114 ELEMENTS OF BOTANY 
relations to the world about them. They may, therefore, 
be gathered into as many (or more than as many) different 
groups as there are important factors influencing their 
modes of life. We may classify plants as light-loving and 
darkness-loving, as requiring free oxygen and not requir- 
ing it, and so on. 
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) Hydrophiytes, or water-loving plants. 
(2) NXerophytes, 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.? 
(5) Halophytes, or salt-marsh plants and “alkali” plants, species 
which can flourish in a very saline soil. 
/134. Leaves in Relation to Ecological Classes. — Although 
the roots and stems of plants which belong to extremely 
specialized ecological types offer many modifications which 
adapt them to the kind of life which they have to lead, yet 
the leaves are still more important in their adaptations. 
A good botanist can often decide merely by looking at the 
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. 
