104 PLANT GEOGRAPHY 



tains, and the desert-plateaux of Tibet, the Tarim, 

 Dzungaria, and Gobi. The Chenopodiaceous Haloxylon; 

 many species of Astragalus; the grey-green Artemisia, 

 torn up into " wind-witches " by the unchecked gales 

 characteristic of deserts; shrubs such as Tamarix and 

 Reaumur ia ; spinous plants such as Alhagi camelorum 

 Fisch., Xanthium spinosum L., and Eryngium campestre 

 L. ; bulbous species of Allium, and in Dzungaria of 

 Tulipa ; species of Rheum ; and, in Tibet, dwarf 

 Hippophae and Potentilla, make up the scanty flora. In 

 the oases is an interesting assemblage of familiar types, 

 Poplars, Ash, Elm, Willow, Raspberry, etc. 



NORTH AMERICAN DESERT. In North America also 

 desert conditions extend far north of the line of the 

 Tropic, both where the rainfall is intercepted by the 

 various Cordilleran ranges and so shut out from the 

 " Central Basin " of Nevada, Utah, Western Arizona, 

 and Southern California, and where an extremely 

 permeable soil drains off what rainfall there is, as in the 

 " Bad Lands " of South Dakota and Nebraska. On the 

 far loftier Mexican Highlands to the south, though there 

 is some snow in winter, there is but little summer rain, 

 and the added desiccating effect of altitude produces 

 conditions and vegetation similar to those of the north. 

 As in the Sahara, there is, from February to April, a 

 short-lived rain-flora; but the more permanent vege- 

 tation is typically xerophilous. Reduced leaf -blades, 

 hairiness, and spines are as prominent as in Old World 

 deserts, of which we are also reminded by species of 

 Atriplex, Lycium, and Artemisia. Sarcobatus vermicu- 

 latus Torr., Suceda, and other Chenopodiacece; Plantago; 

 Aster and many other Composite; spinous Prosopis; the 

 "Pepper-grass" (Lepidium intermedium A. Gray); 

 Eschscholtzia; Cucurbita, and interesting northward ex- 

 tensions of the South American Loasacece are charac- 

 teristic. It is, however, by its Cacti, Agaves, and 

 Yuccas that this region is rightly best known. Opuntia 

 and Cereus extend northward ; Echinocactus is more 

 southern ; Yucca belongs emphatically to the high ground ; 

 whilst Agave and Fourcroya, the arborescent liliaceous 

 Dasylirion and Beaucarnea, with various species of 

 Sedum and Echeveria, are mainly Mexican. The remark- 

 ably close identity of outward form and vegetative 

 structure between the Cacti, with fluted fleshy stems 



