INTRODUCTION. 13 



the other roles which it plays. Although the texture of the soil is of 

 prime importance with relation to the penetration, movement, and con- 

 servation of a water-supply for plants, it is fundamentally the climatic 

 elements of rainfall and evaporation that determine what the soils of 

 a given texture are able to do in presenting a moisture-supply of a 

 given amount. We are compelled, therefore, to regard the soil as a 

 medium through which the climate acts upon plants. The supply 

 of moisture to the plant, due primarily to climatic conditions, is 

 secondarily determined by the soil, just as the loss of moisture from 

 the plant is determined through the medium of the atmosphere. The 

 soil is a medium which differs from place to place independently of the 

 climate, while the atmosphere is alike in all places, except in so far as 

 it is directly affected in its movements, temperature, and moisture- 

 content by the primary conditions of climate. 



Such a view of the role of the soil in forming a portion of the environ- 

 ment of plants takes no account of the cases in which the chemical 

 nature of the soil and the amount and character of the salts and other 

 solutes in the soil-water become factors of great moment. In the con- 

 sideration of saline and alkaline areas, and certain limestone and ser- 

 pentine regions, it is necessary to do more than investigate the texture 

 of the soil in its role as a stabilizer of the climatic moisture conditions. 



It has been customary to speak of competition as if it were a distinct 

 condition of elemental character, capable of admitting or excluding a 

 given plant to a given area in much the same manner as that in which 

 a purely climatic condition would operate. The results of competition 

 are registered upon a plant, however, in exactly the same manner as 

 the results of a given climatic condition or set of conditions. Com- 

 petition may exclude light, may restrict water-supply, or may operate 

 in anj^ one of a number of ways. The end-effects upon the processes 

 of the plant are exactly such as might be exerted through climatic 

 agencies, except it be in those cases in which there is an addition of toxic 

 root-excretions to the soil. Even in such cases, the toxic substances 

 act as chemicals and the plants producing them are not directly effec- 

 tive. Competition may be of importance in determining the com- 

 position of small areas of vegetation, but even then the competing 

 plants must be regarded as struggling not with each other, but with 

 physical conditions which are of precisely the same general nature as 

 the conditions due in other places solely to climatic causes. The 

 cases in which plants grow so closely as to exert an effect on the environ- 

 mental conditions are similar to the cases in which the major plants 

 modify the climate for the minor plants. Both of these cases must be 

 left out of consideration in an attempt to determine the larger features 

 of the role of climate in relation to vegetation. 



It is possible to lay down a program for the study of distribution and 

 its controlling conditions, applicable almost equally well to a given 



