SEASIDE WEEDS. 161 



eat all unprotected specimens. Hence, as a 

 rule, only those survive which happen to have 

 developed some unpleasant personal peculi- 

 arity. Many sand-haunting or desert plants 

 are more or less pungent or have disagreeable 

 alkaline essences stored up in their leaves ; 

 and these alkaline constituents, which they 

 easily obtain from the soil, formerly caused 

 many of them (saltwort and glasswort among 

 the number) to be burnt for barilla. In 

 avoiding the Scylla of animal tastes such 

 plants fell into the Charybdis of human in- 

 dustrial usages. But most sand-loving weeds 

 have solved the difficulty in another way by 

 simply acquiring thorns or prickles. In the 

 saltwort, each leaf ends in a stout spine, which 

 of course runs into the nose of any too inquir- 

 ing cow or donkey. In many cactuses, again, 

 the leaves have been reduced to sharp thorns, 

 which cover the surface of the cylindrical 

 stem, and form the most effectual possible 

 protection against the attacks of animals. In 

 the West Indies, cactus hedges line all the 



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