THE LEAF AND ITS FUNCTIONS 77 



at maturity, thoro have boon transpired through its leaves 400 

 grams of water. Species vary markedly in their water-require- 

 ment, and so do plants of the same species when grown under 

 different conditions. 



The Significance of Transpiration. — Excessive water-loss is 

 an ever-present danger to land plants, and many structural modi- 

 fications have been developed by various species, or may appear 

 in particular individuals growing under dry conditions, which 

 tend to reduce this loss. The question therefore arises as to 

 whether transpiration is an unmixed evil, made necessary by the 

 fact that the stomata must be open to allow the exchange of 

 gases which take place in photosynthesis; or whether it is really a 

 function of the leaf and performs a useful part in the plant's 

 economy. It was long thought that water must be taken in 

 through the roots in large quantities to insure an abundant 

 absorption of nutrient materials from the soil, but a fuller under- 

 standing of the phenomena of osmosis and root absorption shows 

 the fallacy of this conclusion. It has also been contended that 

 transpiration is useful in concentrating the very dilute solutions 

 of nutrient salts taken from the soil — "boihng them down," 

 so to speak. We have seen, however, that the factors which 

 determine the entrance of nutrient materials into the plant 

 preclude such an explanation; and, indeed, experiment shows 

 that the amount of salts absorbed is practically independent of the 

 amount of water transpired. Transpiration from the leaves, 

 however, is evidently what causes the transpiration stream, 

 or continuous movement of water from root to leaf through the 

 lifeless ducts in the wood of the stem. We shall consider this 

 movement more fully when discussing the functions of the stem ; 

 but it is well to note here that by this stream the dissolved 

 substances are transported bodily from the central cylinder of 

 the root upward throughout the plant as far as the ramifications 

 of the dead conducting elements of the wood extend. This 

 movement is probably far more rapid than would result through 

 diffusion from cell to cell, and in tall plants, particularly, the 

 transpiration stream probably performs a distinct service in 

 distributing rapidly throughout the plant the supply of nutrient 

 materials absorbed from the soil. 



Transpiration is also of distinct usefulness in regulating the 

 temperature of the leaf. The blade absorbs much more energy 

 than it uses in photosynthesis, particularly in bright light; and 



