peters: moisture requirements of seeds. 25 



was practically constant. It made no difference as to the 

 plants used, being a fixed quantity for that soil. Furthermore, 

 they worked out formulae by which this wilting coefficient for 

 any soil could be calculated from either of four factors : its 

 moisture equivalent, its hygroscopic coefficient, its moisture- 

 holding capacity, or its texture as determined by mechanical 

 analysis. Their wilting coefficient was the standard when this 

 work was begun. Since then, the work of Caldwell (2) has 

 come to hand. He carried on his experiments at the desert 

 laboratory of the Carnegie Institution at Tucson, Arizona. 

 Here, transpiration was excessive as the result of the heat, 

 low humidity, and the hot, dry winds. When he produced con- 

 ditions similar to those of Briggs and Shantz, his results tallied 

 with theirs. When conditions were natural for his location, he 

 found the wilting coefficient always higher (even 30 to 40 per 

 cent) than theirs or than that calculated from their formulae. 

 Further, "under any set of aerial conditions the observed 

 soil moisture content at permanent wilting is approximately a 

 constant for each of the soils used, and its value increases with 

 the increase in the rate of transpiration, being greater under 

 conditions of high evaporation intensity and declining with 

 the decrease of the evaporating power of the air. For a series 

 of plants grown in any soil, and wilted under different aerial 

 conditions, all with relatively high evaporation rates, as many 

 different soil moisture contents at permanent wilting are ob- 

 tained as there are sets of conditions." 



Russell (6) has shown that the rate of supply of soil water 

 is simph' the speed at which water can move in the soil, and 

 this depends upon the amount of clay and colloidal matter 

 present. Livingstone (5) calls attention to another factor 

 which complicates the problem still more. In a set of experi- 

 ments carried on in the Johns Hopkins' greenhouses where he 

 had plants grown with their roots in vessels of water and sub- 

 jected to varying aerial conditions, he found that with the 

 "back pull" of the soil thus cut out, temporary and even perma- 

 nent wilting occurred. His conclusion is that the trouble is " 

 internal, the absorbing power of the roots is inadequate to 

 supply moisture as fast as it is lost by evaporation. Hence, he 

 thinks permanent wilting need not depend upon soil moisture 

 conditions necessarily, although it frequently does. Caldwell's 

 higher results are thus evidently due to the rapid transpira- 

 tion of water from the leaves, associated with the slowness of 



