RELATIONS OF ANIMALS AND PLANTS 129 



A large weevil {Larinus maculatus) and a Rose 

 Chafer {Protcetia libani) bite into the flower-heads 

 of this thistle, and feed on the interior, leaving an 

 open wound from which the sap continues to run. 

 This sap is drunk greedily by smaU bees, and wasps, 

 and ants, and would not be available to them 

 imless the weevil or chafer had first bitten into 

 the thistle. Such a method of obtaining drink is 

 not, of course, peculiar to desert cUmates ; the sap 

 which exudes from tree-trunks injiu*ed by Goat 

 Moth or other wood-borers, in Great Britain, attracts 

 butterflies, beetles, flies, and other insects. 



Cases in which the animal depends on a plant 

 for food and water might be multiphed. The 

 relationship between Dwarf Gerbille and Suaeda, in 

 which protection as well as food and water is in- 

 volved, leads us on to another group of associations, 

 in which the principal or only bond between plant 

 and animal is shelter from climate or protection 

 from enemies. 



Such an association was studied by Hartert in 

 the northern Sahara, a short distance south of 

 Biskra, in Algeria. He describes it thus : " After 

 leaving Bordj Saada the tamarisk bushes disappear 

 and a wide sandy plain covered with innumerable 

 stones extends to far beyond Bordj Chegga, where 

 the sand begins to prevail more and more. The 

 sandy plain, however, is not smooth or even, but 

 covered with countless little sandhills, the nucleus 

 of which is, or has been in each case, a bush of 

 Limoniastrum, Salsola, or other plant. Each iso- 

 lated plant arrests the sand that flies across the 



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