CHAPTER XL 



PLANT ASSOCIATIONS: ECOLOGICAL FACTORS. 



120. Definition of a plant association. — From the previous 

 chapters it has been learned that every complex plant is a. 

 combination of organs, and that each organ is related in 

 some special way to its environment. It follows, there- 

 fore, that the whole plant, made up of organs, holds a very 

 complex relation with its environment. The stem demands 

 certain things, the root other things, and the leaves still 

 others. To satisfy all of these demands, so far as possible, 

 the whole plant is delicately adjusted. 



The earth's surface presents very diverse conditions in 

 reference to plant life, and as plants are grouped according 

 to these conditions, this leads to definite associations of 

 plants, those adapted to the same general conditions being 

 apt to live together. Such an assemblage of plants living 

 together in similar conditions is a plant association, the con- 

 dition's forbidding other plants. It must not be understood 

 that all plants affecting the same conditions will be found liv- 

 ing together. For example, a meadow of a certain type will 

 not contain all the kinds of grasses associated with that type. 

 Certain grasses will be found in one meadow, and other 

 grasses will be found in other meadows of the same type. 



The rivalry of closely related plants living in the same 

 association is apt to be intense, on account of their similar 

 demands, and unrelated plants are able to live together with 

 the least rivalry. A plant association, therefore, may con- 

 tain a wide representation of the plant kingdom, from 

 plants of low rank to those of high rank. 

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