WATME OF SOIL IN ITS BELATION TO PLANTS 259 



relationship of the physical constants of the soil to the 

 wilting point depends on the rate at which the plant 

 loses water, showing that the soil factors are not entirely 

 dominant in this respect. This work seemed, neverthe- 

 less, to indicate that the conclusions of Briggs and Shantz 

 were correct for plants of humid regions, where the wilt- 

 ing occurred in a saturated atmosphere. If such is the 

 case, it can be accounted for only by the fact that the 

 soil forces in their effect on the wilting point are so power- 

 ful as to override any distinguishing characteristics that 

 the plant itself may possess, or at least reduce such an 

 influence within the error of actual experimentation. 



180. Determination of the wilting point. — Briggs and 

 Shantz,^ in their investigations, devised a very accurate 

 method for making determinations of the wilting point. 

 Glass tumblers holding about 250 cubic centimeters of 

 soil in an optimum condition were used. The seeds were 

 placed in this soil, after which soft parafHn was poured 

 over the surface in order to stop evaporation, thus re- 

 moving this disturbing factor in the capillary equilibrium 

 of the moisture. The seedlings on germination were 

 able to push through this paraffin. While the plants 

 were developing, the tumblers were kept standing in a 

 constant-temperature vat of water in order to prevent 

 condensation of moisture on the inside of the glass. The 

 vegetative room was under temperature control. When 

 definite wilting occurred, as determined in a saturated 

 atmosphere, a moisture test was made on the soil. The 

 resulting figure, within experimental error, represents 

 the wilting point for the soil used. 



^ Briggs, L. J., and Shantz, H. L. The Wilting Coefficient 

 for Different Plants and its Indirect Determination. U. S. 

 D. A., Bur. Plant Indus., Bui. 230, pp. 10-14. 1912. 



