228 



PLANT PHYSIOLOGY AND ECOLOGY 



families have been invaded by plants of another species. A large 

 number of prairie and meadow species occur in more or less definite 

 communities, e.g.,Anemo}ie, Draba, Lithospermum, Rosa, Laciniaria, 

 etc. Communities appear in nearly all formations, but they are 

 most abundant in the open ones which develop in new or denuded 

 habitats. 



242. The family is a group made up entirely of individuals 

 belonging to a single species. In many instances it springs from 

 a single parent plant, but this is not necessarily the case. In size 

 a family may consist of a few individuals or it may cover a large 

 area. It passes into a community as soon as one or more individ- 



FiG. 88. A Senecio family in the rock clefts on Pike's Peak. 



uals of another species enter it. In consequence, families are 

 usually small, since they are readily invaded when large. Species 

 whose seeds are numerous and little or not at all modified for 

 dissemination often form families. For this reason annuals 

 are found to form families much more frequently than perennials 

 do. Families are more or less characteristic of new or denuded 

 soils, in which each pioneer usually serves as a center for its off- 

 spring. As the individuals become more and more numerous, 

 they invade the neighboring families formed by other species 

 and change them into communities. In the ordinary vegetation 

 of forest and grassland, families are relatively rare, since they are 

 readily turned into communities by the movement of their crowded 



