Distribution of Plants in Nature 305 



on the surface of the ground, and others below the surface. All 

 these plants are in one way or another affected by the other plants, 

 and so we may very properly speak of the forest as an organization. 



When we study forests still further, we find that there are many 

 kinds in North America, and that when a particular kind of tree 

 is dominant a definite group of low trees, shrubs, and herbs usu- 

 ally grow with these dominant trees, and that when another kind 

 is dominant, a different set of plants make up the undergrowth. 



Plant associations. Even a brief study of a mature forest, 

 such as we have just described, brings out clearly the fact that 

 plants in nature do not live as isolated individuals, but in com- 

 munities, more or less definitely organized. The organization, 

 to be sure, develops gradually through the carrying of seeds into 

 the area, and through the elimination of those species of plants 

 that cannot endure the environment. Each year some new plants 

 are starting and others are dying ; the population is continually 

 changing, but in an orderly w^ay which is determined by the 

 conditions in the community and the kinds of plants whose 

 seed is carried into it. Such communities of plants are called 

 plant associations. 



All the plants in one association have somewhat similar re- 

 quirements, but usually each plant differs from the others in 

 some one or more requirements. For example, some are shallow- 

 rooted, others are deep-rooted ; some complete their growth, 

 flower, and produce seed in the spring ; others require a longer 

 growing season and flower in the summer or autumn ; some re- 

 quire full sunlight, while others need partial shade. Plants that 

 are associated, then, are species that have certain water, soil, 

 fight, or temperature requirements in common, and are enough 

 different in other respects not to interfere materially with one an- 

 other. In the naming of associations, we use the name of the 

 plants that are most prominent and that dominate the community 

 {dominant species) . The other, less prominent plants are spoken 

 of as secondary species of the association. 



