74 VICTOR E. SHELFORD. 



from the soil. The water holding power of different soils is 

 different. It increases with the decrease in size of the soil par- 

 ticles and with the addition of humus which takes up water by 

 imbibition. The amount of water in the soil is usually expressed 

 in terms of per cent, of weight but a soil with 8 per cent, of 

 moisture may not give up water to an organism as readily as 

 another soil with only 2 per cent. It is necessary therefore, to 

 determine the capacity of a soil to retain or give up moisture. 

 This has been determined for a number of soils by Briggs and 

 McLane ('07) and Briggs and Shantz ('12), in terms of what 

 they call the moisture equivalent. The moisture equivalent of a 

 soil is the percentage of water which it can retain in opposition 

 to a centrifugal force 1 ,000 times that of gravity. This has been 

 determined for a number of soils (1. c., '12, p. 57). The main- 

 tenance of turgor in plants is believed to be a purely physical 

 matter. If the roots of a plant are in a mass of soil, the plant 

 gradually reduces the water content until the permanent wilting 

 occurs. The wilting coefficient of a soil is the moisture content 

 (in percentage of dry weight) at the time when the leaves of the 

 plant growing in the soil first undergo a permanent reduction 

 in moisture content, as a result of a deficiency of moisture supply. 

 The moisture equivalent of a soil is 1.84 times the wilting coefficient 

 for wheat, used as a standard plant. Fuller ('12) states that the 

 wilting coefficient of dune sand is about 0.75 per cent, while the 

 usual moisture content of the cottonwood dune sand is two or 

 three times this amount. For the clay soil of the oak-hickory 

 forest, according to McNutt and Fuller ('12) the coefficient is 

 about 8 per cent. These standards of soil moisture indicate the 

 amount of water available to animals through direct contact with 

 the soil or available for evaporation into the air of cavities 

 which they construct for themselves beneath the surface of the 

 soil. The soil inhabiting animals of the cottonwood area live in 

 the presence of a greater amount of available wacer than do the 

 animals of the oak hickory forest. 



(b} Plants and Animals. Cowles ('n) mentions the impor- 

 tance of soil bacteria which increase with the increase of the 

 humus, and the development of substances toxic to the plants 

 producing them (Schreiner and Reed, '07). Little is known of 



