Plants choosing
such habitats as they have adopted, usually in tropical or
semi-tropical regions, had to resort to various expedients to
save loss of water through transpiration and evaporation. Now, as
leaves are the natural outlets for moisture thrown off by any
plant, manifestly the first thing to do was either to reduce the
number of branches and leaves, or to modify them into sharp
spines (not surface prickles like the rose's); to cultivate a low
habit of growth, not to expose unnecessary surface to sun and
air; to thicken the skin until little moisture could evaporate
through the leathery coat; and, finally, to utilize the material
thus saved in developing stems so large, fleshy, and juicy that
they should become wells in a desert, with powers of sustenance
great enough to support the plant through its fiery trials. A
common expedient of plants in dry situations, even at the north,
is to modify their leaves into spines, as the gorse and the
barberry, for example, have done. That such an armor also serves
to protect them against the ravages of grazing animals is an
additional advantage, of course; but not their sole motive in
wearing it.
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