62 ANIMAL LIFE IN DESERTS 



little need for a reserve of water, for they are only- 

 active after rain when the soil is damp ; they store 

 food in the form of starch and allied substances, 

 so that when the rain is received they can at once pro- 

 duce active growth. Members of the third group are 

 active at all seasons ; they therefore are able to 

 manufacture their own food as they need it : many 

 of them possess storage organs, but they store water 

 not food. 



The sap of many succulent plants is saUne or 

 bitter : this appears to protect the plant from 

 animals in many instances, but it does not follow 

 that the primary function of the bitterness is 

 protective. In certain instances it is known that 

 the water accumulated by succulent plants is of use 

 to animals ; for instance, the donkey can be watered 

 with the juice of the giant cactus (Cereus giganteus), 

 which is too bitter for man's use, and the barrel 

 cactus (Echinocactus) furnishes several pints of fluid 

 which even man drinks readily. 



Thorniness is possibly more characteristic of the 

 third group of desert plants than succulence : it is 

 a character common to the plants of deserts in all 

 countries, and of every type of desert, and it is 

 exhibited by herbs and bushes belonging to a very 

 great number of different natural orders. It is 

 found on the most varied parts of the plant body ; 

 the stem may be thorny, and the leaves may be 

 covered with thorns, or may themselves be trans- 

 formed into thorns ; the fruits also may be thorny. 

 It is believed by many that the thorns protect the 

 plant against herbivorous animals. This point will 



