102 Plant Physiology 



61. Transpiration and growth. - - It has long been evi- 

 dent that there is, under certain circumstances, a relation 

 or fairly definite ratio between transpiration and growth. 

 As a result of various series of water cultures with wheat 

 and other grasses, Livingston has attempted a further 

 analysis of this relationship. He finds that the transpira- 

 tion data are frequently as instructive as a comparison of 

 total increase in weight or growth. It is observed, then, 

 that transpiration and relative growth vary with weight 

 and area of the leaves. The amount of transpiration is 

 regarded as a simple function of the leaf surface, which 

 again varies directly with leaf weight, or, practically speak- 

 ing, with the weight of the entire tops. It follows, of 

 course, that total transpiration is a more or less accurate 

 measure of the total growth. 



This relationship, however, is limited by several factors. 

 It is necessary to have conditions favorable for fairly rapid 

 transpiration and favorable for growth. Again, increasing 

 the salt content of the solution in which plants are grown 

 measurably affects transpiration and may not increase 

 growth materially, so that plants growing in diverse 

 concentrations may show extreme variations with respect 

 to the amount of water-loss. Reed has also recently 

 demonstrated that potassium in any combination exerts 

 a depressing effect upon transpiration, while a small 

 quantity of tannic acid facilitates it. In other words, the 

 relation applies to a relatively narrow set of conditions. 



62. Water transport. - This is a convenient but 

 scarcely an accurate expression, since, except in the dif- 

 fusion of water-vapor and in the formation of ice-crystals, 

 there is, perhaps, within the plant no such thing as the 



