FUNCTIONS 219 



56. Functions. It will be easier to understand the 

 structure of leaves if we consider their functions first; 

 it would be easier, also, to understand their functions if 

 we were to consider their structure first. Something is to 

 be said in favor of either way. You have already read 

 something about the structure of leaves, however, and it 

 may be better to consider some of the processes with 

 which leaves are concerned before going on to consider in 

 more detail the structures concerned with these processes. 



A. Transpiration. Transpiration is the evaporation 

 of water from living plants. Strictly speaking, it is not 

 a function at all. A function is something which an organ 

 does, and transpiration, strictly speaking, is not a thing 

 which leaves do. It is rather a thing which is done to them. 

 If transpiration is a function of leaves, then we may speak 

 of the evaporation of perspiration as a function of our 

 skins. Transpiration from leaves, like evaporation from 

 our skins, is a thing which happens to them rather than a 

 thing which they cause to happen ; with reference to evapo- 

 ration the leaves (and our skin) are passive rather than 

 active. Transpiration is a process of great importance in 

 plant life, however, and, since it occurs principally in 

 leaves, it is customary to consider it among their functions. 



We have considered the large quantities of water absorbed 

 by roots (see page 47) and have noted that the continuous 

 entrance of this water is permitted by continuous trans- 

 piration (see page 108). We must now consider some 

 of the relations of this phenomenon to plant life in general. 

 We must bear in mind that the transpiration stream passes 

 out into all the veins of the leaves, and from these into the 

 mesophyll. Here the water is contained in cells whose 



