''DEFENCE, NO! DEFIANCE. i55 



to very great danger from herbivorous animals, 

 against which they are accordingly compelled to 

 protect themselves by some hostile device. In the 

 first place, there is comparatively little vegetation on 

 sandy spots, so that each plant runs an exceptional 

 chance of being eaten. Then, again, the succulence 

 and juiciness of sand -haunting weeds makes them 

 particularly tempting to thirsty animals, which are 

 sure to eat all unprotected specimens. Hence, as a 

 rule, only those survive which happen to have 

 developed some unpleasant personal peculiarities. 

 Many sand -haunting or desert plants are more or 

 less pungent, or have disagreeable alkaline essences 

 stored up in their leaves ; and these alkaline con- 

 stituents, which they easily obtain from the soil, 

 formerly caused many of them. Saltwort and Glass- 

 wort among the number, to be burnt for barilla. 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 in- 

 quiring cow or donkey. In the West Indies, Cactus 

 hedges line all the roads in the plains, and rise in a 

 solid wall to the height of i 5 or 20 feet. No animal 

 on earth dare attempt to pass through such a hedge, 

 and the task of cutting one down, when necessary, is 

 extremely difficult. On bare dry expanses, like the 

 Mexican plain. Cactuses and Agaves run wild in every 

 direction, collecting what little moisture they can in 



