PERMANENT WILTING IN PLANTS 121 



to the kind of plants nor to the size of the containers employed 

 in the experiments. The constant difference values are so nearly 

 in accord that it is possibly safe to conclude that, within the 

 limits of all the conditions of our studies, the relation of soil 

 moisture residue to evaporation intensity is approximately ex- 

 pressed, for the three plants here used and for a wide range of 

 physicall}^ different soils, by the equation, 



X 



y 



)0.45 



The quantitative relations holding between the magnitude 

 given to the calculated soil moisture residue at permanent wilting, 

 by the Briggs and Shantz formula, and the observed magnitudes 

 as shown in these experiments, remains to be considered. Cald- 

 well has shown that, for lighter soils (with low water holding 

 powers), the observed values are always much larger than the 

 calculated, this discrepancy being greater, of course, with high 

 evaporation intensities than with lower ones. On the other 

 hand, the same author has established the fact that, with heavier 

 soils (of high water holding powers), the observed values ap- 

 proach much more closely the calculated ones, being about 

 equal to them under conditions of low evaporation intensities. 

 These observations of Caldwell are clearly substantiated by 

 the work here reported, excepting that the present experiments 

 furnish a number of cases where the observed values fall markedly 

 below the calculated ones (series V). As might be expected 

 from Caldwell's results in this connection, these cases occur with 

 low evaporation intensities and with the soil of highest water 

 holding power. It is unnecessary here to enquire further into 

 this relation, since it is quite obvious that the formula of Briggs 

 and Shantz cannot be considered as of general application, but 

 only as expressing a relation obtaining under some as yet undeter- 

 mined range of external and internal conditions, within the 

 limits of which must have lain the experimental conditions em- 

 ployed by these workers. 



