PLANT SOCIETIES: ECOLOGICAL FACTORS 
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. 
129. 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) 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 meso- 
