228 



TYPICAL EXAMPLES OF PLANT SUCCESSION 



usually much retarded. The accumulation of humus is impossible 

 and the pioneer stage is likely to persist until physiographic changes 

 have brought about more favorable conditions. This usually begins 

 with a collection of soil at the base of the cliff. As this soil increases 



Fig. 103. — Remnant of a climax beech-maple forest. (Photograph by A. G. 



Eldredge.) 



in amount the plants of later stages climb higher and higher and at 

 the same time plants from the surrounding upland descend as the 

 ledges near the top of the cliff become covered with soil. Eventually, 

 of course, the whole rock surface will be covered with soil and plants, 

 but this takes a very long time. 



A rock canyon usually presents very complex and interesting 

 ecological conditions. The head of a canyon is constantly eating its 

 way back into the upland and so is always young, while farther down 

 it is middle-aged, and at the mouth may be relatively mature. The 

 pioneers in the young canyon are likely to be liverworts instead of 



