9 8 
THE GARDEN OF THE WORLD 
Leaves make food from water and carbon 
dioxide. The leaves also give of? water which 
evaporates into the air, a process called Tran¬ 
spiration . Trees drop their leaves in winter 
so they will not lose too much moisture. The 
cactus plant prevents the transpiration of water 
by not having any leaves. The stems have some 
green coloring, and they do the work of making 
food. Naturally they do not make it very fast, 
and for that reason the cactus grows slowly. 
The water is stored in the stem, which is very 
thick, sometimes over a foot across. All over 
the stem is a heavy skin which prevents evapo¬ 
ration. There are also hairs and thorns which 
keep the moisture inside. They serve to keep 
away prowling animals who might break the 
stem and so let some of the precious water 
escape. The skin, in addition to being thick, 
is rather waxy. Wax prevents evaporation. 
That is why we wrap food in wax paper to keep 
it fresh. The only way water can get out of 
this armored plant is when some outside agent 
like the prospector with his ax, breaks through. 
