298 BOTANICAL GAZETTE [October 



shrub association of Symphoricarpus and later Rhus glabra advance, 

 preparing the way for the bur oak-slippery elm association, which 

 likewise makes its greatest progress up these slopes from ravines and 

 flood plains. 



7. The floral activity of the formation may be approximately 

 recorded in the following five aspects, set off by marked chmatic and 

 floral changes: prevernal, April i to April 25, 6 species; vernal, 

 May 3 to May 31, 28 species; estival, June i to July 7, 21 species; 

 serotinal, July 7 to August 7, 13 species; autumnal, August 7 to 

 September 21, 22 species. 



8. The prairie elements show a marked grouping into vertical 

 layers, which correspond approximately with the floral aspects. 

 Overtopped by the autumnal layer the sub-layers are successively 

 those of the serotinal, estival, vernal, and prevernal. 



9. There is a marked distinction in the chresard of base, slope, 

 and crest in the prevernal, which becomes less marked in the sub- 

 sequent aspects, approaching equality in the autumnal. As a result, 

 the floral covering shows a corresponding difference upon base, slope, 

 and crest in earUer aspects; the influence of position gradually 

 declines, the floral covering presenting a striking similarity over the 

 entire formation in the autumnal. 



The University of Chicago 



