110 Field, Forest, and Wayside Flowers 
dried out, and Nature guards against this by ex- 
posing the least possible proportionate surface to 
the rays of an ardent sun. The plant substance, 
instead of being spread out into a great number of 
thin, flat leaves, is collected into a solid mass, al- 
most globular in some varieties, and this living 
lump is covered with a skin, which is richly col- 
ored with chlorophyll, and acts as one all-enfold- 
ing leaf. 
The real leaves, superseded in their original 
work, have become converted into spines or 
prickles, and act as a deterrent to vegetarian ene- 
mies. 
A member of the widely-differing family of the 
spurges, which lives on dry ground under an Af- 
rican sun, has adopted like habits. Its branches 
are succulent, spiny prongs, whose surfaces contain 
chlorophyll, and the plant, when not in bloom, 
might be mistaken for one of the many varieties of 
cactus, while the exigencies of the South African 
climate have driven a native milkweed to do as 
the cactuses do. In all three of these plants the 
vegetable substance is condensed into a mass, the 
inner tissues are full of juice, the bark is converted 
into an all-enfolding leaf, and the plant body is 
protected from thirsty vegetarians by thorns, hairs, 
or prickles (Fig. 22). 
