Aridity and Evolution. 



227 



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

 very important proportion of the flora would consist of moisture - 

 loving species, or mesophytes, and the remainder would be in- 

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

 leaves, and the succulents with shoots reduced but with the addi- 

 tional development of storage structure 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 number of individuals is less, the inter-relations of the indi- 

 viduals and species are widely different, and the competitive 

 struggle for existence is of a nature much unlike that of 

 a tropical forest. Increase in aridity tends to localization in 

 distribution, and humidity to diffuseness. 



XEROPHYTES ARE OF RECENT ORIGIN. 



Evidence of the existence of xerophytes in previous periods 

 of desiccation is extremely scanty. Calamites and Lycopods, 

 with a slender central cylinder and a heavy enclosing cylinder 

 of thin walled tissue, have been alluded to in this connection, but 

 their great sporophytes probably stood in swamps, or at least 

 were hygrophytic in habit, and by the requirements of their 

 separated gametophytic reproduction could not exist on land 

 areas independently. It is also to be noted that many forms 

 peculiar to swampy areas at the present time display reduced 

 shoots and leaves of a specialized structure due to the action of 

 certain constituents in the substratum, being known as 

 "swamp xerophytes" and if brought to light as fossils might give 

 the impression of having lived in an arid climate. It is true, of 

 course that desert conditions are not favorable for fossilization, 

 yet many opportunities for such action undoubtedly occur in the 

 carrying and burying action of the torrential floods of desert 

 stream ways, while wind blown deposits might preserve the more 

 indurated forms. Many of these and the skeletons of the 

 Cactaceae would seem well adapted for preservation in this 

 manner. The view that such forms are of recent origin, that is 

 since the Cretaceous period, within the present period of advanc- 

 ing desiccation, would predicate a very great phylogenetic activ- 

 ity, unprecedented, perhaps, but by no means impossible. 

 Among earlier types of plants capable of withstanding aridity, 

 most successfully, the cycads, Bennetitales, and conifers may 



