CHAPTER XII 

 ECOLOGY OF LEAVES {continued) 



132. Plant Formations. — A little observation is enough to 

 show the beginner in botany that plants are not scattered 

 indiscriminately over the surface of the earth, but that 

 hills, meadows, fresh-water marshes, salt marshes, and many* 

 other kinds of localities have their characteristic assem- 

 blages of plants. Any such group is called & plant formation. 

 It may consist of only a few species, but more commonly 

 comprises several score or even a hundred or more of 

 flowering plants (seed-plants) alone, not to speak of the 

 multitudes of lower forms, such as ferns, mosses, and 

 simpler microscopic plants. 



It will generally be found that the members of a plant 

 formation are growing under what is, for them, nearly the 

 best en\'ironment, since they -cannot usually be made to 

 exchange places with each other. If a square mile of land 

 in Louisiana were to be planted with Minnesota species, 

 and a square mile in Minnesota with Louisiana species, 

 it is very improbable that either tract, if left to itself, 

 would long retain its artificial flora. To this rule there 

 are, however, important exceptions. 



133. Ecological Classification of Plants. — The ordinary 

 classification of plants is based, as far as possible, on their 

 actual relationships to each other. But when plants are 

 classified ecologically they are grouped according to their 



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