256 THE PLANT WORI.D 



water and easily detachable from the plant. In this instance a large 

 number of animals make use of the leaves as a water supply, eating both 

 leaves attached to the branches and those which have fallen to the ground. 



VII. Species adapted to soils containing large proportions of soluble 

 salts. — Great areas in deserts have soils from which the soluble salts are 

 not washed by reason of lack of rainfall, and in certain places around saline 

 springs and alkaline pools the concentration of the salts is extremely high. 

 Many of the plants capable of living in soils of this character show the 

 same structural adaptations as those of the saline districts near the sea- 

 shore, being truly halophytic in character. The stems and the leaves of 

 some species are succulent and are brittle, being easily broken into frag- 

 ments. A good illustration of the features of halophytes may be obtained 

 by a comparison of the two eastern species of Tissa, one of which, 

 T. marina, is an inhabitant of sandy salt-saturated soils and the other is 

 found on arid ridges and on dry hills. Quite a number of forms are 

 characteristic of the saline and alkaline regions in western deserts, among 

 which may be mentioned the salt-bushes {,Atriplex) and Allenrolfea, 

 yvhile: /uncus Cooperi may be cited as the case of a plant which finds 

 suitable environment only in the briny wet soils around saline springs and 

 has been collected only in the Mohave and Colorado Deserts, and in 

 the Death Valley region. (PL 32.) 



The most important features then of the environment encountered by 

 desert vegetation consist in scanty and unequally distributed water sup- 

 ply, coupled with high concentration of soil salts in most instances; an 

 extremely dry atmosphere, with the occurrence of high air and soil-sur- 

 face temperatures. The actual difference of temperature between the 

 root and shoot is quite unlike that of plants in moist regions, and as I 

 have previously pointed out, must be of great influence in all of the vege- 

 tative processes.* 



The general features of desert vegetation offer some most alluring 

 problems in the study of the origin of species. Here a general interpre- 

 tation of the face of nature might lead one directl}^ to the conclusion that 

 xerophytic species are examples of a direct adaptation to environmental 

 factors and the consequent development of forms adapted to these con- 

 ditions. This alluring generalization has the academic advantage and dis- 

 advantage of being most difficult of proof and disproof and may be supported 

 only on theoretical grounds, for no one has ever actually seen a species arise 

 in this manner, and for every argument brought forward to support the 

 theory, the advocate of natural selection will adduce equally cogent, and 

 quite as theoretical proof, that desert forms arose otherwise. The study 

 of the extreme types presented by the vegetation of arid regions might 



* MacDougal, D. T. Soil Temperatures and Vegetation. Monthly Weather- Review, 31:375, 1903. 

 Washington. 



