WATER-RELATION BETWEEN PLANT AND SOIL. 27 



is assumed for the cases to be dealt with below, although the value of 

 R is there quite unknown. The logical analysis, along the lines just 

 followed, may be carried farther when need arises. 



In our actual employment of the various powers above mentioned 

 we shall not attempt any adequate treatment of the internal conditions; 

 as has been stated, root absorbing power and the conducting power of 

 the plant as a whole have not yet been brought under experimental 

 observation, so that the experimental approximation of effective envi- 

 ronmental aridity (involving these as well as transpiring power) is not 

 now possible. We shall confine attention to the external conditions 



T 



I and E and to the single internal one, transpiring power -, and the 



main interest of our study will lie in the daily marches of the relative 

 magnitudes of these conditions, measured in terms of their respective 

 values for some selected hour period. 



THE MEASUREMENTS. 



The numerical data derived from one of each of our pairs of duplicate 

 experiments will now be presented. 1 The other one of the pair showed 

 results essentially similar, in all three cases. In this presentation we 

 shall deal with the march of evaporation (E), transpiration (T), trans- 



/T\ 



piring power -, attraction of the soil for water (/), and environmental 



aridity (El). In tables 2, 3, and 4, the hourly change in weight of 

 plant and soil and the hourly loss of water from the irrigator and from 

 the atmometer are the observed data. The corresponding absolute 

 transpiration increments are obtained, as has been stated, by sub- 

 tracting the change in weight from the irrigator loss. Columns 6, 7, 

 and 8 of the tables, give the three values, irrigator loss (7), absolute 

 transpiration (T 7 ), and atmometer loss (E) for every hour of the two 

 series of observations, reduced to a basis which considers the rate for 

 one of the night hours of the first series as unity. Column 9 gives the 



/T\ 

 relative transpiration ratios -, also reduced to the same uniform 



basis, and the last column shows the values of the product of irrigator 

 loss and atmometer loss, also on that basis. These last values may be 

 considered to represent environmental aridity (following our preceding 

 discussion), relative to the value of this condition for the standard hour 

 of the earlier series. Each table represents a single plant and includes 

 the results from the two series of observations, one beginning February 

 23 and the other March 23. 



Since the weighings lacked accuracy, owing largely to the crude 

 method of suspending the looped rubber tube of the apparatus instead 



*In the calculation and tabulation of the great mass of measurements resulting from this study 

 the authors have been ably assisted by Mrs. Grace J. Livingston and Miss Aleita Hopping. 



