168 PLANT RELATIONS. 



the plants have been compelled to adapt themselves to this 

 loss of water ; and in some regions the prevailing winds are 

 so constant and violent that the force of the wind itself has 

 influenced the appearance of the vegetation, giving what is 

 called a characteristic physiognomy to the area. 



These five factors have been selected from a much larger 

 number that might be enumerated, but they may be re- 

 garded as among the most important ones. It will be 

 noticed that these factors may be combined in all sorts 

 of ways, so that an almost endless series of combinations 

 seems to be possible. This will give some idea as to the 

 possible number of plant societies, for they may be as 

 numerous as are the combinations of these factors. 



126. The great groups of societies. It is possible to re- 

 duce the very numerous societies to three or four great 

 groups. For convenience, the water factor is chiefly used 

 for this classification. It results in a convenient classifica- 

 tion, but one that is probably more or less artificial. The 

 selection of any one factor from among the many for the 

 purpose of classification never results in a very natural 

 classification when the combination of factors determines 

 the group. However, for general purposes, the usual 

 classification on the basis of water supply will be used. 

 On this basis there are three great groups of societies, 

 as follows : 



(1) Hydrophytes. The name means "water plants," and 

 suggests that such societies are at that extreme of the water 

 supply where it is very abundant. Such plants may grow 

 in the water, or in very wet soil, but in any event they are 

 exposed to a large amount of water. 



(2) Xerophytes. The name means "drought plants," 

 and suggests the other extreme of the water supply. True 

 xerophytes are exposed to dry soil and dry atmosphere. 



(3) Mesopliytes. Between the two extremes of the water 

 supply there is a great middle region of medium water 

 supply, and plants which occupy it are known as meso- 



