PLANT ASSOCIATIONS: ECOLOGICAL FACTORS. 163 



Before considering some of the common associations, it 

 is necessary to note some of the conditions that deter- 

 mine plant associations. Those things in the environment 

 of the plant which influence the organization of an associa- 

 tion are known as ecological factors. 



121. Water. — Water is certainly one of the most im- 

 portant conditions in the environment of a plant, and has 

 great influence in determining the organization of associa- 

 tions. If all plants are considered, it will be noted that the 

 amount of water to which they are exposed is exceedingly 

 variable. At one extreme are those plants which are com- 

 pletely submerged ; at the other extreme are those plants 

 of arid regions which can obtain very little water ; and be- 

 tvreen these extremes there is every gradation in the amount 

 of available water. Among the most striking adaptations 

 of plants are those for living in the presence of a great 

 amount of water, and those for guarding against its lack. 



One of the first things to consider in connection with 

 any plant association is the amount of water supply. It is 

 not merely a question of its total annual amount, but of its 

 distribution through the year. Is it supplied somewhat 

 uniformly, or is there alternating flood and drought ? The 

 nature of the water supply is also important. Are there 

 surface channels or subterranean channels, or does the 

 whole supply come in the form of rain and snow which 

 fall upon the area? 



Another important fact to consider in connection with 

 the water supply has to do with the structure of the soil. 

 There is what may be called a water level in soils, and it is 

 important to note the depth of this level beneath the sur- 

 face. In some coils it is very near the surface ; in others, 

 such as sandy soils, it may be some distance beneath the 

 surface. 



Xot only do the amount of water and the depth of the 

 water level help to determine plant associations, but also the 

 substances that the water contains. Two areas may have 

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