38 



THE ECOLOGY OF STEMS 



with numerous large branches well down toward the base while an 

 individual of the same species growing in a dense forest has a long 

 trunk with no branches on the lower half (Fig. 13). The same state- 

 ment is true of most species of trees and a similar statement would be 

 true of many herbaceous plants. 



The two factors that seem most likely as causes of these stem varia- 

 tions are light and moisture. Light has usually been regarded as 

 having a retarding effect upon growth, and especially upon elonga- 

 tion, while absence of light favors elongation. Recent work has 



Fig. 13.— Sugar maple forest showing long trunks with the first branches far 



from the base. 



cast considerable doubt upon this general statement concerning 

 light and in any case it is certain that light has often been over- 

 emphasized as a factor affecting the elongation of stems. 



Moisture is much more important in this respect. Nearly all seed 

 plants absorb water through their roots and lose water by transpira- 

 tion from their leaves and stems. In general there must be a balance 

 between these two processes and the height to which any plant can 

 grow is limited by the height to which it can transport water rapidly 

 enough to counterbalance the loss by transpiration. This fact is 

 strikingly shown by certain water plants with floating leaves in 



