PLANT SOCIETIES 311 



Indeed, one of the most useful classifications of bacteria, 

 for practical purposes, is into species which must have free 

 oxygen, that is, oxygen not chemically combined with other 

 substances, in order to grow and increase, and those which 

 can live without it. 



The most important consideration in classifying seed- . 

 plants on ecological grounds is based on their require- / 

 ments in regard to water. Grouped with reference to ' 

 this factor in their life all plants may be classed as : 



(1) Hydrophytes, or water-loving plants. 



(2) Xerophytes, or drought-loving (or perhaps drought-tolerating) 



plants. 



(3) Mesophytes, or plants which thrive best with a moderate supply 



of water. 



These three classes do not fully express all the relations 

 of plants to the water supply, so two others are found 

 convenient. 



(4) Tropophytes, or seasonal plants which are hydrophytes during 



part of the year and xerophytes during another part. 1 



(5) Halophytes, or salt marsh plants and " alkali " plants, species 



which can flourish in a very saline soil. 



383. Difficulties in Ecological Classification. It seems 

 at first sight a simple matter to group plants in regard to 

 their need of water. There can be no difficulty in classi- 

 fying as hydrophytes all plants like the bladderworts, water 

 cresses, certain mosses, and many lower spore-plants which 

 live only in water. Cactuses, aloes, and similar plants are 

 recognized at sight as xerophytes. But the chief difficulty 



1 The plants which E. Warming, one of the foremost authorities, classes as 

 mesophytes are many of them grouped by another great authority, A. F. W. 

 Schimper, as tropophytes. 



