PLANT ASSOCIATIONS 279 



do not commit race suicide, and the deciduous (hard-wood) 

 trees are not favored by the winter rains and dry summers. 



154. Some conspicuous plant associations. It has be- 

 come customary to group all plant associations under three 

 heads, based on the water-supply of the habitat. Hydro- 

 phytes (" water plants ") are plants that grow in water or 

 in very wet soil, and their associations are " hydrophytic 

 associations." Xerophytes (" dry plants ") are plants that 

 grow in conditions of scanty water-supply, and their associa- 

 tions are " xerophytic associations." Mesophytes (" medium 

 plants ") are plants that grow in conditions of medium 

 water-supply, and their associations are " mesophytic asso- 

 ciations." 



It should be noted that the mesophytic conditions are 

 those used for cultivating plants, such land being said to be 

 " arable." If the conditions are hydrophytic, the land is 

 drained before cultivation ; if the conditions are xerophytic 

 and there is sufficient soil accumulation, the land is irrigated. 

 The same processes are going on in nature. If a succession 

 of societies starts in a pond or swamp (hydrophytic), the 

 conditions gradually become more and more mesophytic, 

 until finally mesophytic associations appear. If a succes- 

 sion starts on a bare rock or sand (xerophytic) , through the 

 accumulation of humus and the increase of shading, the con- 

 ditions gradually become more and more mesophytic, until 

 finally mesophytic associations appear. This means that 

 where conditions are not mesophytic already, the natural 

 succession of societies tends to make them so. 



From what has been said of plant associations, it is evident 

 that they are far too numerous to permit in this connection 

 a description of even the most conspicuous ones. The best 

 that can be done is to select from among the most con- 

 spicuous associations a few types that will emphasize what 

 is meant by plant associations. It must be understood that 

 this is not to be a study of these associations, for such study 



