1914] FULLER— EVAPORATION AND SOIL MOISTURE 23 1 



soil, due largely to its increased humus content, must play no 

 inconsiderable role in causing the succession here culminating in 

 the mesophytic beech-maple forest. Little need be said concerning 

 the rank of the prairie, in respect to its soil moisture in comparison 

 with the other associations under consideration, further than to 

 point out that its soil has a very great water capacity, and that 

 over against its large amount of growth-water should be placed the 

 fact that the supply completely fails at intervals. More investiga- 

 tion must be made before more definite comparisons can be made. 

 Another and still more important comparison may be instituted 

 among the associations under investigation by considering the ratio 

 between the average mean weekly evaporation rates for the 10 mid- 

 summer weeks of the years 1910-1912 and the mean growth-water 

 for the same period. These ratios are expressed in the final column 

 of table III. In determining these ratios it is recognized that the 

 units of measurement in the case of the evaporation rates and the 

 soil moisture are not directly comparable. Still it is thought that 

 the comparison is a legitimate one, and institutes a quantitative 

 summation of the mesophytism of the habitats which is valuable and 

 exceeds in accuracy anything hitherto attained. It is true that 

 these habitats are limited to the lower stratum of the aerial and 

 the upper strata of the subterranean vegetation, but, as previously 

 pointed out, these are the portions of the habitat that are of critical 

 importance in the establishment and maintenance of the associa- 

 tions, because in them the seedlings, both woody and herbaceous, 

 develop. An extension of the habitats by the addition of the higher 

 strata of the air and the lower strata of the soil containing all the 

 aerial and subterranean portions of the vegetation would doubtless 

 modify and perhaps decrease the steepness of the gradient between 

 the various ratios. The ratios may either be compared directly, 

 remembering that the mesoDhvtism of the various habitats varies 



numbers expressin 



m 



meso 



esponding 



pine dune, and the cottonwood dune will be respectively 65, 20, 17, 

 and 15. The prairie expressed in similar terms will be 62. These 



