
88 BOTANICAL GAZETTE [FEBRUARY 
though the ash, elm, and other trees are frequent. The most 
characteristic undershrub is the witch hazel (Hamamels Vzr- 
giniana). The herbaceous plants are notoriously vernal forms, 
such as Hepatica, Thalictrum, Trillium, Mitella, Dicentra, 
Sanguinaria; mosses abound and liverworts are frequent. A 
ravine with the above vegetation is shown in jig. 2. We can 
explain this flora only by regarding it as having reached a tem- 
porary climax. Ravine conditions are more favorable for plants 
than those that precede or follow. The instability and exposure 
of the gully have gone; in their place there is protection from 
wind and exposure. The shade and topography favor the collec- 
tion and conservation of moisture, and as a result there is.a rapid 
development intoa high-grade forest, as outlined above. 
Rock ravines are much less common in the Chicago area than 
are those of clay, since the underlying limestone rarely comes 
near the surface. Excellent illustrations of stream gorges are 
to be seen at Lockport, and also inivarious tributaries of the IIli- 
nois river near: Starved rock, A striking difference between 
these rock’ gorges or cafions and the clay ravines is in the slope 
of the sides.; The physical nature of the rock excludes landslide 
action, hence the sides are often nearly vertical for a long time. 
Lateral cutting is also relatively slow as compared with clay. 
Thus the conditions for vegetation at the outset are much more 
favorable than in a clay ravine. Rock-bound gorges are very 
shady and often dripping with moisture, hence liverworts and 
many mosses find here a habitat even more congenial than in 
the clay. Among the higher forms are found the most extreme 
shade plants that we have, such as Impatiens, Pilea, and shade- 
loving ferns, plants whose leaves are broad and remarkably thin. 
figs. 3 and 4 represent cafions of the above description, whose 
rocks drip with moisture. 
_ The stages of development pass much more slowly in caiions 
than in clay ravines, largely because the primitive conditions of 
shade and moisture remain for a long period of time. Nor do 
the steep slopes permit the development of a wealth of trees and 
shrubs, since a secure foothold is not easily found. However, 



