220 BOTANICAL GAZETTE [September 



Those of the clay soils made by the centrifuge, using the formula 



moisture equivalent ., . rr . , 

 = wilting coefficient, 



i . 84 

 the moisture equivalent being the amount of water which the soil 

 retains against a centrifugal force of 1000 times gravity, showed 

 very constant results. 



The results are represented graphically, the wilting coefficients 

 being represented by broken lines and the range of soil moisture in 

 percentages of the dry weight of the soil by graphs with the weekly 

 intervals as abscissas; while the ordinates represent the percentage 

 of soil moisture present at the weekly determinations. Throughout 

 the diagrams the vertical scale is denoted by figures at the left; 

 and heavy lines are used for the determinations at 7 . 5 cm., and light 

 lines for 25 cm. depth. The intervals between the graphs and the 

 line denoting the wilting coefficient give the amount of the growth- 



water. 



RANGE IN THE DIFFERENT ASSOCIATIONS 



An examination of the accompanying diagrams will show at a 

 glance some of the peculiarities of the water supply of the various 

 associations. The results for the cottonwood dune association for 

 1911 have already been published (18), and show rather surprising 

 results (fig. 16). The water content is rather constant, but seldom 

 more than 5 per cent, a decidedly small amount. However, the 

 wilting coefficient was here found to be less than 1 per cent, the 

 average for many determinations by the indirect method being 

 0.8 per cent at both depths, the absence of humus and the insta- 

 bility of the soil accounting for the same coefficient at both depths. 

 Considered in relation to this wilting coefficient, the soil moisture is 

 seen to be continually at least twice the amount of water necessary 

 for the growth of such a plant as wheat. This is in striking contrast 

 to the desert-like aspect of this association, due, to the almost 

 complete absence of herbaceous undergrowth. Doubtless this 

 constancy in soil moisture is largely due to the conserving action 

 of a dry mulch of 3 to 5 cm. thickness maintained by the action of 

 the wind upon the sand and to the small quantities withdrawn by 

 the sparse vegetation. The record for 191 2 (fig. 17) differs in no 



