274 EREMOPHYTES SECT, xi 



to arid lowlands, inasmuch as they are the sources of rivers, which 

 may descend more or less into the lower-lying land and provide this 

 with permanent ground-water, even after it has become parched nearer 

 the surface. 



According to the minimum moisture with which the vegetation can 

 content itself, the following formations may be distinguished : 



1. Desert prevails in very dry places, and its soil shows very sparse 

 vegetation. 



2. Shrub-steppe occurs where the rainfall is very scanty, but never- 

 theless regular. 



3. Grass-steppe is found where the rain is moderate in amount, but 

 falls only in a few days in the year ; grass-steppe, as a rule, can be utilized 

 for cultivation without artificial irrigation. 



A transition between grass-steppe and forest is provided by the 

 bushland that Adamovicz 1 terms 'sibljak*. 



Vegetation growing on saline soil has already been described in 

 connexion with halophytic vegetation (Section VII). 



CHAPTER LXXII. DESERT 



WHERE rain falls at quite irregular times, and is continuously lacking 

 for a long time during the season otherwise most favourable to plant- 

 growth, only the scantiest vegetation can exist. Here and there are 

 dotted little greyish-green plants which stand wide apart, while interven- 

 ing large stretches of soil are utterly devoid of vegetation. Only when a 

 shower of rain casually moistens the soil do numerous annual herbs 

 germinate, and then merely bear fruit and die a few weeks later. 



On the desert of Atacama in South America rain scarcely ever falls. 

 At Walfisch Bay on the coast of South Africa the mean annual rainfall, 

 which descends on only six days in the year, is seven millimetres. In 

 regard to the rainfall in several stations on the northern fringe of the 

 Sahara statistics have been published. Here rain falls in winter ; but 

 even at this season several months may pass with only a single day's 

 rain, or possibly without any. 



In all deserts there are, however, more favoured localities, whether 

 these be mountains, where the rainfall is heavier, or river-beds, where 

 water flows after falls of rain and where the subsoil may remain moist 

 for a longer period. 



The soil in desert is by no means uniform. 2 



There are pure sandy regions vast stretches of sand and dunes which 

 are gradually transformed by the wind. The vegetation of such places 

 has been discussed in the preceding section. 



In other regions of the world, such as parts of North Africa, the soil 

 is practically solid rock ; the lithophilous vegetation on this has been 

 dealt with in Section VIII. 



In yet other regions the combined action of sun and wind have dis- 

 integrated rock into stones and gravel. In Egypt one encounters ' silica r 



1 Adamovicz, 1902. 



a See Schirmer, 1893; Hochreutiner, 1904; Massart, 1898; Flahault, 19066. 



