226 



The Plant World. 



TWO TYPES OF VEGETATION RESULTING FROM DESERT CONDITIONS. 



Two types of vegetation may be definitely connected with 

 arid conditions', representing fairly distinct stages of develop- 

 ment due to the influence of aridity. In one form the chief 

 operation has been one of reduction and protection of surfaces. 

 Leaves have been reduced to linear vestiges representing va- 

 rious parts of the foliar organ, branches to spines or short rudi- 

 ments, stomata have special constructions and all parts of the 

 shoot are heavily coated and hardened ; root-systems have been ex- 

 tended horizontally and the individuals thus isolated, becoming 

 more or less accommodated to soils containing a large proportion of 

 salts. The spinose, stubby and switch-like perennials which 

 result from such action, are characteristic of low enclosed desert 

 basins, like that of the Salton and those of southern Africa and 

 central Asia, where the scanty rainfall does not occur within 

 such regular limits as to make distinct moist seasons. 



The second type of desert vegetation includes forms which 

 have not only been modified in the manner described, but in 

 addition have developed the storage function as a further step 

 in the same direction. An increased capacity for rapid and eff- 

 fective absorption, together with an enormous development of 

 storage mechanisms, in xerophytically modified stems, branches, 

 leaves or roots, has resulted in such groups as the cacti, the 

 mesembryanthemums, the euphorbias, agaves, yuccas, sotols, 

 crinums, crassulas, and others in which the individuals often 

 accumulate sufficient surplus water to meet its vegetative needs 

 for a decade, while species are not unknown in which the supply 

 on hand is sufficient to carry on the annual seasonal activity of 

 shoot extension and reproduction for a quarter of a century. 

 These forms are desert plants par excellence, showing two dis- 

 tinct stages of modification, the latter consequent upon the first. 

 Marloth's view, therefore, that the regions characterized by 

 succulents, of which he names the Karroo of South Africa as an 

 example, and which by implication would also apply to most 

 of Arizona, Sonora, Chihuahua, and southern Mexico, are not 

 true deserts, is directly controverted by the evidence obtained 

 from a consideration of the evolutionary history of the con- 

 stituents of the floras. 



