290 D. T. M AC DOUG A L 



stomata show special constructions, and all parts of the shoot heavily 

 coated and hardened; root-systems have been extended horizontally 

 and the individuals thus isolated, being more or less accommodated 

 to soils containing a large proportion of salts. The spinose, stubby, 

 and switchlike perennials which result from such action are char- 

 acteristic of low, inclosed desert basins, like 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 form of desert vegetation is one in which the absorp- 

 tive function has become highly developed and the capacity gained 

 for conserving the surplus water taken up during the moister seasons. 

 The Cactaceae are the most prominent representatives of this type 

 in North America, and some of this group, as well as other species 

 representing a wide range of families, have the capacity for sufficient 

 water to meet the needs of the individual for a decade, while forms 

 are known which might carry out their cycles of reproduction for a 

 quarter of a century by the use of the surplus accumulated within 

 their bodies. Such succulents display not only the reduction of the 

 shoot and of the foliar surfaces together with induration of the 

 epidermis, but have also this capacity for accumulating water and 

 are hence desert plants par excellence, representing the apex of 

 specialization to desiccation. 



As a total result of the slow desiccation of any region, therefore, 

 a very important proportion of the flora would consist of moisture- 

 requiring species, or mesophytes, and the remainder would be 

 included in two classes, the spinose forms with reduced shoots and 

 roots, and the succulents with atrophied shoots, but with the addi- 

 tional development of storage structures in some organ of the shoot 

 or root. The total number of species within an arid region is not 

 less than that of the most densely closed tropical area, but the num- 

 ber of individuals is less, the interrelations of the individuals and 

 species are not identical, and the competitive struggle for existence is 

 of a nature widely different from that of a tropical forest. Increase 

 in aridity tends to localization in distribution, and increase in humid- 

 ity to diffuseness. 



Evidence of the existence of xerophytes in previous periods of 

 desiccation is extremely scanty. Calamites and lycopods with a 



