•PERMANENT WILTING IN PLANTS 101 



and Shantz formula) of the moisture residue is indicated by the 

 horizontal broken line, y = 3.80. It is obvious that all of the 

 observed lesidues are much greater than the values thus cal- 

 culated, although (as Caldwell has already clearly shown), with 

 low e\'aporation rates the two values differ less markedly than 

 they do with high rates. With an evaporation rate of 0.15 cc. per 

 hour from the standard instrument, the theory of the curve above 

 considered demands that the soil moisture residue become 3.8 

 per cent., thus agreeing with the calculation based on the formula 

 of Briggs and Shantz. It seems improbable that the evaporation 

 intensities prevailing in the glass chamber of the last-named 

 authors should have been as low as 0.15 cc. per hour, and it is 

 almost certain that their other exposures must have all given 

 higher intensities of evaporation than did their glass chamber. No 

 attempt is as yet possible, to explain this apparently pronounced 

 disagreenlent between the results of the Washington workers 

 and those here brought forward. It may be merely suggested 

 that the explanation sought may have to do with different physi- 

 cal conditions within the plant, depending upon the environ- 

 ment in which it grew; thus the plants with which Caldwell and 

 the present writers worked may have been markedly and definitely 

 different from those with which the experiments of Briggs and 

 Shantz were carried out. 



Series II. Phaseolus was employed in this experiment, the 

 plants being six weeks old at the beginning. Three plants com- 

 prised each culture. The soil used was the same mixture of 

 sand and clay loam as that employed in series I. The experi- 

 ment was begun September 8, 8 a.m. The first permanent wilting 

 occurred in the chamber in the open, on September 8. On Sep- 

 tember 11, permanent wilting was attained in the chamber in 

 the shelter and in the open; on September 13, permanent wilting 

 occurred in the glass box. 



The numerical data obtained from this series make up table 

 III, in which the arrangement and notations are quite similar 

 to those used in table 11. 



It is again generally clear, in table III, that the higher evapora- 

 tion rates accompany high soil moisture residues and that low 



