THE PLANT AND ITS ENVIRONMENT 



169 



thrive on saline soil where the majority of plants are quite unable 

 to exist. Others grow only where the soil is markedly acid, 

 and still others only where lime is abundant. Some ubiquitous 

 types are able to exist in soils of almost any chemical composition. 

 The distribution of plant species over the earth's surface is influ- 

 enced to no small degree by their reactions to particular chemical 

 substances in their environment. 



A number of species may be all adapted in a similar way to 

 these various physical conditions of the environment which we 



Fig. 87. — Galls produced by the attack of a fungus {Gymnosporangium) on 

 red cedar. 



have just been discussing and may thus grow side by side together, 

 forming an easily recognizable group. Such a group is known as 

 a Plant Association. We may distinguish, for example, a Swamp 

 Association (Fig. 88), a Desert Association (Fig. 89), a Meso- 

 phytic Forest Association, a Sea Beach Association, and many 

 others, each with its characteristic species and its characteristic 

 structural and functional modifications. As the environment 

 changes, one association may encroach upon another and the 

 vegetation of a region may thus be gradually altered. A study 

 of plant associations and their history is an important part of the 

 science of ecology. 



