Water 99 



duty instead. The stems, too, are protected against 

 evaporation by being enveloped in a peculiar leathery 

 skin, which is thickest in the species inhabiting the 

 hottest and driest regions ; and they lose little water 

 therefore, except through the pores, which are but 

 few in number. Thus protected, they not only exist, 

 but flourish, in dry sand, where for three-quarters of 

 the year they are exposed to the blazing, parching 

 sun. 



The tall, fluted columns of the species of cactus 

 called the ' Torch Thistle/ sometimes fifty feet high, 

 are to be seen springing out of crevices in the hard 

 rock, and standing up like telegraph-posts on the 

 mountains and in the rocky valleys all over the hot, 

 parched, almost desert regions of New Mexico. 



This tall cactus seems to be so fully protected by its 

 thick skin that it may venture to expose its whole 

 surface to the sun without risk ; but other species are 

 less bold, and keep close to the ground, growing in the 

 form of large cushions or great globular masses, and so 

 diminishing the extent of exposed surface. Some, too, 

 are set all over with long, slender, needle-like spines, 

 and are also covered with what look like dense masses 

 of floss silk, both of which protect the plant from the 

 hot air and sun. . 



But though these special means of defence are more 

 striking in the tropics than elsewhere, they are em- 

 ployed more or less everywhere, our own moist land 

 not excepted. Besides the bark, and the cork, and 

 the more or less thick skin of the leaves, and the wax, 

 which we have already mentioned as the ordinary 

 means by which evaporation is checked, these other 

 measures are also frequently adopted for securing the 



