DESERTS 605 



The amount of rainfall naturally differs in these various desert districts, 

 but it never exceeds 300 mm., and usually remains considerably lower. 

 According to Woeikof s computations, the point on the earth that is poorest 

 in rain, as far as meteorological observations are available, is Copiapo in 

 Chile, with an annual average of 10 mm. According to the same author, 

 the following places have less than 100 mm. : Fort Mohave in Arizona, 

 60 mm. ; San Juan in Argentina, 70 mm. ; Serena in Chile, 40 mm. ; 

 Suez, 60 mm. ; Nukus on the Amu-Daria, 70 mm. ; Petro-Alexandrovsk, 

 60 mm. ; Leh in Kashmir, 70 mm. ; Aden, 50 mm. ; the island of Ascension, 

 80 mm. The distribution of the scanty rainfall throughout the year also 

 differs. In the Sahara the atmospheric precipitations are irregular, though 

 taking place chiefly during spring. On the Amu-Daria and in North Chile 

 the maximum is in autumn, in Australia in summer. But owing to the 

 smallness of the rainfall such maxima and minima have no practical signifi- 

 cance ; the vegetative periods depend on the heat, which increases the 

 injurious effects of drought and therefore brings plant-life to a state of rest 

 at the time of its maximum. The atmospheric dryness acts in the same 

 manner as the heat, and is usually much greater in deserts than in woodland 

 and grassland districts ; its maximum is attained in summer. 



The oecology of most desert-floras is at present but little known, so that 

 the following descriptions are quite fragmentary in character. 



1. THE DESERTS OF THE EASTERN HEMISPHERE. 



i. THE DESERT DISTRICT OF NORTH AFRICA AND SOUTH-WEST 



ASIA. 



Between §£ and 20 N., 18 W., and 70 E., a district, chiefly formed of 

 high plateaux, and extending like a belt right across North Africa, Arabia, 

 South Persia and Baluchistan, to the country beyond the river Indus, has 

 an annual rainfall of less than 200 mm., so that its vegetation throughout 

 exhibits a desert character. As regards its thermal conditions, this, the 

 greatest of all desert districts, belongs to the belt of mild winters ; only at 

 its most northerly points do slight frosts and transitory falls of snow occur 

 in winter. The summer temperatures are among the highest in the world 

 and coincide with the season of greatest drought ; over a large part of this 

 desert district the temperature in July is 36 C. and upwards. 



The following tables give more detailed data of an extreme western point 

 (Cape Juby). two central points (Ghardaia and Cairo), and an extreme 

 eastern point (Multan) of the North African and South-West Asiatic 

 desert district : — 



