22 WATER-RELATION BETWEEN PLANT AND SOIL. 



In this equation Z)« represents the total tendency of the internal com- 

 plex of conditions to give up water to the surroundings, d; denotes the 

 factor of this tendency due to transpiring power, and dr stands for the 

 factor due to the ease with which roots might part with water. But 



T 



transpiring power (d,) may be measured by the ratio -^, and the recip- 

 rocal of absorbing power (dr) may be represented by -rj. Therefore, 



T 1 T 1 

 Dt= -^ X -7j = -7 X -^j. In words, the tendency of the plant body to 



part with water during anj'- time period, as related to this tendency for 

 any other period, is the ratio of absolute transpiration to absolute 

 absorption for the second period (these being stated in terms of the 

 corresponding rates for the first period) multiplied by the reciprocal 

 of the product of the atmometric and irrigator losses for the second 

 period (these latter being also stated in terms of the corresponding 

 losses for the first period). It is to be emphasized that such a state- 

 ment rests on the assumptions that the rate of water loss from the 

 irrigator is a measure of the power of the soil to retard water absorp- 

 tion by the root system, that its reciprocal is proportional to the water- 

 supplying power of the soil, and that the atmometric loss is a measure 

 of the desiccating power of the aerial surroundings. Furthermore, 

 this statement is of no great practical value at the present time, on 

 account of the fact that no method is yet available for approximating 

 absorption rates. 



If the reasoning of the last paragraphs be correct, then the tendency 

 of any plant to lose water, with any given set of surroundings (which 

 tendency may be represented bj^ D), should be proportional to the 

 product, D, X D/, the product of the internal and external tendencies 

 toward plant desiccation. This product may be regarded as a relative 

 measure of the effective environmental desiccating power or effective 

 aridity; it is the power of the environment tending toward plant desic- 

 cation, as far as this power is influential when operating in conjunction 

 with the given internal conditions. 



We may thus write: D = D,XDi = daXd,XdiXdr or, for our tests, 



T 1 . T T 



D = EX I X-tX -pTr , which finally becomes, D = -j. The ratio -j 



is, then, a relative measure of the tendencj^ of the plant (with its 

 internal conditions as they may happen to be) during any time ]')oriod 

 to lose water to the surroundings (with whatever complex of conditions 

 the surroundings maj^ present). This is exactly the proposition with 

 which we started. If this ratio is greater than unity the environment 

 is not well adapted to the plant, or the plant is not well adapted to its 

 surroundings, as far as the water-relation is concerned, and, if such a 

 state of affairs be continued, growth must cease and finally death or 



