RELATIONS OF ANIMALS AND PLANTS 137 



also that certain plants which are without thorns 

 spring up in the midst of a thorny bush of another 

 species : in the Algerian Sahara the Terebinth or 

 Betoum tree (Pistacia) is shielded by the Zizyphus, 

 and in the Indian desert the Zizyphus protects the 

 grass Andropogon in a similar manner. It is 

 generally supposed that the Terebinth and Andro- 

 pogon are protected from grazers by the Zizyphus, 

 but one must remember that any plant which suc- 

 ceeds in estabhshing itself tends to shelter the soil 

 around it, and to hold together the particles of soil 

 if they are light and inclined to blow away. One 

 well-established perennial plant mitigates the cUmatic 

 conditions, and becomes the centre of a group of 

 other plants of several species, and this wiU happen 

 whether the perennial is thorny or not. It is possible, 

 therefore, that the shelter which the Zizyphus affords 

 from wind and heat is more important than the pro- 

 tection which it gives from grazing animals. Against 

 the theory that thorns afford protection from grazing 

 animals one may advance the fact already quoted 

 that near human habitations, or frequented tracks, 

 aU desert vegetation, whether thorny or not, is 

 stunted and deformed, or exterminated, because of 

 the animals which graze upon it. From Central 

 AustraHa we have Baldwin Spencer's evidence that 

 the camel can eat every plant, from the spiny Acacia 

 farnesiana to the juicy Claytonia. In the semi- 

 deserts of Palestine camels prefer some of the more 

 thorny shrubs, such as the Caper (Capparis), to others 

 which are almost unarmed, but which are beheved 

 to be bitter, or nauseating by reason of taste or 



