286 Eduard Riibel 



sagebrush deserts in western North America — an Artemisietum 

 tridentatae with a Kochietum vestitae on loam and an AtripUce- 

 tum confertifohae on a lower, moister level. The corresponding 

 communities in Algeria-Tunisia are the Artemisietum herbaeal- 

 bae and the Stipetum tenacissimae halfa grass-scrub desert. The 

 halfa grass is stiffened by cellulose; its leaves are pluriennial, and 

 therefore ecologically it is a shrub. 



Where conditions take on a more extreme character, there 

 are deserts of dry squarrose shrubs with reduced foliage. Most 

 shrubs are thorny and have small, gray-green, leathery, ever- 

 green or only raingreen leaflets. These are the thornbush deserts 

 of Algeria, Tunisia, Egypt, and a most unusual one in the alpine 

 belt of Teneriflfa, the Retama Blanca desert of Spartocytisus 

 supranubius. In South Africa is the large Karoo desert. Still 

 more exacting on the vegetation is dryness and salt content com- 

 bined. We find salty dry deserts in southern Russia, in Tunisia, 

 Algeria, and around Great Salt Lake in Utah. 



Prominent in this class are the succulent deserts. In Africa, suc- 

 culent Euphorbiaceae are building up communities; in America 

 (that is, in Mexico, Texas, Arizona, and New Mexico ),Cactaceae. 

 In the Bajada, Larrea tridentata is dominant, accompanied by 

 the oddest forms of Opuntias. On the slighdy inclined deep-soil 

 talus stands the Parkinsonietum microphyllae including the 

 local miracles, Cereus giganteus, the fleshy telegraph pole, and 

 the stem-assimilating thorny ocatilla, Fouquiera splendens. 



21. Frigorideserta: Cold deserts. — The vegetation of these des- 

 erts consists mostly of herbaceous perennial, rarely woody, 

 plants, usually of fresh green color, and frequently of tufted, 

 rosette, cushion, or creeping habit. They are climatically condi- 

 tioned. Cold has the effect of dryness, since water for a great part 

 of the year is in the form of ice and snow, unavailable for plant 



