PLANT ASSOCIATIONS: ECOLOGICAL FACTORS 175 



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 regarded 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 associations, for they may be as 

 numerous as are the combinations of these factors. 



129. The great groups of associations. — It is possible to 

 reduce the very numerous associations to three or four 

 great groups. For convenience, the water factor is chiefly 

 used for this classification. It results in a convenient 

 classification, but one that is certainly more or less arti- 

 ficial. 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 asso- 

 ciations, as follows : 



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

 suggests that such associations 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 " drouth plants," and 

 suggests the other extreme of the waiter supply. True 

 xerophytes are exposed to dry soil and dry atmosphere. 



(3) Mesophytes. — 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 



