TRANSPIRATION 263 



but when water is to be conserved they close the pores 

 by loss of turgidity, and a corresponding alteration of 

 form. 



Continued excess of transpiration would, of course, be 

 disastrous, and in such plants as those which live in 

 deserts, or in high latitudes, the stock of water must 

 needs be conserved. In desert conditions where there 

 are long-continued droughts, such plants as can flourish 

 there must take the fullest advantage of rain when it 

 falls, and retain sufficient water to carry them over a 

 drought; by no means must they permit an excess 

 of transpiration. The Prickly Pears 

 {Opuntia) and Cacti occur in great 

 variety in the high plains of Mexico; 

 they flourish in very dry sandy and 

 stony places, and also in rock crevices 

 where there is the smallest modicum of 

 soil. For about nine months in the year 

 no rain falls, yet the plants do not suffer 

 and always appear " succulent." They Fi«- 78— Species 



J rr _ •'OP Melon CAOnrs. 



are perfectly adapted to their environ- 

 ment. Their stem structure is remarkably developed ; 

 it is swollen, green, and interiorly fleshy, and it is respon- 

 sible for carbon assimilation. The epidermis is thickened 

 and almost cartilaginous, and in some instances oxalate 

 of lime is deposited in great quantity so as to form a 

 species of armour protecting the green tissue beneath 

 the epidermis. Indeed the fleshy stem is a reservoir of 

 water, the evaporation of which is prevented by the 

 thickened epidermis and the mineral underclothing, 

 and the store of water is sufficient to last the plant during 

 the dry season. There are no foliage leaves; in their 



