1916] 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 Briccs 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 Xanthium pennsylvanicum 
absorb water has been measured by two methods: (a) osmotic 
