Chap. IX] 



DESERTS 



6 39 



On the rocky slopes of the mountain chains the only characteristic species 

 are found growing in fissures or in the shade of the rocks : — 



Shrubs such as Aplopappus cuneatus, Bigelovia teretifolia, Coleosanthus atracty- 

 lioides (Compositae), and Hofmeisteria pluriseta (Compositae) ; perennial herbs such 

 as Arenaria macradenia, Eucnide urens (Loasaceae), and annual plants such as 

 Macrocalyx micranthus (Hydrophyllaceae), Parietaria debilis, and Pterostegia dry- 

 marioides (Polygonaceae). 



Very peculiar stations are the so-called dry lakes — places where the 

 water accumulates after a heavy shower of rain, but soon evaporates. The 

 soil consists of very hard clay and is rich in alkalis, yet not sufficiently 

 so to give rise to a crystalline efflorescence. In wide valleys between 

 high mountains, the soil in such stations remains continuously moist, and, 



Fig. 376. North American desert in Arizona. Artemisia tridentata (sage-brush). 

 From a photograph. 



wherever it is not covered with water, exhibits saline efflorescence. Such 

 places are entitled swamps. The dry lakes are occupied by plants only 

 along their margins, and there the vegetation consists chiefly of a brush- 

 wood of Atriplex polycarpon, often accompanied by A. confertifolia and 

 Suaeda suffrutescens. The swamps likewise exhibit vegetation only on 

 their margins. The vegetation however is often very luxuriant and dif- 

 ferentiated into very distinct belts, which are sometimes several hundred 

 meters wide : — 



Nearest to the swamp is a belt of the chenopodiaceous Allenrolfea occidentalis ; 

 then follow in belts, all growing on a clay soil covered with salt, Juncus Cooperi, 

 then Sporobolus airoides and Pluchea in company. The plants of the fourth belt 

 have accumulations of sand round their bases ; they are Prosopis juliflora and Atri- 

 plex canescens (Fig. 382), sometimes accompanied by Suaeda suffrutescens. The 



