60 ANIMAL LIFE IN DESERTS 



function performed in ordinary plants by chlorophyl 

 in the leaves. When leaves are present they are 

 often extremely tough and leathery ; the cuticle, in 

 particular, is greatly thickened and the surface is 

 protected in many species either with wax or with 

 resins, or with incrusted salts, or with densely 

 matted hairs : it is beHeved that all these modifica- 

 tions, which occur in desert plants of the most 

 widely different famiUes, enable the plant still 

 further to reduce the amount of water which it 

 loses by transpiration. In all deserts a certain 

 proportion of the plants become succulent, that is 

 to say, develop the capacity of storing a considerable 

 quantity of water. Succulence is not only a char- 

 acteristic of desert plants, but of those growing in 

 any environment in which the available moisture 

 is limited: some succulents, for instance, grow on 

 salt-marshes, others on rock surfaces and roofs and 

 walls, even in damp cHmates. Most of the desert 

 succulents are found in areas with a rainfall which is 

 sHght but fairly regular, and they are not generally 

 found in intense deserts and in cUmates without any 

 regular precipitation. They are developed in an 

 apparently capricious manner in certain deserts and 

 not in others. For instance, the cacti, nearly all 

 of which are succulent, some of them extremely so, 

 are numerous both as species and as individuals in 

 the deserts of the southern United States and of 

 Mexico. In the arid parts of South Africa their 

 place is taken by plants of the Spurge family 

 (Euphorbiaceae), and some of these plants so closely 

 resemble the cacti of the New World that they can 



