THE ORIGINATION OF XEROPHYTISM 



, D. T. MacDOUGAL and H. A. SPOEHR 



The Desert Laboratory, Tucson, Arizona 



The part played by aridity as an evolutionary factor in the 

 derivation of land forms is well recognized by students of phy- 

 logeny, and the general changes in morphology which ensue in 

 plants which move into areas with a lessened water supply 

 have been described in a systematic manner by Bower, ^ while a 

 discussion of certain features of the subject was presented by 

 one of the authors in this Journal in 1909.2 



It is clearly recognized that only plants showing specialized 

 habits coupled with well defined anatomical features may con- 

 tinue to exist in places having pronounced desert conditions. 

 Furthermore, the idea that aridity stands in a causal relation 

 to the characters of desert vegetation bulks large in determining 

 our thinking of thdie forms. 



It is to be seen however that the matter has been dealt with 

 hitherto as if the effect of aridity did not make an impression 

 upon the plant, its living matter, or accessory structures di- 

 rectly. The combined effects of rapid evaporation and unde- 

 veloped drainage in desert regions has resulted in the accumula- 

 tion of a greater proportion of salts in the soils than in well 

 watered and freely drained soils, and the strand habitats of the 

 sea-shore succulents are also high in salts, and this has caused 

 attention to be directed to the possible effects of these substances 

 in inducing the succulence of plants both on strands and in the 

 desert. The formation of the spiny plants of the desert was also 

 attributed to the possible osmotic action of these substances. 

 Meanwhile the senior author has consistently looked to the direct 



1 Bower, F. O. Origin of a land flora. 1908. 



2 MacDougal, D. T. Influence of aridity upon the evolutionary development 

 of plants. The Plant World 12: 217. 1909. 



245 



THE PLANT WORLD, VOL. 21, NO. 10 

 OCTOBER, 1918 



