XXXVII 
THORNS THAT ARE LEAVES 
I SN’T IT ODD that desert plants tend to do away with 
those leaves without which ordinary trees cannot live, 
but that they still get along quite well! 
Different cactus plants like the sahuaro or the prickly 
pear have turned their leaves into thorns. These thorns 
have come to be used to protect the plant, whereas leaves 
ordinarily are factories in which its food is made. From 
the standpoint of the mass of the vegetation of the world 
cacti are plants without leaves. 
These desert plants have trained their bark to do the 
work of leaves. It is the green in leaves that breaks up 
certain of the sun’s rays and uses their power in extracting 
carbon dioxide from the air and combining it with water 
from the ground to make sugar, which is the chief build¬ 
ing material of plants. 
If you examine one of these desert plants that in part 
or in whole has done away with its leaves, you will prob¬ 
ably find that it has a green skin or bark. The cacti have 
these green skins. The palo-verde is a desert tree with 
a green bark. Palo-verde means green pole in Spanish. 
Palo-verde and the thorny ocotillo have small leaves, var¬ 
nished air-tight so that no moisture can get out through 
them, but their green bark does most of the work of food¬ 
making. 
Desert trees like the mesquite, the catclaw, and the 
creosote bush are able to suspend life for long periods be- 
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