COOLING EFFECT OF TRANSPIRATION 213 



Just those plants which are most exposed to extreme 

 insolation get the least good of the cooling effect of tran- 

 spiration. We must, however, look at the economy of the 

 plant as a whole, and, in any case, this is evidently not the 

 only means by which plants meet high temperatures. The 

 cactuses do survive temperatures which would be fatal to 

 other plants. Stahl (1909) has shown this experimentally, 

 and we are bound to suppose that their protoplasm is capable 

 of accommodating itself to a greater range. There is nothing 

 extraordinary, though much that is not explained, in this. 

 The same is very probably true for many other xerophytic 

 types, though we must remember that a plant which reduces 

 its total transpiration by reducing the size of its leaves also 

 reduces the heat and light absorbing area, and may at the 

 same time transpire as vigorously, area for area, as a meso- 

 phyte. In fact, it is very likely that the lowering of temper- 

 ature by transpiration is of greatest importance for the freely 

 transpiring mesophyte. 



The importance of this second role of transpiration we 

 may take as established much more firmly than the role in 

 salt supply. We should always remember, however, that, 

 whether transpiration has useful functions or not, it must 

 occur in advanced types of land vegetation. 



§ 35. Inter-relation of Transpiration and 

 Assimilation 



We have insisted at the beginning of the chapter on the 

 close relation between assimilation and transpiration ; and 

 throughout our discussion it will have become increasingly 

 evident that, in the first place, the opportunity for tran- 

 spiration is enhanced by the necessities of the photosynthetic 

 processes, and, in the second place, that assimilation must 

 often be limited when transpiration is in any way reduced. 

 The necessity of reducing water loss must frequently clash 

 with the work of building up organic substances. It might 

 be expected that some general relation should exist between 



