SECTION XI 



CLASS IX. EREMOPHYTES. FORMATIONS ON 

 DESERT AND STEPPE 



CHAPTER LXXI. OECOLOGICAL FACTORS. FORMATIONS 



In regions where the atmospheric precipitations are less than a certain limit, 

 the vegetation acquires a xerophytic structure, excepting in spots where the 

 soil is rendered wet by flowing or subterranean water. Loam is there as dry- 

 as sand or stony soil, for all kinds of soil ahke are inadequately supphed 

 with moisture by rain. The soil varies greatly in nature, but apparently 

 is always very rich in nutriment, as the salts that arise by weathering 

 of the soil are only very incompletely washed out. Yet dryness of the 

 soil checks absorption of nutriment by the roots ; and this necessarily 

 reacts upon the dimensions and numbers of the plants present. The 

 abundance of nutriment may be such that the soil is supersaturated 

 with salt, and the amount present increases with increasing aridity of 

 climate. The various kinds of soil behave differently in this connexion. 

 Salts are readily washed out of sandy soil, so that, other conditions 

 remaining constant, sand is poorer in salts than is clay, which is relatively 

 impervious to water. Also the form of the terrain is responsible for 

 distinctions. Slopes of hills are more completely elutriated ; on the 

 other hand water collects in depressions, and, after evaporation has taken 

 place, leaves behind deposits of salt. In this way steppes and deserts may 

 display salt-impregnated spots, where incrustations of salt carpet the 

 soil.^ Excepting in some grassy steppes, the soil is utterly devoid of 

 humus ; for sun and drought burn it up completely. The air is very 

 dry, and the variations in its temperature are very great. Frosts 

 occur in deserts even of low latitude. In summer the temperature 

 may exceed 50 C. In the interior of Arabia in the winter of 1893, 

 according to Nolde, at night minimal temperatures of 5 to ioC. 

 were reached, while by day the thermometer rose to more than 25 C. 

 The soil during daytime becomes exceedingly hot, and its temperature in 

 the Sahara rises above 70 C. Similar high temperatures in the soil have 

 been observed in tropical savannah ; for instance one of 85 C. was 

 registered at Chinchosho in West Africa. In dry regions mountain-slopes 

 are favoured with greater atmospheric humidity and heavier rainfall 

 than are the plains, because they experience an alternation of an ascending 

 wind at daytime, and a descending one at night-time. Especially 

 favoured in this respect are slopes exposed to the prevailing wind here 

 mesophytic vegetation may reign whereas on lee-slopes and plains dry 

 steppe prevails. Mountains are of additional significance in relation 



* See p. 219. 



WARMING J 



