24 BOTANICAL GAZETTE [july 



residing in the seed substance at any moisture content between 

 saturation and air-dry. The actual values for this range in Xan- 

 thium seeds are given in the last column of table III. 



The results obtained by measuring the soils with measured seeds 

 are of the greatest interest. While it is important to have means 

 of determining in terms of atmospheric pressure the water-retaining 

 power of soils at any degree of dryness between saturation and air- 

 dry, it is still more important to understand the moisture relations 

 of the plant to the soil at and just below the wilting coefficient. 

 Does wilting occur because capillary soil forces and osmotic root 

 forces reach equilibrium ? If so, why should Briggs and Shantz 

 (8) have found a uniform wilting coefficient for all kinds of plants 

 when we know that root cells vary somewhat in osmotic concen- 

 tration from species to species ? And why should this uniformity 

 fail in the presence of intense evaporation, as shown by Caldwell 

 (9), and by Shive and Livingston (32)? These questions, and 

 the discrepancies between the excellent work done at Washington 

 and at Tucson can probably be answered intelligently, and explained 

 in the light of these experiments. 



The moisture equivalent of a soil is the percentage of water left 

 in the soil after centrifuging for a certain time under a force of 1000 

 gravities, a force about equal to one atmosphere. We have been 

 accustomed to find the wilting coefficient empirically, or to divide 

 the moisture equivalent by 1 . 84. But we have not known how 

 much greater are the soil forces at the wilting coefficient than at 

 the moisture equivalent. From the results obtained, the pressure 

 value of the wilting coefficient seems to be about 4 atmospheres. 



As the soil becomes drier and drier, the forces become greater 

 and greater, until on the approach of air-dry conditions a very 

 small change in moisture content makes a very large change in the 

 forces involved. The increase of the force with decreasing soil 

 moisture is shown graphically in fig. 5, which shows the curve of 

 increasing force for the Oswego silt loam subsoil, and for no. 2/0 

 sand. The difference in the curves for clay and sand is striking. 

 In the clay, where the surface films are relatively thick, the force 

 decreases very slowly for a considerable distance. But as the films 

 become very thin, the surface force and forces of adhesion of water 



