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 toa 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 the 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 
