292 



PLANT PHYSIOLOGY AND ECOLOGY 



mation is represented in one or more areas suitable to it by a 

 similar or corresponding species, group, or formation. 



Numerical and corresponsive alternation are variations of the 

 normal process. They arise out of slight differences in physical 

 factors, or the course of competition, or out of migration from 

 the surrounding vegetation. 



316. Normal alternation of formations, consocies, etc. The 

 alternation of formations or of minor areas is especially character- 

 istic of greatly diversified regions, such as mountains. It is 



Fig. 119. Normal alternation of consocies of the foothill thicket forma- 

 tion near Colorado City. Quercus is fovmd on the black soil, Ccrcocarpus 

 on the limestone soil, and Rhus in the valleys. 



naturally much less conspicuous in lands with a more uniform 

 surface. A xerophytic formation alternates from ridge to ridge, 

 a mesophytic one from one valley to another. Aquatic vegeta- 

 tion alternates from pond to pond, or from stream to stream. 

 The appearance of new or denuded soils upon which successions 

 are established is the most important source of the alternation of 

 formations. This is true in general of other causes of succession, 

 such as erosion, flooding, burning, and cultivation, especially 

 when they occur in areas physically similar. 



