The following paper, by Frederic E. Clements, was read by D. T. 

 MacDougal : 



The Real Factors in Acclimatization. 



BY FREDERIC E. CLEMENTS, 

 Department of Botany, University of Michigan, 



The exact study of plant environments by means of instru- 

 ments, which has been carried on in the Colorado mountains 

 for the past ten years, has thrown light upon a number of cur- 

 rent ideas as to acclimatization and hardiness. It is hoped that 

 a brief statement of some of the results obtained in nature may 

 prove of interest and value to the scientific horticulturist actually 

 engaged in acclimatizing plants. 



The advantages of a mountain region for studies of ac- 

 climatization are decisive. On Pike's Peak, for example, as 

 many climates are found in ten miles as occur between the for- 

 tieth and seventy-fifth parallels, a distance of 3,000 miles. The 

 climatic zones of the mountains are further broken up into a 

 great number of local climates, owing to the extremely irregu- 

 lar surface and the varying maturity of the different land areas. 

 The result is a diversity of climates not to be found elsewhere, 

 and an accessibility to different climates that is unique. 



In experimenting to determine the causes of the well-known 

 dwarfing of alpine plants, it was found that the influence of 

 light is negligible, and that water, in the form of soil water, 

 is more important than temperature. Since dwarfing is merely 

 the characteristic sign of alpine adjustment, i. e., acclimatization, 

 it seems evident that for all dry regions at least, water, and not 

 temperature, must be regarded as the controlling factor in ac- 

 climatization, and hence in hardiness and winter-killing. This 

 conclusion has been repeatedly tested with the alpine shrubs and 

 trees at timber line. On many of the Colorado peaks, as else- 

 where, the timber line is fringed with a zone of thickets, chiefly 

 willows and birches. These regularly reach their highest alti- 

 tudes in moist depressions, especially where the exposure to bit- 



