THE CACTUS TRIBE. 47 



while the arid earth has nothing to part with, and then 

 the leaves wither and fall off, the stems shrink up and 

 crack with the dry heat, and the roots themselves, in 

 many cases, follow the same fate. With Cactuses this 

 is different ; they have so tough and thick a hide that 

 what liquid substances they contain can only pass 

 through it in minute quantities ; the breathing pores 

 of their surface are comparatively few, and so small as 

 to act with extreme slowness when the air is dry ; so 

 that in proportion to the aridity of the air, and the 

 heat to which such plants are exposed, is their reluct- 

 ance to part with the food they contain. They digest 

 and re-digest it, with extreme slowness, and may be 

 truly said to live upon themselves during all those 

 months when they cannot feed upon the soil or the 

 atmosphere. 



This statement applies more particularly to the 

 species consisting of solid fleshy masses, like the 

 Melon-thistles, the Hedgehog-thistles, and the like ; 

 but requires to be modified with reference to the 

 thinner-stemmed species, such as Cactus speciosus, 

 speciosissimus, and truncatus ; of them it is equally 

 true, but in a less degree. 



The property which the Cactuses thus possess of 

 living where few other plants can exist, sometimes 

 renders them of great utility to man. On Mount 

 ^tna, for instance, and its volcanic fields, it is the 

 Indian Fig (Opuntia) which the Sicilians employ to 

 render such desolate regions susceptible of cultivation. 

 This plant readily strikes into the fissures of the lava, 

 and soon, by extending the ramifications of its roots 



