480 PLANT ASSOCIATIONS 



distribution of such plants as those just mentioned, but it is 

 one factor, at any rate, and it is certain that, on the whole, most 

 of our native and thoroughly naturalized plants are growing 

 under what is, for them, the best environment, since they can- 

 not usually be made to exchange places with one another. If a 

 square mile of land in Louisiana were to be planted with Min- 

 nesota 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. 



458. Associations of few species. It is not uncommon to find 

 tracts of land or water inhabited by great numbers of seed 

 plants of the same species, so as almost to exclude all other vege- 

 tation except microscopic spore plants. Ponds and slowly flow- 

 ing streams are often filled in this way with the water hyacinth, 1 

 the water cress, or the American lotus. 2 The canebrakes of the 

 South and the wild rice swamps along northern lakes and rivers 

 are other examples of an extremely simple flora spread over 

 large areas. Prairies not infrequently for many square miles 

 are covered mainly (not entirely) with a very few kinds of 

 grasses. The arid plains of the Rocky Mountain region, over 

 thousands of square miles, contain little vegetation except sage- 

 brush (Artemisia tridentata), and immense tracts of snow in 

 the arctic regions are destitute of plant life except for the 

 red-snow alga (Sphcerella nivalis, Sec. 215), by which they are 

 colored pink. 



In all such cases it is evident that the single species or the 

 few species which populate the area can endure the conditions 

 of existence there so well that other plants which migrate into 

 their territory cannot compete with them. 



1 Eichhornia. a Nelumbo. 



