114 ESSENTIALS "OF BOTANY 



relations to the world about them. They may, therefore, 

 he 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 tlieir require- 

 ments in regard to water. Grouped with reference to this 

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



.(1) i/^rfrojO%(es, or water -loving plants. 



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



plants. 



(3) Mesophytes, or plants whioh 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) Halophyles, 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. 



