. But though they bend, the hollow stems of 

 grass and grain do not break easily; they bend without 

 breaking, and a storm may do more damage to the high 

 stiff stems of a forest than to the swaying stems of a field 

 of wheat. Hollow stems, however, generally endure for 

 but one season and never attain the great heights of solid 

 stems. 



B. Prostrate Stems. — If to secure light for leaves is the 

 main work of stems, why should any stems which bear 

 green leaves be prostrate? Surely this is a position un- 

 favorable for leaf display; it seems about the most un- 

 favorable position possible. Yet there must be some 

 advantage in it, since prostrate stems are very common. 

 They lie sprawHng on the ground and run the risk of 

 being shaded by even the lowest of plants with erect stems. 



An answer to our question may be found in the expres- 

 sion, " Many kinds of places, many kinds of plants." In 

 other words, the principal reason why we find so many 

 kinds of plant forms appears to be that there are so many 

 kinds of places in which plants grow. A plant form which 

 is sure to fail in one place may be just the thing to succeed 

 in another. 



