CHAPTER XXII 



PLANT ASSOCIATIONS 



In the previous chapters plants have been considered as sep- 

 arate individuals or as groups of closely related individuals 

 known as species. In nature, however, each plant is a member 

 of some plant community, living together with other plants which 

 vitally affect its growth and development. The student will also 

 find upon investigation that the vegetation of each region is not 

 uniform in character, but is made up of smaller units, such as 

 meadow, pond, and cultivated field and lawn, each of which is 

 composed of plants requiring similar conditions of soil and cli- 

 mate for their best development. It will be found that these 

 various plant communities, known as plant associations, although 

 they seem to the uncritical observer to be stable, are never- 

 theless constantly changing, owing to competition among the 

 existing members, to the advent of new species by migration, or 

 to changes in the earth's surface caused by fires, floods, and 

 other agencies, which have a far-reaching effect on vegetation. 



Kinds of plant associations. The most general classification 

 of plant associations is that mentioned in the first part of the 

 text, namely, the mesophytic (Fig. 282), Kydrophytic (Fig. 286), 

 and xerophytic (Fig. 71) plant associations, based upon the avail- 

 able water in the soil. To this general classification a fourth 

 should be added, to include the halophytic plant associations, 

 which inhabit salt marshes and ponds where the amount of 

 various salts in the water is excessive. These larger associations 

 are usually subdivided into smaller ones, which reflect more 

 accurately the particular environmental conditions prevailing in 

 particular habitats. 



Thus, the hydrophytic association comprises smaller pond, lake, 

 stream, and swamp associations, which differ either in the kinds 



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