THE RELATION OF DESERT PLANTS TO SOIL MOISTURE 



AND TO EVAPORATION. 



INTRODUCTION. 



Every observer of desert vegetation has had his attention drawn to 

 the question of how certain plants of the arid regions are able to main- 

 tain a more or less active transpiration during long periods of absolute 

 lack of precipitation, when the soil in which they are rooted becomes 

 not only apparently air-dry but also attains exceedingly high tempera- 

 tures. It seemed that careful quantitative studies of the moisture con- 

 ditions in desert soil and desert atmosphere, and of the relation of these 

 conditions to the transpiration and life of desert plants, might throw 

 considerable light not only upon this problem of extreme xerophytism, 

 but also upon the limitations of plant life in general. Just as the alpine 

 summits of high mountains in all parts of the earth and the frozen 

 tundras of the arctic regions exhibit vegetable life under temperature 

 conditions which almost render it impossible, so the arid desert with its 

 centimeters of annual rainfall and its meters of annual evaporation 

 exhibits plant life under conditions of extreme dryness which similarly 

 approach a limit to the very existence of such life. It is thus plausible to 

 suppose that certain fundamental truths regarding the vital activities 

 of plants may be more advantageously studied in the case of organisms 

 existing under these extreme conditions than by confining attention to 

 what are considered the more normal circumstances of life and growth. 



With the aid of a grant from the Carnegie Institution of Washington 

 the writer was able to spend the summer of 1904 at the Desert Botanical 

 Laboratory of that Institution at Tucson, Arizona, in carrying out a 

 series of quantitative studies on desert plants. The results of these 

 studies are embodied in the present paper. 



Thanks are due to Prof. Frederic V. Coville and Dr. D. T. MacDougal, 

 who constituted the Advisory Committee of the Laboratory when this 

 work was done, as well as to Dr. W. A. Cannon, resident investigator, 

 for the excellent facilities provided at the Laboratory, without which 

 the work could not have been carried out. Mrs. Grace Johnson Liv- 

 ingston has rendered very valuable assistance in the preparation of this 

 paper, especially in the tabulating of the data and in the construction of 

 the curves. 



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