CHIEF ENVIRONMENTAL CONDITIONS AND THE GENERAL 

 NATURE OF THEIR EFFECTS UPON PLANTS. 



I. GENERAL CLASSIFICATION OF ENVIRONMENTAL FACTORS. 



The enwonmental conditions that are commonly most potent in 

 the determination of plant development, and that therefore appear 

 most important in distribution, may be classified under the following 

 headings: (1) Moisture conditions; (2) temperature conditions; 

 (3) light conditions; (4) chemical conditions; (5) mechanical conditions. 

 The present section will be devoted to a brief summary of the nature 

 and effects of these factors as they vary in quality, intensity, and dura- 

 tion. No attempt is made to denote more by the order of items in 

 the above list than a very general estimate of the relative importance 

 of the various factors as they are usually operative in limiting plant 

 distribution. There is, of course, nothing to be gained in a discussion 

 of the relative importance of a number of factors, all of which are 

 necessary in order that a given phenomenon may occur. Such con- 

 sideration were as bootless as a discussion of the relative importance 

 of the hub, spokes, felloes, and tire of a wheel. The reader is there- 

 fore requested to make nothing of our order of arrangement. 



Students of ecological plant distribution have usually classified these 

 sets of conditions according to their origin or source, rather than 

 according to their mode of physically affecting the plant. Thus, the 

 literature contains many references to climatic and edaphic condi- 

 tions, physical and biotic ones, and the like. Such groupings seem, 

 however, not to have led much farther than to the mere description 

 and arbitrary classification of distribution conditions, and, since by 

 their very nature they point to the causes of the factors imme- 

 diately involved rather than to the real nature of these factors or their 

 effects upon the plant, they promise little for our present purpose. 

 Thus, shade produced by a natural rock arch or overhanging cliff may 

 be impossible of physical or physiological differentiation from that 

 produced by a tree ; j^et the former is said to be a physical factor and 

 the latter a biotic one. Again, the mechanical relation of the physical 

 separation of plant and soil, together with the accompanying ruptures 

 and lesions of the plant tissues, might arise equally well from the 

 action of animals (a biotic factor) and from that of wind or torrential 

 water (undoubtedly physical factors). In a study of the ultimate 

 causes that bring the proximate, effective factors into being, such 

 classification has its value, but in such studies we have assuredly left 

 our field of plant distribution for the adjoining one of climatology, 

 physiography, and the rest. In the beginning it appears more promis- 



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