INTRODUCTION 11 



plants are concerned, this is, however, an uninvestigated 

 subject, one which offers many attractions to the physiolo- 

 gist.* 



* In the year book of the U. S. Dept. of Agriculture, 1897, Whitney re- 

 ports (p. 129) a comparison of the soils and subsoils of the eastern and 

 western States. From this he draws the conclusion that the absence of a 

 heavy subsoil, and the spontaneous formation of a "mulch" of dust on 

 the surface, cause the plants of arid regions, and those living where there 

 is a long dry season, to secure for their needs a much larger percentage of 

 the total annual rainfall than is available for eastern plants. In the East, 

 the heavy subsoil drains off about half of the total rainfall, and the re- 

 mainder is still further reduced by evaporation. But though all this is un- 

 doubtedly true, yet the perennial plants themselves must also show adap- 

 tations to the climatic conditions of the far West and of California, and 

 these deserve study by physiologists. 



