PROTECTIVE ADAPTATIONS IN PLANTS 



125 



The species of Cactus are almost the only plants which grow on the 

 stony, hard, and hot plateaux of Mexico, and they are protected Prom 

 desiccation by the thickness of their epidermis. But, enticing aa is 

 the food promised by the juicy stem, animals rarely venture to 

 approach them, and it is only when tortured by thirst that hors 

 and asses occasionally knock off the spines with their hoofs, and so 

 reach the soft tissues rich in water. For this attempt, however, as 

 Alexander von Humboldt pointed out, they often suffer, as the sharp 

 spines are apt to pierce the hoof. In any case, the cactuses are 

 effectively protected from the danger of extermination by grazing 

 animals. 



It must certainly strike every one that many districts, especially 



Fig. 22. Tragacanth {Astragalus tragacantha). A, two spring shoots. B, a single 

 leaf, from which the three uppermost leaflets have fallen off. C, leaf midrib, 

 from which all the leaflets have fallen off. After Keraer. 



those which are dry, hot, and stony, are conspicuously rich in thorny 

 plants, and it has often been supposed that the production of thorns 

 must be a direct result of these peculiar conditions of life; indeed, 

 the hard, thorny habit of many of these plants has even been 

 regarded as a protection against desiccation. This, however, is con- 

 tradicted by all those thorny plants which, like the cactuses, possess 

 tissues extremely rich in sap, and in which desiccation is prevented, 

 not by the thorns, but by the thick epidermis. The only satisfactory 

 explanation is that afforded in terms of natural selection. In such 

 hot, and at the same time dry regions, the plant-growth is often very 



