272 EVOLUTION OF PLANTS 



tion thoroughly xerophytic in character. The giant 

 cacti, Yuccas, sage-brush, and century plants give the 

 scattered desert vegetation a peculiar aspect, which is 

 not soon forgotten. The cacti are probably as perfect 

 examples of adaptation to extreme desert conditions as 

 can be found. In these the leaves have entirely dis- 

 appeared and the plant in some forms is reduced to 

 a single, enormously enlarged, often nearly globular 

 stem, thus presenting the least possible surface, and 

 reducing the loss of water to a minimum. The green 

 tissues are protected by several overlying layers of cells 

 with thick walls, the outer ones strongly cutinized so 

 as to be waterproof. Nearly the whole inner mass of 

 tissue is made up of thin-walled cells gorged with 

 water, and forming a reservoir from which the slight 

 loss of water at the surface, due to transpiration, is 

 made good. Branches cut off and thrown upon the 

 ground will remain alive for weeks before tjie water 

 stored up in them is finally exhausted. 



The Yuccas and the century plants (Agave) present 

 a type somewhat different from that of the cacti. This 

 is best seen in the. century plant, where the leaves, in- 

 stead of being absent, are very large ; but like the stem 

 of the cactus they are enormously thickened, and effi- 

 ciently protected from loss of water by the heavily 

 cutinized walls of the superficial cells. 



Most of these desert plants, as we have indicated in 

 a former chapter, are very efficiently protected against 

 the attacks of herbivorous animals by their thorny 

 armor. The terrible spines developed upon the cacti, 

 and the dagger-like leaves of the Yuccas and Agaves, 

 are quite sufficient to keep the hungriest animals at 



