CHAP. LXXII DESERT 275 



deserts ' (' serir '), where rounded, blackish-brown, resonant, siliceous 

 pebbles clothe the essentially sandy expanses and stand out as dark 

 objects from the reddish-yellow desert-sand ; gravel-steppes x occur in 

 Algeria, and extensive stony plateaux (which are termed ' hammada ' by 

 the natives), carpeted with sharp flints and calcareous stones, form the 

 greatest part of the Sahara ; again, in the upper terraces of the Karroo 

 in Cape Colony one encounters waterless stony desert, and, in the 

 Kalahari, ' stone-fields.' Finally, there are deserts, for instance on the 

 Mexican Plateau, whose soil consists of a firm, reddish clay which is rich 

 in stones : in the dry season the clay becomes as firm and hard as rock, 

 and consequently fissured, so that it may almost be regarded as equivalent 

 to a rock substratum. 



EGYPTO-ARABIAN DESERT 



As a type we may select the Egypto-Arabian desert described by 

 Volkens. 2 This includes rocky, gravelly, and sandy deserts, where 

 eight or nine months often elapse before a drop of rain falls. Rain 

 descends almost exclusively in winter, from December to April. Nowhere 

 else has the atmosphere been observed in daytime to be drier than in 

 North Africa, where the relative humidity may be 10-25 P er cent. ; yet 

 at night the temperature may sink very considerably, often below o C., 

 and there may be a rich fall of dew, which is the sole atmospheric source 

 of water during the prolonged dry season. During this season the tem- 

 perature of the air may exceed 50 C., and at daytime the soil is consider- 

 ably hotter than the atmosphere. In general, too, there is not a breath 

 of wind, particularly in the valleys. 



The vegetation during the dry season presents the following appear- 

 ance. Most of the plants are greyish-white or dirty green stumpy 

 shrubs, which sometimes attain a height of three feet, and are rounded 

 and hemispherical in form : some of the plants are low, mainly prostrate, 

 caespitose herbs : rarely do there occur herbs that twine or are provided 

 with larger long-lived leaves. 



Scarcely have the first showers of rain fallen, about the commencement 

 of February, before the shrubs shoot forth foliage and soon burst into 

 blossom ; seeds of numerous ephemeral species (Odontospermum 

 pygmaeum, the ' rose of Jericho', for instance) germinate, thus initiating 

 an active life that will last for only one or two months, and some few 

 juicy and therefore longer-lived annual species (of Mesembryanthemum 

 for instance) 3 develop also ; there emerges thereafter a crowd of bulbous 

 plants, whose shoots and flowers had already been initiated and were 

 only awaiting the rain to reach full development. In addition there 

 are many other perennial herbs with hypogeous shoots, most of them 

 possessing a multicipital primary root ; many of these have rosulous 

 shoots and spread their leaves flat over the ground. 



Among the ephemeral and annual species there is but little structural 

 adaptation to the dry climate ; for the active life is passed under favour- 

 able circumstances, and the adaptive feature is brevity of existence. 

 But in all other species structural adaptation reveals itself. The construc- 

 tion of succulent and bulbous plants has been described in Chapter XXXI, 



1 Trabut's ' Steppes rocailleux '; Flahault, 19066. * Volkens, 1887. 



8 See p.i2i. 



T 2 



