48 



TEXTBOOK OF BOTANY 



Fig. 26. One way of destroying a valuable heritage. The fertile top soil that 

 was formed during thousands of years has been eroded from the hill in the center 

 of the picture through bad farm practices. Photo by G. S. Growl. 



cal analysis, but also to an understanding of their economic, esthetic, and 

 recreational possibilities. For the beginner those interests and appeals 

 are most valuable which come to him through his own observations. 



Summary. The vegetation of any soil area is a mixed population of 

 several kinds of plants, some of which are readily visible, others micro- 

 scopic in size. The relative abundance of the different kinds of plants in 

 the mixed population, and also the kinds or species of plants present in 

 it, vary from place to place. These variations in the composition of plant 

 populations result in the formation of many different kinds of plant com- 

 munities, which we may recognize and name on the basis of the more 

 abundant or dominant species. The composition and distribution of these 

 communities are dependent upon the influence of the plants upon each 

 other, upon the animals that are present, and upon the factors of soil 

 and climate. Owing to all these factors, the composition of a plant com- 

 munity on a given area changes in time. This change may be rather 

 rapid or almost imperceptible, but it finally leads to one community 

 being succeeded by another. The change in composition may consist 

 only of changes in the relative abundance of the species already present 

 in the community, or it may be due in part to the invasion of the com- 



