PLANT SOCIETIES: ECOLOGICAL FACTORS. 169 



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. 



