54° THE POPULAR SCIENCE MONTHLY 



THE WATER RELATIONS OF DESERT PLANTS 



By Dr. D. T. MacDOUGAL 



DESERT BOTANICAL LABORATORY, TUCSON, ARIZ. 



WHILE working as a student in the laboratory of Professor 0. P. 

 Jenkins at DePauw University twenty-three years ago, a type- 

 written schedule of experiments in plant physiology by Professor J. C. 

 Arthur was placed in my hands as a guide in some practical work that 

 was to extend throughout the collegiate year. The program in question 

 probably constituted the first attempt of its kind in an American school, 

 and its series of demonstrations may be taken to represent with fair 

 accuracy the concepts and assumptions which might be safely presented 

 to a student at that time. 



Sachs and his students had made contributions of immense im- 

 portance in growth, organogeny, irritability and tropisms in general, 

 but the first serious efforts at analysis of the physical phenomena under- 

 lying the action of organisms may be assigned to Pfeffer and DeVries. 

 Pfeffer established the principles of osmosis by a study of the behavior 

 of crystalloidal substances toward membranes, the results of which 

 were published in 1877, and in the same year DeVries brought out his 

 contributions on turgidity. To the latter we owe the first systematic 

 analysis of turgor, and of the mechanism by which the rigidity and 

 firmness of soft-bodied organisms are maintained and by which move- 

 ments are executed. The plasmolytic method for the detection of the 

 differential action of substances and membranes, and the establish- 

 ment of the principle of isotonic coefficients were also the work of 

 DeVries. Both of these authors were intent on finding the solution of 

 problems in plant physiology, in which they were notably successful, 

 but their results form the basis of the dissociation theory of Arrhenius, 

 and theory of pressure in solutions of van't Hoff, which together may be 

 regarded as the basis of modern physics and chemistry. 



It seems highly characteristic of research in plant physiology that 

 devotion to many of its problems may lead the student far afield from 

 botany, or the stricter domain of biology. The worker in this subject 

 frequently finds it necessary to build cantilever bridges across chasms 

 which yawn in front of him to find that the farther ends of his spans 

 comes down to the solid ground of chemistry, physics, climatology or 

 geology. At present, however, he has come upon rifts which he can 

 not cross without aid from the farther side. 



