6l2 



ZONES AND REGIONS [Pt. Ill, Sect. II 



as Malcomia aegyptica, Spr., Matthiola livida, DC, Roemeria dodecandra, 

 Stapf., various Papilionaceae (for instance species of Astragalus), many 

 inconspicuous Compositae, some Boraginaceae, and grasses. 



Other likewise annual plants owe their power of temporarily enduring the 

 dry season to the supplies of water that they have collected during the 

 rainy season, and the exhaustion of which marks the termination of their 

 existence. Among these, according to Volkens, are included the few 

 Aizoaceae of the Sahara (Mesembryanthemum crystallinum, Linn., Fig. 349, 



Aizoon canariense, Linn.), also 

 several Paronychieae, species 

 of Reseda, and Cruciferae. 



The rain-perennials, that is 

 to say perennials which have 

 an existence above ground only 

 during the rainy season, are 

 partly bulbous Monocotyledones 

 — for instance, in the Egyp- 

 tian Sahara, Pancratium Sicken- 

 bergeri, Aschs. et Schweinf., 

 Urginea undulata, Steinh., Al- 

 lium Crameri, Aschs. et Boiss. — 

 and partly, but to a less degree, 

 Dicotyledones, such as species 

 of Erodium and Heliotropium. 

 Plants of tJic desert dependent 

 upon subterranean water. This 

 second category of desert plants 

 exhibits its dependence on sub- 

 terranean water nearly uni- 

 versally by the immense length 

 of its root-system, which the 

 depth of level of the subterra- 

 nean water renders vitally 

 necessary. Only a small mi- 

 nority of these plants are 

 annuals, for instance Monsonia 

 nivea, according to Volkens ; most of them have lignified axes and markedly 

 xerophilous structure. Yet among them there is a large and even 

 herbaceous plant, clearly hygrophilous in the structure of its epigeous parts 

 and transpiring freely, and it shows better than any other desert plant 

 the importance of subterranean water to the vegetation of the desert. It is 

 Citrullus Colocynthis, a cucurbitaceous plant resembling our cultivated 

 pumpkin, and its long juicy, relatively thickly foliaged, and large-leaved 



Fig. 343. Flora of the Sahara. Zilla myagroides, Forsk. 

 Natural size. After Prantl in Engler ivnd Prantl, Die 

 naturlichen Pflanzenfamilien. 



