6i6 



ZONES AND REGIONS [Pt. Ill, Sect. II 



and Asclepiadaceae, are rare, and are confined to the extreme western and 

 southern points of the Sahara. 



As essential constituents of the flora of the Egyptian Sahara, according to Volkens, 

 the following are prominent : Cocculus Leaeba, DC, with slender shoots up to four 

 meters long, creeping on the ground with scanty foliage ; Farsetia aegyptica, Desv., 

 an under-shrub up to a meter in height with felted hairs ; Zilla myagroides, Forsk. 

 (Fig. 343), which in valleys forms leafless, thorny, and frequently globose bushes, 

 a meter high ; Capparis spinosa, Linn., var. aegyptica (Fig. 347), chiefly growing in 

 the clefts of rocks, effectively protected against transpiration by a coating of wax ; 

 Gymnocarpos decandrum, Forsk., a plant belonging to the Paronychiae with succulent 

 leaves, and yet leafless in the dry season; scaly-leaved hard Tamaricaceae with 

 exudations of salt ; Genista (Retama) Raetam, Forsk. (Fig. 341), a large leafless 



Fig. 350. South Arabian rocky desert. Aden. From a photograph. 



besom-like shrub ; thorny, rain-green shrubs ol Astragalus ; Alhagi maurorum, 

 Medic. (Fig. 344), a thorny small-leaved papilionaceous plant ; Acacia tortilis, 

 Hayne, a thorny shrub or small tree ; Convolvulus lanatus, Vahl, a felted, squarrose, 

 much ramified shrub ; Lycium arabicum, Schweinf., an evergreen, perpetually 

 flowering, thorny shrub with uncommonly long, partly tuberous roots ; Lavandula 

 coronipifolia, Poir., a nearly leafless shrub, also other shrubby, poorly leaved hairy 

 Labiatae ; Statice pruinosa, Linn., a herb bearing leaves for a short time only, and 

 covered with a dense calcareous powder ; Atriplex Halimus, Linn., a large shrub, 

 with leaves which, owing to their abundant vesicular hairs, remain green for a long 

 time during the dry season ; also various other Salsolaceae with internal aqueous 

 tissues ; grasses of the sandy soil with involute leaves and immensely long roots 

 (Cynodon Dactylon, Pers., Danthonia Forskalii (Vahl), Trim, Sporobolus spicatus 

 (Vahl), Kth., species of Andropogon, Aristida (Fig. 346), and others). 

 In the tropical Sahara, between 16 and 20 N.— in Air, for instance— the character 



