96 BOTANICAL GAZETTE [FEBRUARY 
slow as it may be, is nevertheless revealed by occasional leaning 
or even falling trees. From the above it is easy to understand 
that cycles of vegetation often pass much more rapidly than 
cycles of erosion, but never more slowly. During one erosion 
cycle the mesophytic forest develops at least twice, once on 
the ravine slopes and then finally on the gentler slopes that 
betoken approach toward base level. 


Fic. 8.—General view of the Illinois valley near Starved rock, showing islands 
and an extensive flood plain with trees along the margin. Young islands in the fore- 
ground, older islands in the background, 
C. The flood plain— We may now follow the successive stages 
in the development of the flood plain vegetation. While the 
ravine is still young, as in fig. 2, there is no permanent stream, 
but merely torrents which remain but a short time. As the 
ravine deepens, widens, and lengthens, thus approaching the 
underground water level and increasing the drainage area, the 
water remains for a longer and longer time after each rainfall. 
As the ravine conditions thus become more and more hydro- 
phytic, the original flora, perhaps of shade mesophytes (as 
Impatiens), becomes replaced by amphibious shade plants, such 
lente eee 
