CHAPTER XII 
ECOLOGY OF LEAVES (continued) 
132. Plant Formations.—A little observation is enoughto 
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 a 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 environment, 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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