344 THE NORTH AMERICAN DESERT FLORA. 



mineral contents to be concentrated in the lower depressions. The 

 intervening rocky ridges and isolated mountain peaks, when not of 

 sufficient elevation to act as condensers of the iipper currents of the 

 atmosphere, exhibit the same characters of arid vegetation, though 

 comprising a larger proportion of shrubby and dwarf tree growth. 



In attempting an enumeration of North American desert plants, my 

 aim has been not so much completeness of detail as to exhibit the 

 main features of desert vegetation, as here brought to view, and to 

 afford the means of comparison with corresponding districts in other 

 portions of the earth. One of the most striking features of the desert 

 flora may be noted in the very marked distinction between the annual 

 and perennial vegetation. Thus the annual desert plants, whose 

 period of growth is strictly confined to a short and uncertain period 

 of spring or fall rains, require for their continued preservation a safe 

 deposit for their usually minute seeds during the prolonged dry 

 season. This condition is, in great measui'e, supplied by the porous, 

 sandy and gravelly soil, or rock crevices, into which they fall and 

 are safely buried, not only out of the reach of climatic influences, but 

 also safe from destruction by animals. Their growth is necessarily 

 rapid and evanescent ; and no sooner do warm rains moisten the 

 ground, than they spring forth from their hitling-places and clothe the 

 barren soil with their scanty verdure, rapidly flower and mature 

 their seeds, which are again deposited in the earth, while their slight, 

 evanescent forms dry up and are blown away, hardly leaving any 

 visible trace of their existence. These characteristics are plainly ex- 

 hibited in ordinary herbarium specimens, and are further exemplified 

 in the specific name of ' exiled so often very appropriately applied. On 

 the other hand, the perennial desert plants either store up a large 

 amount of surplus nourishment in their thick tuberous or tap-roots, 

 or, in the case of trees and shrubs, possess exposed stems and foliage 

 of the most scant and starved character ; spine-clad branches and 

 green-barked stems are, in many instances, made to supply the office 

 of leaves, or where these latter are present, they are often thickly coated 

 with resinous varnish, or clothed with tomentose hairs or scales, serv- 

 ing, in either case, to check evaporation, and thus limit the usual pro- 

 cesses of growth. The preservation of species in perennial plants being 

 less dependent than in annuals on the production of seed, these are 

 generally scanty, often mature late, and are frequently protected by 



