1 6 THE PLANT SOCIETIES OF 



canons 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 



Fig. 3. — Head of a canon in the St. Peters sandstone at Starved Rock. Erosive forces 

 prominent, and vegetation sligfit on tlie dripping slopes. 



gorges are very shady and often dftipping with moisture, hence liver- 

 worts 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, j and 

 4 represent canons of the above description, whose rocks drip with 

 moisture. 



The stages of development pass much more slowly in canons 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 



