i 9 1 6] SHULL— SOILS 27 



of movement of film water toward the plant. It is a question of 

 rates. Evaporation continues from the leaves in accordance with 

 atmospheric conditions somewhat regardless of conditions below 

 the soil surface. At the wilting coefficient the film water becomes 

 so stable, and the friction of movement becomes so great, that the 

 rate of movement of water toward the root is quite inadequate to 

 meet the needs of the plant, and permanent wilting ensues. 



It becomes clear at once why Briggs and Shantz, working 

 under rather uniform conditions of evaporation, found the same 

 wilting coefficient for all kinds of plants in a given soil, regardless 

 of variability of root sap concentration and other variable factors, 

 for these variables do not affect the point at which capillary move- 

 ment of water over the soil particles ceases to be effective for the 

 plant. This is determined by the physical properties of the soil, 

 the fineness of the particles being the chief factor. We should 

 expect, therefore, this uniform behavior under moderate conditions. 

 On the other hand, when the evaporation rate is very intense, the 

 plant might be caused to wilt permanently before this wilting 

 coefficient is reached, owing to the fact that after all it is a question 

 of rates. The rate of movement in the soil fails to be adequate 

 sooner. 



In concluding this discussion, may I suggest that the methods 

 used and the conclusions reached in this work should receive very 

 critical consideration by plant physiologists, soil physicists, and all 

 others interested in these problems. We have lacked even the 

 most elementary facts concerning these important moisture rela- 

 tions of the soil. This is a first attempt to throw light upon an 

 unexplored region of soil physics. It is hoped that other methods 

 may be devised for testing the correctness of the conclusions 

 reached by the methods presented here. The apparatus is quite 

 simple and easily used. If the results obtained can be fairly 

 substantiated by other methods, the method will be exceedingly 

 valuable in physiological and ecological investigations of many 

 kinds. 



VI. Summary 



1. The force with which the seeds of Xanthiiim pennsylvanicum 

 absorb water has been measured by two methods: (a) osmotic 



