810 rOtnSTDATIONS OF BOTAinr 



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 iind 

 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. 



