PLANT SOCIETIES: ECOLOGICAL FACTOKS. 163 



Before considering some of the common societies, it is 

 necessary to note some of the conditions which determine 

 plant societies. Those things in the environment of the 

 plant which influence the organization of a society 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 societies. 

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

 of water to which they arc exposed is exceedingly variable. 

 At one extreme are those plants which are completely 

 submerged; at the other extreme are those plants of arid 

 regions which can obtain very little water ; and between 

 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 society 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 soils it is very near the surface ; in others, 

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

 surface. 



Not only do the amount of water and the depth of the 

 water level help to determine plant societies, but also the 

 substances which the water contains. Two areas may have 



