310 FOUNDATIONS OF BOTANY 
native and thoroughly naturalized plants are growing 
under what is, for them, 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 (see Sect. 457). 
381. Plant Formations. — It is not uncommon to find 
tracts of land or water inhabited by great numbers of 
plants of the same species so as almost to exclude all 
other plants except microscopic cryptogams. Ponds and 
slowly flowing streams are often filled in this way with 
the water hyacinth or the American lotus. The cane- 
brakes of the south and the wild rice swamps along north- 
ern lakes and rivers are other examples of an extremely 
simple flora spread over large areas. [Prairies not infre- 
quently for hundreds of square miles are covered mainly 
(not entirely) with a very few kinds of grasses. Such 
assemblages are called plant formations or plant colonies. 
382. Ecological Classification of Plants. — The ordinary 
classification of plants, as set forth in Chapter XIX, 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 relations to the world 
about them. They may, therefore, be gathered into as 
many (or more than as many) different groups as there 
are important factors influencing their modes of life. We 
may classify plants as light-loving and darkness-loving, 
as requiring free oxygen, and not requiring it, and so on. 
