12 THE VEGETATION OF THE UNITED STATES. 



III. MANIFOLD OPERATION OF ENVIRONMENTAL CONDITIONS. 



Our knowledge of the modes in which the nature of the environment 

 may affect the activities of plants, and thereby affect or determine 

 their distribution, is sufficiently great to give a profound impression of 

 the complexity of the problem of the physical control of distribution. 

 The experiences of agriculturists and horticulturists, the work of plant 

 physiologists, and the observations and deductions of ecologists, have 

 all combined to give us a very large body of facts relative to the mani- 

 fold features of the physical environment which may be of critical 

 importance for the spread or survival of particular plants under par- 

 ticular conditions. We are very well informed, both empirically and 

 experimentally, with regard to many of the conditions which are 

 harmful to cultivated crops and the climatic conditions which make it 

 impossible to cultivate them beyond certain well-defined boundaries. 

 A large part of this information relates to the ability or inability of 

 plants to withstand frost or freezing temperatures, and to their ability 

 to survive and grow under given limitations of water-supply. 



Enough has been done in the selection and breeding of economic 

 plants to show that closely related forms may often exhibit great 

 differences in ability to withstand low temperatures, low soil-moisture 

 content, great ranges of soil texture, and the like. Enough is known 

 of the distributional limits of plants which are associated in given 

 regions to indicate that these are not limited by the same sets of condi- 

 tions; they range to different distances and into regions of diverse 

 character in such manner as to indicate that they have very dis- 

 similar distributional controls. 



In spite of the manifold nature of the environmental controls and 

 the well-known diversity among plants with respect to the nature and 

 intensities of the conditions which control them, it is possible, never- 

 theless, to distinguish certain classes of controlling conditions. The 

 broadest line that may be drawn is the one separating the simple or 

 direct factors of a climatic character and those which are not directly 

 attributable to the climate. Later pages will be devoted to an elabora- 

 tion of some of the major types of direct climatic conditions, but little 

 will be said hereafter regarding the non-climatic conditions, such as 

 the nature of the soil, such "biotic factors" as competing plants and 

 preying animals, and such mechanical factors as the influence of wind 

 (in causing mechanical injury), lightning, fire, landslips, inundations, 

 active erosion, and other agencies of very real importance but usually 

 of local or comparatively infrequent occurrence. So much is known 

 regarding the importance of the character of the soil that it is custom- 

 ary to speak of "climatic and soil conditions" as if the two were of co- 

 ordinate importance. The role of the soil in maintaining a water- 

 supply for plants is of vastly greater importance to them than any of 



