FACTORS AFFECTING PLANT DISTRIBUTION 25 



have an effect on the climate of North America in summer. Their 

 action is indicated in the diagram. 



Thus the interplay of air currents plowing into and out of the 

 grand centers of action can be seen to influence greatly all the liv- 

 ing vegetation. Wind, from the standpoint of plants, is a moisture- 

 temperature-relation factor. Increased wind velocity and increased 

 temperatures both accelerate transpiration. In full sunlight the 

 cooling effect of transpiration is the greatest possible benefit to the 

 plant in preventing coagulation of protoplasm. Rapid transpiration 

 probably also aids the plant greatly in the transfer of minerals 

 after absorption from the soil. But rapid transpiration in the ab- 

 sence of equally rapid water absorption is undoubtedly a constant 

 menace to the life and rapid growth of plants. 



2. Soil Factors 



In dealing with the soil factors which influence plant succes- 

 sion, the only measure which we may use for comparing different 

 soils is plant growth. Obviously, it would be incorrect to talk of 

 "fertile soils" or "barren soils" unless we had the climate as well as 

 the soil expressed in some way. We cannot say that a soil will or 

 will not support a dense cover of vegetation if we consider the soil 

 simply inert mineral material or if we base our notion of produc- 

 tivity on the physical structure of the soil alone, or upon the chemi- 

 cal nature of the soil alone. The temperature and the moisture must 

 both be included in our notion of soils. In the arid southwestern 

 regions of the United States the soils contain salts in more than 

 the amounts required for plant growth and the temperatures are 

 high enough to support a subtropical vegetation. Water is, how- 

 ever, the limiting factor. In the sandy barrens of southeastern 

 United States, the rainfall is high, the temperature high, and the 

 growing season long, but mineral salts and available water seem 

 to be the limiting factors. 



The mineral portions of soils formed under arid or humid con- 

 ditions are entirely different. In arid regions the mineral parts of 

 the soil are formed through mechanical processes of weathering or 

 by disintegration. Soluble materials are found in abundance in the 

 soils of arid regions. In humid regions the mineral portion of the 

 soil is the insoluble or only sparingly soluble material, since the 

 most soluble portions have been carried off in the surface streams 

 and ground water. The mineral portion of the soil in a humid re- 



