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. 
(5) 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 durin 
popny p yarop § 
part of the year and xerophytes during another part. 
5) Halophytes, or salt marsh plants and “alkali” plants, species 
phy I p P 
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. 
