ned to clothe themselves in prickly
mail, to store in their loose tissues an abundant supply of sticky
moisture, and to set at defiance the persistent attacks of all external
enemies. The prickly pear, in fact, is a typical instance of a desert
plant, as the camel is a typical instance of a desert animal. Each lays
itself out to endure the long droughts of its almost rainless habitat
by drinking as much as it can when opportunity offers, hoarding up the
superfluous water for future use, and economising evaporation by every
means in its power.

If you ask that convenient fiction, the Man in the Street, what sort of
plant a cactus is, he will probably tell you it is all leaf and no
stem, and each of the leaves grows out of the last one. Whenever we set
up the Man in the Street, however, you must have noticed we do it in
order to knock him down again like a nine-pin next moment: and this
particular instance is no exception to the rule; for the truth is that
a cactus is practically all stem and no leaves, what looks like a leaf
being really a branch sticking out at an angle. The true leaves, if
there are any, are reduced to mere spines or prickles on the surface,
while the branches, in the prickly-pear and many of the ornamental
hot-house cactuses, are flattened out like a leaf to perform foliar
functions. In most plants, to put it simply, the leaves are the mouths
and stomachs of the organism; their thin and flattened blades are
spread out horizontally in a wide expanse, covered with tiny throats
and lips which suck in carbonic acid from the surrounding air, and
disintegrate it in their own cells under the influence of sunlight. In
the prickly pears, on the contrary, it is the flattened stem and
branches which undertake this essential operation in the life of the
plant--the sucking-in of carbon and giving-out of oxygen, which is to
the vegetable exactly what the eating and digesting of food is to the
animal organism. In their old age, however, the stems of the prickly
pear display their true character by becoming woody in texture and
losing their articulated leaf-like appearance.

Everything on this earth can best be understood by investigating the
history of its origin and development, and in order to understand this
curious reversal of the ordinary rule in the cactus tribe we must look
at the circumstances under which the race was evolved in the howling
waste of American deserts. (All deserts have a prescriptive right t