PERMANENT WILTING IN PLANTS 



99 



only values that closely approach an intensity of zero. The 

 region of the curve where differences of evaporation rate pro- 

 duce the smallest differences in the soil moisture residue at 

 permanent wilting Hes in its upper portion, with high values 

 of X. But even here, for the highest intensities of the atmos- 

 pheric evaporating power experienced in this experiment, the 

 relation between the two conditions is still obvious enough. 



P.O. 



e.o 



6.0 _ 



4.0 _ 



3.a 



SERIES I 



(TABLE ll.> 



Fig. 1 



A further observational study of the curve denoted by the 

 five main averages brings out the apparently important fact 

 that this curve very closely approximates that of a logarithmic 

 function in which, for each geometrical increase in the evapora- 

 tion rate (x), there is an arithmetical increase in the value of 

 the moisture residue (y) . The general equation of such a curve 



y 



referred to tbe axes, x = 0, y =0, is: x =/c", where k is the 

 constant ratio by which x increases and a is the constant differ- 

 ence by which y increases. If k be taken as 2 in the present case. 



