14] vegetational types of tropical lands 449 



Tropical and Subtropical Deserts 



The hot deserts are areas in the tropics and subtropics having 

 such very sHght precipitation that they typically support at best 

 only a scanty growth of scattered plants. Even if the nights are 

 cool, with dew and mists occurring especially in central regions, 

 the days are normally blazing hot. The environment is thus severe 

 and the general run of habitats so unfavourable that they can only 

 be colonized by appropriately adapted plants and animals. Bald 

 expanses of flat or rolling plains predominate, all sunbaked and 

 windswept, with wide tracts of yellowish sand-dunes or browner 

 gravels or more rugged rocky floors, and in places broken scarps 

 of naked hills. Sparingly dotted in the less inimical situations may 

 be low and dry, strange-looking plants, or, in the very occasional 

 damper situations, luxuriant but limited oases. 



Hot deserts chiefly occur well to the north and south of the 

 equatorial zone, which imaginative generalizers are apt to say is 

 ' hemmed in ' by them. Although probably more extensive than 

 they used to be, owing to interference by Man and his domestic 

 animals, they are often not nearly so vast and invariable as is popularly 

 supposed, great areas being occupied by other types which are only 

 relatively speaking ' desertic '. The main examples are the Sahara 

 and Arabian Deserts, occupying, respectively, much of northern 

 Africa and southwestern Asia, whence there are extensions eastwards 

 into northwestern India and northwards and then eastwards into 

 temperate central Asia. Extensive hot desert and near-desert^ areas 

 also occur in central Australia and the southwestern portions of 

 North America, and smaller ones in southwestern Africa and western 

 South America. 



The primary cause of hot deserts is paucity or even perennial 

 absence of rain, though an excessively clear and dry atmosphere, 

 with scorching sun, usually contributes to the general aridity. 

 Thus the relative humidity in the daytime is commonly less than 

 50 per cent, and may drop as low as 5 per cent. The rainfall usually 

 averages less than 20 cm. per annum, often very much less, while 



^ To many who have experienced the real deserts of, for example, northern 

 Africa and southwestern Asia, it seems unreasonable to refer to the more pro- 

 ductive of the so-called deserts of North America as properly desertic — hence 

 the use of this designation here. It was one of the present author's earliest 

 experiences in Iraq to show pictures of such North American ' desert ' areas to 

 students who asked ' But, Professor, how can you call those areas desert, when 

 the vegetation is so great ? ' 



