CHAP. Lxxii DESERT 275 



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

 pebbles clothe the essentially sandy expanses and stand out as dark 

 objects from the reddish-yellow desert-sand ; gravel-steppes ^ 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-ARABIAX DESERT 



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

 Volkens.- This includes rocky, gravelly, and sandy deserts, w'here 

 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^r cent. ; yet 

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

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

 lof 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 

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

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

 < aespitose 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 



I if 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 



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



juicy and therefore longer-lived annual species (of Mesembryanthemum 



: tor instance) ^ develop also ; there emerges thereafter a crowd of bulbous 



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



)nly awaiting the rain to reach full development. In addition there 



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



. possessing a multicipital primary root ; many of these have rosulous 



phoots and spread their leaves flat over the ground. 



Among the ephemeral and annual species there is but little structural 



Captation to the dry climate ; for the active hfc is passed under favour- 



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



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



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



' Trabut's ' Steppes rocailleux '; Flahault, 1906^. ' Volkens, 1887. 



' See p. 1 2 1 . 

 T 2 



