PAPERS BY MEMBERS 



95 



those reported by Fuller (2) for the cottonwood dune, the piuv^ 

 dune, the oak dune and the beech-maple forest, and also those 

 of McXutt and Fuller (3) for the oak-hickory forest. Though 

 the figures themselves are not repeated, fig. 2 will show the 

 relative average evaporation of these different associa- 



Fig. 2. Diagram showing average daily rates of evap- 

 oration in various plant associations of the Chicago region. 



tions. It is evident at once that the evaporation 

 conditions of this edaphic prairie are between those 

 of the cottonwood dune and the pine dune, though 

 more nearly equivalent to the latter. But the sum- 

 mer maximum of evaporation on the prairie is as low as the 

 corresponding maximum of any of the forest associations yet 

 studied in this region, excepting the beech-maple forest which 

 has a maximum of ortly 13.2 c. c. per day. It corresponds 

 rather closely in this respect to the pine dune, the oak dune, 

 and the oak-hickory forest that has not been pastured. It 

 seems somewhat surprising that the summer maximum on 

 this prairie should be found so low as compared with the 

 forest types, considering how slight is the apparent protection 

 here from wind and sun. So the results would tend to show- 

 that, contrarv to what one might expect, there must be a con- 

 siderable resistance ofifered by the covering of grasses to the 

 movement of air, this becoming evident in the stratum in 

 which the cups were placed. The moist air blanket, thus 

 tending to develop among and just above the grass covering, 

 would largely account for the rather unexpected low summer 

 maximum rate of evaporation. 



III. Soil moisture. 



The determinations of the water content of the soil were 

 made weekly from April 11 to October 25 from samples of 

 about 200 grams taken at the depth of 7.5 cm. and 25 cm. at 

 each of the stations 1 and 2. The soil was brought into the 



