phytism was the paucity of tender mesophytic forms and the less 

 luxuriant vegetation in general on the south-facing slopes. In the 

 early spring the north-facing slopes of the ravine were covered more 

 abundantly with the various plants of the vernal flora and certain 

 mosses than the south-facing slopes. 



Plants of the; Ravine and the Oak Uplands 



The plants of the ravine are recorded under two more or less 

 distinct physiographic areas; the xeromesophytic slopes of the gul- 

 lies (Region i of the table) and the mesophytic slopes of the ravine 

 (Region 2). Some of the plants of the oak uplands (Region 3) are 

 listed, to emphasize the differences between it and the ravine. The 

 vegetation of the oak uplands may be thought of as either antedating 

 or succeeding that of the ravine. The vegetation in these three suc- 

 cessional regions is not entirely distinct, but even a superficial ex- 

 amination of the following list shows striking differences. As this 

 study covers only the period from June 21 to October 18, 19 13, much 

 of the vernal vegetation is omitted. 



In the examination of this list of plants it should be borne in mind 

 that the vegetation of the slopes of the gullies and ravine is noted 

 much more completely than that of the oak uplands ; that the in- 

 dividuals of certain species were generally more numerous on the 

 slopes of the ravine than on the slopes of the gullies ; and that in 

 certain localities of the ravine such plants as Osmorhiza longistylis 

 (PI. Ill, Fig. 5), Amphicarpa monoica (PI. II, Fig. 4), and Aralia 

 nudicaulis (PI. IV, Fig. 6) had developed pure growths to the ex- 

 clusion of all other species. Furthermore, and most important of all 

 from the standpoint of this study, the list of plants for any of the 

 three regions must not be considered as static but as dynamic in char- 

 acter. As already emphasized in an earlier part of this paper, with 

 the changes in the physiography of the ravine the vegetation of its 

 slopes tends to assume a xerophytic aspect, but only until stability of 

 topography favors the succession of forest types, whose climax in 

 the Evanston-Waukegan region is essentially the same as that of the 

 mesophytic slopes of the ravine, the hard-maple type. 



An investigation which is to determine whether or not the differ- 

 ences in the evaporative power of the air and in the soil-moisture 

 content account for these successions and the minor differences within 

 each of the areas where plants have been listed, involves, as the first 

 step, the selection of certain typical stations — stations significant from 

 the standpoint of exposure and ravine development ; as the second 

 step, the measurement, at regular intervals, of the evaporative power 



