CHAPTER XI. 



PLANT SOCIETIES : ECOLOGICAL FACTORS. 



120. Definition of plant society. 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, therefore, 

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

 plex 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 ref- 

 erence 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 association of plants living together 

 in similar conditions is a plant society, the conditions for- 

 bidding other plants. It must not be understood that all 

 plants affecting the same conditions will be found living 

 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. 



Very closely related plants generally do not live in the 

 same society, as their rivalry is apt to be intense. Closely 

 related plants are likely to occur, however, in different 

 societies of the same type. A plant society, therefore, may 

 contain a wide representation of the plant kingdom, from 

 plants of low rank to those of high rank. 



