CHAPTER XXI 



PLANT ASSOCIATIONS 



186. Definition. The earth's surface presents such di- 

 verse conditions for plant life that plants become grouped 

 according to the conditions favorable for their growth. 

 These groups of plants, living together in similar condi- 

 tions, are called plant associations, or sometimes plant so- 

 cieties or plant communities. For example, a meadow is a 

 plant association growing in conditions that favor certain 

 grasses; a forest is an association growing where certain 

 trees are favored, etc. In these associations grasses and 

 trees are simply the conspicuous types; but numerous other 

 plants, which the same conditions favor, are associated with 

 them. Each plant association, therefore, indicates a special 

 set of conditions for plant growth, and to discover these 

 conditions is a very important kind of field work. 



187. Water. Water is probably the most important con- 

 dition that determines plant associations. The available 

 amount of water for plants varies in different areas, from 

 the very small supply in deserts to the abundant supply in 

 swamps and lakes. The character of the soil has a very 

 important effect upon water-supply; for some soils retain 

 water and others do not, so that what is called the water- 

 level is of varying depths ( 39). Not only are the amount 

 of water and the depth of the water-level important, but 

 also the substances that the water contains in solution, 

 which may prevent certain plants from growing and permit 

 others. 



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