128 ILLINOIS ACADEMY OF SCIENCE. 



prairie region and as the association next below the climax in 

 areas farther removed from the grasslands where the beech-maple 

 forest association forms the climax. The composition and rela- 

 tionships of these associations have been fully discussed by 

 Cowles,^ Whitford- and others, but almost nothing has been 

 known quantitatively of the physical factors (other than rainfall) 

 that determine their character and extent. Some preliminary 

 studies upon the evaporating power of the air in some of the 

 associations involved have already been reported to this Acad- 

 emy,^ but the oak-hickory forest was not included among the 

 associations studied. To supply this deficiency, the evaporating 

 power of the air and the soil moisture have been determined dur- 

 ing the season extending from April 22 to October 28, 191 1, in 

 a fairly undisturbed forest situated upon the Valparaiso moraine, 

 about twenty miles southwest of Chicago, near the little village of 

 Palos Park. 



In the evaporation determinations the porous cup atmometer 

 was employed and the directions given by its inventor, Livings- 

 ton,* for its operation were so closely followed that any extended 

 account of its management becomes quite unnecessary. All instru- 

 ments were standardized before being set up and the standard- 

 ization repeated at intervals of six to eight weeks. By the coeffi- 

 cients thus obtained all readings were reduced to a common unit. 

 Readings were made weekly throughout the season and the 

 results expressed as the average daily rate of loss for the interval 

 between the readings. These results have been graphically rep- 

 resented with the weekly intervals as abscissae, and the amount 

 of daily loss by the standard atmometer, in cubic centimeters, as 

 ordinates. 



Four stations were established, three upon the upland and one 

 in a small depression, a wedge-shaped flood-plain of a small 

 stream, usually without water during the summer. The upland 

 stations were upon the low sloping hills of the moraine about 

 twelve meters above the level of Lake Michigan, where the soil 

 was of a fine texture, being composed of boulder clay with a 

 small admixture of sand. In the depression there was a mixture 

 of alluvium and humus, no clay appearing at a depth of 30 cm. 



1 Cowles, H. C. The physiographic ecology of Chicago and vicinity. Bot. Gaz., 31: 

 73-108, 145-182, 1901. 



8 Whitford, H. N. The genetic development of the forests of northern Michigan. 

 Sot. Gaz., 31: 289-325, 1901. . 



"Fuller, Geo. D. Evaporation and plant succession. Trans. 111. Acad. Sci., 4: 119- 

 125, 1911, and Bot. Gaz. 52: 193-208, 1911. 



♦Livingston, B. E. Operation of the porous-cup atmometer. Plant World, 13: iii- 

 J19, 1 910. 



