e surface is as nothing compared
with an ordinary leafy plant of the same bulk. Compare, for instance,
the largest Cactus known, the Giant Cereus of the Gila River (Fig. 111,
in the background), which rises to the height of fifty or sixty feet,
with a common leafy tree of the same height, such as that in Fig. 89,
and estimate how vastly greater, even without the foliage, the surface
of the latter is than that of the former. Compare, in the same view, an
Opuntia or Prickly-Pear Cactus, its stem and branches formed of a
succession of thick and flattened joints (Fig. 111, _a_), which may be
likened to tubers, or an Epiphyllum (_d_), having short and flat joints,
with an ordinary leafy shrub or herb of equal size. And finally, in
Melon-Cactuses, Echinocactus (_c_), or other globose forms (which may be
likened to permanent corms), with their globular or bulb-like shapes, we
have plants in the compactest shape; their spherical figure being such
as to expose the least possible amount of substance to the air. These
are adaptations to climates which are very dry, either throughout or for
a part of the year. Similarly, bulbous and corm-bearing plants, and the
like, are examples of a form of vegetation which in the growing season
may expand a large surface to the air and light, while during the period
of rest the living vegetable is reduced to a globe, or solid form of the
least possible surface; and this protected by its outer coats of dead
and dry scales, as well as by its situation under ground. Such are also
adapted to a season of drought. They largely belong to countries which
have a long hot season of little or no rain, when, their stalks and
foliage above and their roots beneath early perishing, the plants rest
securely in their compact bulbs, filled with nourishment and retaining
their moisture with great tenacity, until the rainy season comes round.
Then they shoot forth leaves and flowers with wonderful rapidity, and
what was perhaps a desert of arid sand becomes green with foliage and
gay with blossoms, almost in a day.

[Illustration: Fig. 111.]

Section VII. LEAVES.

118. STEMS bear leaves, at definite points (nodes, 13); and these are
produced in a great variety of forms, and subserve various uses. The
commonest kind of leaf, which therefore may be taken as the type or
pattern, is an expanded green body, by means of which the plant exposes
to the air and light the matters which it imbibes, exhales certain