176 PLANT STUDIES 
phytes, the plants of the middle region. It is evident that 
mesophytes gradually pass into hydrophytes on the one 
side, and into xerophytes on the other ; but it is also evi- 
dent that mesophyte societies have the greatest range of 
water supply, extending from a large amount of water to 
a very small amount. 
It should be understood that these three groups of socie- 
ties, which are distinguished from one another by the amount 
of the water supply, are artificial groups rather than natural 
ones, for they bring together unrelated societies, and often 
separate those that are closely related. For example, a 
swampy meadow is put among hydrophyte societies by this 
classification ; and it may shade into an ordinary meadow, 
which belongs among the mesophytes. Probably the largest 
fact which may be used in grouping plant societies is that 
certain societies are so situated that they seek for the most 
part to reduce transpiration, and that others are so situated 
that they seek for the most part to increase transpiration. 
However, the factors which determine societies are so 
numerous that they cannot be presented in an elementary 
book, and the simpler artificial grouping given above will 
serve to introduce the societies to observation. 
Upon a different basis another great group of societies 
has been suggested as follows : 
(4) Halophytes. The word means " salt plants." The 
basis of classification here depends not so much upon the 
water supply as upon the fact that the water contains 
certain, salts which make it impossible for most plants to 
live. Such societies may be found near the sea-coast, 
around salt springs, on alkaline flats, or wherever the soil 
contains these characteristic salts. 
