Water y^ 



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 diminish- 

 ing 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 defense are more 

 striking in the tropics than elsewhere, they are employed 

 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 same end. Our plants do not indeed go 

 so far as to drop their leaves altogether, but some of them 

 do greatly diminish both their number and size; and some 

 clothe themselves with hairs, partly, as it would seem, lest 

 they should be deprived of too much moisture, though 

 partly also, probably, as a protection against insects. An 

 example or Iwo of these- must suffice, and we will take, 

 first, that of the amphibious persicaria — a particularly 

 interesting plant, as it grows both on land and in water, 

 and adapts itself to its situation in a very marked manner. 

 When it grows in water, where, of course, it does not 

 matter how much it may lose, it has smooth, lance-shaped 

 leaves ; but when it grows on land the leaves are narrower, 

 and not only this, but they are covered as well with a 

 quantity of long hairs, pressed close upon the surface, 

 which they protect against evaporation. 



