PLANT STUDIES 
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. 
124. 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 are 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 
