45© INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



in some areas there may be no rain at all for several years on end. 

 Such areas may be wholly devoid of macroscopic plants over some 

 tracts. Intense radiation and considerable changes of temperature 

 are also common, so that where the ground is rocky or clayey it is 

 liable to be much fissured. Often it is gravelly, sandy, loamy, or 

 stony — but none the less arid. Thus although the substratum and 

 even the topography may change greatly in a single desert area, 

 the abiding influence is the paucity of water. Tracts of different 

 type may exhibit different forms and degrees of vegetative develop- 

 ment, or sometimes virtually none, but all have the desert character, 

 the stamp of aridity. For example in the western Sahara there are 

 the pebbly-clayey areas with cushion-plants and succulents, the 

 sandy or gravelly beds of dry watercourses populated with Tamarisks 

 {Tamarix spp.), the sand-dunes dotted with heath-like bushes and 

 grass-tussocks, and the rocky plateaux, most desolate of all, consisting 

 of split stones and broken rocks with an occasional spiny or other 

 xerophyte anchored in the fissures. In addition there are saline 

 depressions which at best support relatively sparse colonies of 

 dwarfed shrubby halophytes. 



Similar ranges of type are found in other deserts, or extremes from 

 absolutely bare moving sand-dunes to fairly dense heathlands char- 

 acterized by switch-plants. There may even be open miniature 

 woodlands, with or without large bushy succulents. The American 

 near-deserts^ are remarkable for their giant Cacti such as the Saguaro 

 or Sahuaro [Carnegiea gigantea) and their small pebble-like Pin- 

 cushion Cacti {Mammillaria spp.), as well as for their glutinous 

 Creosote-bushes {Larrea spp.) and characteristic Ocotillo {Fouquiera 

 splendens) (Fig. 155). The South African desert is famous for the 

 unique gymnospermous Tumboa (Wehvitschia mirabilis) and the 

 Desert Melon [Acanthosicyos horrida), as is the Australian for other 

 highly peculiar plant forms. Thus whereas the desert populations 

 are normally limited to relatively small and scattered plants, many 

 of which are thorny, each area may have its own particular character 

 given by the plants themselves. 



Desert plants are adapted in various ways to withstand the adverse 

 conditions under which they have to establish themselves, grow, 

 and ultimately reproduce. Many, particularly among the shrubs 

 and more occasional shrubby trees, have long roots that are said to 

 reach down sometimes to a depth of 10 or more metres (they may 

 certainly exceed 15 metres in length) to subterranean water or at 



^ See footnote on preceding page. 



