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 climatic and 
floral changes: prevernal, April 1 to April 25, 6 species; vernal, 
May 3 to May 31, 28 species; estival, June 1 to July 7, 21 species, 
serotinal, July 7 to August 7, 13 species; autumnal, August 7 1 
September 21, 22 species. 3 
8. The prairie elements show a marked grouping into vertical 
layers, which correspond approximately with the floral ese 
Overtopped by the autumnal layer the sub-layers are successive y 
those of the serotinal, estival, vernal, and prevernal. 
g. There isa marked distinction in the chresard of base, ee 
and crest in the prevernal, which becomes less marked in the 
sequent aspects, approaching equality in the autumnal. As a resus 
the floral covering shows a corresponding difference upon base, i 
and crest in earlier aspects; the influence of position grad zs 
declines, the floral covering presenting a striking similarity overt 
entire formation in the autumnal. 
THE UNIVERSITY oF CHICAGO 
