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.
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