276 EREMOPHYTES SECT, xi 



while aqueous tissue and water-storing hairs have been dealt with on p. 120. 

 The hammadas almost exclusively bear small shrubs whose leaves and 

 stems are clothed with felted hairs. The leaves of the grasses present 

 are short, stiff, of the rolled type, and poor in sap. Many shrubs have 

 aphyllous shoots, or shoots showing only scale-like leaves, as in Tamarix, 

 Ephedra, Polygonum equisetiforme ; many leaves are changed into 

 thorns. 1 Conversion of the inner part of the bark into mucilage is 

 frequent, and in Halimodendron a like conversion even affects the pith. 

 Assimilatory tissue persists for a long time. After the original assimila- 

 tory cells have disappeared chlorophyll appears in the secondary cortex, 

 and may be still seen in quite old branches. 2 



SAHARA 



In regard to the Sahara the works of Schirmer 3 , Massart 4 , Hochreu- 

 tiner 5 , and Flahault 6 should be consulted. Its nature is the same as that 

 of the desert just described, as Volkens has indicated. 



SOUTH AFRICA 



Like North Africa, South Africa has gravelly, sandy, and other 

 deserts, including the Kalahari, Karroo, and Great Namaqualand, which, 

 however, are not so poor in vegetation. Here many remarkable growth- 

 types appear. Among them is Welwitschia mirabilis (Tumboa Bainesii), 

 which was discovered in Damaraland by Welwitsch and Baines. On 

 a parched arid plain these naturalists found, in addition to a little grass, 

 only this species, which spreads its two gigantic leaves over the dry 

 ground, sends its roots down to the depths, and can vegetate without 

 intermission throughout the year, remaining active despite cold or drought. 



The coast of German South- West Africa is an almost rainless desert. 

 On it Euphorbia-steppe may appear, like an oasis, mainly where there 

 is a permanent supply of subterranean water. 



Many plants in South African steppes have epigeous tubers so closely 

 resembling the stones among which they grow that during the dry season, 

 when they are leafless, it is almost impossible to distinguish them from 

 the stones ; Wallace regards this as a case of mimicry. 7 



In South Africa one finds numbers of bulbous and tuberous plants, 

 belonging to the Liliaceae, Amaryllidaceae, Oxalidaceae, and other 

 families. There are also succulent plants, including Mesembryanthemum, 

 Euphorbia, Aloe, and Pelargonium, which exhibit great diversity of form 

 and are represented by numerous individuals (forming about thirty per 

 cent, of the vegetation in certain parts of the Karroos). In addition 

 there occur xerophytes that are poor in sap and belong to many different 

 families, including Proteaceae, Restiaceae, and Mimosaceae (with Acacia). 



EAST AFRICA 



On mountainous country of German East Africa there is succulent- 

 steppe which has been described by Volkens. 8 It is mainly formed of 

 cactus-like species of Euphorbia, Stapelia, Sanseviera, and Kleinia. 

 Growing between these succulent plants are shrubs, including such thorny 



1 See Massart, 1898. 2 Jonsson, 1902. 3 Schirmer, 1893. 



4 Massart, 1898. 8 Hochreutiner, 1904. Flahault, 1906 b. 



7 Seep. 124. * Volkens, 1897. 



