CHAP, iv PLAN OF THIS BOOK 15 



dioxide in the air of forests and of open land there is no difference. 1 The 

 composition of the atmosphere is therefore devoid of geographical signifi- 

 cance. 



Edaphic factors are treated in Chapters IX-XVII as follows : 



Chapter IX. The nutrient substratum its constitution. 



X. Its structure. 



XI. Its air. 



XII. Its water. 



XIII. Its temperature. 



XIV. Its dimensions. 

 XV. Its nutriment. 



XVI. The kinds of soil. 



XVII. The problem as to the chemical or physical 

 action of soil. 



Indirect factors are 



The contour of the Earth's surface, configuration of land and sea, 

 altitude, latitude, and other active and modifying factors. These are 

 treated as follows in Chapters XVIII-XXI : 



Chapter XVIII. The influence of a non-living covering over 



vegetation. 

 XIX. The influence of a living vegetable covering on 



soil. 



XX. The activity of animals and plants in the soil. 



XXI. Exposure. Orographic, and other factors. 



The atmospheric factors coincide approximately with Drude's 'geo- 

 graphically operating ' factors, because they are usually almost constant 

 over large parts of the Earth's surface. Consequently these are 

 the ones that more than all others make their impress upon the 

 vegetation of a country, for all plant communities are subject to their 

 influence. Edaphic and indirect factors, on the contrary, determine the 

 topographical differences in the vegetation. 2 



1 Hann, 1897, vol. i, p. 76. 



1 Further particulars are to be found in the works by Sachsse, Deherain, Vajlot, 

 Ramann, Drude, Grabner, Schimper, and Clements. 



