72 ILLINOIS ACADEMY OF SCIENCE 



The comparative amounts of growth-water indicate even 

 more clearly the relationship existing between the available 

 water supplies of the associations, and should serve to empha- 

 size the fact that the progressive increases in the water retain- 

 ing power of the soil, due largely to its increased humus con- 

 tent, must play no inconsiderable role in causing the succession 

 here, culminating in the mesophytic beech-maple forest. More 

 investigation must be made before more definite comparisons 

 can be made. 



Another and still more important comparison may be insti- 

 tuted among the associations under investigation by consider- 

 ing 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 I. In determining these 

 ratios it is recognized that the units of measurement in the 

 case of the evaporation and the soil moisture are not directly 

 comparable. Still it is thought that the comparison is a 

 legitimate one, and institutes a quantitative comparison of the 

 mesophytism of the habitats which is valuable and exceeds in 

 accuracy anything hitherto attained. It is true that these habi- 

 tats 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 

 associations, because in these the seedlings, both woody and 

 herbaceous, develop. An extension of the habitat by the ad- 

 dition of the higher strata of the air and the strata of the soil 

 containing all the aerial and subterranean portions of the vege- 

 tation would doubtless modify and decrease the steepness of 

 the gradient between the various ratios. 



The ratios may either be compared directly, remembering 

 that the mesophytism of the various habitats varies in inverse 

 ratios with the numbers expressing these ratios, or the beech- 

 maple forest may again be taken as the standard and represent- 

 ed by 100, when the direct ratio of the mesophytism of the 

 corresponding portions of the oak-hickory forest, the oak dune, 

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

 20, 17, and 15. These comparative values of the moisture 

 factors show such a surprising rate of increase as one pro- 

 ceeds from the pioneer to the climax associations, that it cannot 

 be doubted that such a change in water conditions must be one 

 of the chief, if not the most important cause of the succession 

 of associations from the more xerophytic to the mesophytic. 



