CHAPTER ANI 
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- 
cicties or plant communities. For example, a meadow isa 
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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