Chapter X 

 ENVIRONMENTAL FACTORS 



We have now dealt sufficiently with special kinds and systematic 

 groups of plants and their distributions, and must proceed to 

 consider the results of their natural aggregation, namely, vegetation. 

 As a basis for this we must deal in the present chapter with ecological 

 factors and, in the next chapter, with the main habitats these factors 

 constitute. In addition there will be considered in Chapter XI 

 certain fundamental tendencies and attributes of vegetation whose 

 recognition is essential to the full understanding of vegetational 

 changes and types. 



Ecology is the study of the mutual relations among organisms and 

 between them and their environment — environment being the aggre- 

 gate of all external conditions and influences aff^ecting life and 

 development at a given spot. Ecological or environmental factors 

 are many and diverse, and often intricately mixed and interdependent. 

 Either singly or in combination, the various ecological factors may 

 affect the presence or absence, vigour or weakness, and relative 

 success or failure of various plant communities through their 

 component taxa. Although the subject is enormously complex, the 

 immediate vehicles of influence are very few% being chiefly food, 

 light, temperature, water, and dissolved substances ; these are 

 affected by variations in the ecological factors which characterize 

 different habitats and consequently lead to the differentiation of 

 vegetational types. 



The four main classes of ecological factors with which we shall 

 deal below — namely, climatic, physiographic (of topography, etc.), 

 edaphic (of soil), and biotic (due to living organisms) — are them- 

 selves commonly interrelated and intricately mixed. Often they 

 work through one another, acting and reacting together, as in the 

 case of physiographic changes which bring about local climates that 

 in turn may affect the soils and competition-impress. Accordingly 

 this classification, like so many other biological ones, is to some 

 extent artificial. Yet all categories affect the plant either directly 

 or indirectlv through modification of its reactions, bringing about 



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