inly,
awkward, and paradoxical; that would be a point of view almost as far
beneath the dignity of science (which in spite of occasional lapses into
the sin of levity I endeavour as a rule piously to uphold) as the old
and fallacious reason 'because there's a B in both.' But cactuses, like
camels, take in their water supply whenever they can get it, and never
waste any of it on the way by needless evaporation. As they form the
perfect central type of desert vegetation, and are also familiar plants
to everyone, they may be taken as a good illustrative example of the
effect that desert conditions inevitably produce upon vegetable
evolution.

Quaint, shapeless, succulent, jointed, the cactuses look at first sight
as if they were all leaves, and had no stem or trunk worth mentioning.
Of course, therefore, the exact opposite is really the case; for, as a
late lamented poet has assured us in mournful numbers, things (generally
speaking) are not what they seem. The true truth about the cactuses runs
just the other way; they are all stem and no leaves; what look like
leaves being really joints of the trunk or branches, and the foliage
being all dwarfed and stunted into the prickly hairs that dot and
encumber the surface. All plants of very arid soils--for example, our
common English stonecrops--tend to be thick, jointed, and succulent;
the distinction between stem and leaves tends to disappear; and the
whole weed, accustomed at times to long drought, acquires the habit of
drinking in water greedily at its rootlets after every rain, and storing
it away for future use in its thick, sponge-like, and water-tight
tissues. To prevent undue evaporation, the surface also is covered with
a thick, shiny skin--a sort of vegetable macintosh, which effectually
checks all unnecessary transpiration. Of this desert type, then, the
cactus is the furthest possible term. It has no flat leaves with
expanded blades, to wither and die in the scorching desert air; but in
their stead the thick and jointed stems do the same work--absorb carbon
from the carbonic acid of the air, and store up water in the driest of
seasons. Then, to repel the attacks of herbivores, who would gladly get
at the juicy morsel if they could, the foliage has been turned into
sharp defensive spines and prickles. The cactus is tenacious of life to
a wonderful degree; and for reproduction it trusts not merely to its
brilliant flowers, fertilised for the most part by desert moths