Part 3 • Factors Controlling the 

 Community: the Environment 



Vegetational analysis gives the information necessary to de- 

 scribe and name a community and provides data that can be used 

 to compare it with other communities or with itself after a lapse 

 of time or an experimental treatment. This in itself is worth while, 

 but the ecologist has the added objective of correlating the vege- 

 tational record so obtained with the environment. To interpret the 

 vegetational statistics, and to explain them in terms of cause and 

 effect, leads to an analysis of the environment and its relationships 

 to the community. 



Since the environment consists of many factors interacting upon 

 each other and upon the vegetation, its complexity prohibits con- 

 sideration of it as a whole. The interactions are by no means all 

 clearly understood and the effects of a single factor upon an or- 

 ganism may be inadequately known; therefore, it is logical to ap- 

 proach the subject of environment through individual factors and 

 their effects. With information as complete as possible on the 

 operation of individual factors, explanations may often be found 

 for plant responses among the interactions and effects of a few of 

 the variable factors. The chapters of this section deal successively 

 with climatic, physiographic, and biological factors as each may 

 operate in the complex of factors termed environment. 



CHAPTER V 



CLIMATIC FACTORS: THE AIR 



GASES OF THE ATMOSPHERE 



The air surrounding the earth is made up of only a few gases in 



proportions that remain remarkably constant. The average volume 



percentages of dry air are : nitrogen, 78.09; oxygen, 20.95; carbon 



dioxide, 0.03; and argon, 0.93. In addition, there are minute but 



75 



