294 PLANT PHYSIOLOGY AND ECOLOGY 



On some it is as abundant as the facies, while on others it is repre- 

 sented by a few scattered individuals. Such alternation is much 

 more striking in separate examples of the same formation, par- 

 ticularly when the abundance of a facies is normal in one and 

 extremely reduced in another. This is a matter of much impor- 

 tance in the study of formations, since a separate consocies may 

 otherwise be mistaken for the formation itself. 



319. Corresponsive alternation. Owing to the accidents of 

 migration and competition, similar areas within a habitat are 

 not always occupied by the same species or group of species. A 

 species found in one area may be replaced in another by a differ- 

 ent one of the same or another genus. Such genera and species 

 are termed corresponding. They must be essentially alike in 

 habitat form, i.e., in response to the habitat, though they may 

 be entirely unrelated systematically. A good example of corre- 

 sponding alternation is found upon exposed sandy crests of the 

 prairie formation where Loniatium occurs upon some, but is repre- 

 sented upon others by Comandra or Pentstemon. 



Experiment 71. Alternation of species. Select several species of a 

 heterogeneous formation, such as a hilly prairie, for study. Note the 

 places at which each species occurs. From the physical nature and the 

 location of the various places, indicate whether the alternation of each 

 species is due to ecesis and competition or to accidents of migration. 



320. Zonation. Zonation is the common response of plants 

 to the way in which physical factors are distributed through a 

 habitat or a series of them. In nearly all habitats one or more 

 of the physical factors present decrease gradually in passing 

 away from the point of greatest intensity. The result is that 

 the plants of the habitat arrange themselves in more or less defi- 

 nite belts about this point, their position being determined by 

 their relation to the factor concerned. j\Iost formations show 

 zonation of some kind, though the zones are often incomplete or 

 obscure through various causes. In many cases, when the gen- 

 eral structure of the formation reveals no zones, it will be found 

 that some of the species are arranged zonally. As a rule, zona- 

 tion is more characteristic of vegetation, as a whole, than of the 

 formation. This is seen in the zones of continents, though these 

 are often interrupted, owing to climatic and physiographic differ- 

 ences, so that they are not always continuous. 



