climbing is an economical method of obtaining a freer exposure to light and
air with the smallest possible expenditure of material. But twiners have
one disadvantage: to rise ten feet they must produce fifteen feet of stem
or thereabouts, according to the diameter of the support, and the openness
or closeness of the coil. A rootlet-climber saves much in this respect, but
has a restricted range of action, and other disadvantages.

There are two other modes, which combine the utmost economy of material
with freer range of action. There are, in the first place, leaf-climbers of
various sorts, agreeing only in this, that the duty of laying hold is
transferred to the leaves, so that the stem may rise in a direct line.
Sometimes the blade or leaflets, or some of them, but more commonly their
slender stalks, undertake the work, and the plant rises as a boy ascends a
tree, grasping first with one hand or arm, then with the other. Indeed, the
comparison, like the leaf-stalk, holds better than would be supposed; for
the grasping of the latter is not the result of a blind groping in all
directions by a continuous movement, but of a definite sensitiveness which
acts only upon the occasion. Most leaves make no regular sweeps; but when
the stalks of a leaf-climbing species come into prolonged contact with any
fitting extraneous body, they slowly incurve and make a turn around it, and
then commonly thicken and harden until they attain a strength which may
equal that of the stem itself. Here we have the faculty of movement to a
definite end, upon external irritation, of the same nature with that
displayed by Dionaea and Drosera, although slower for the most part than
even in the latter. But the movement of the hour-hand of the clock is not
different in nature or cause from that of the second-hand.

Finally--distribution of office being, on the whole, most advantageous and
economical, and this, in the vegetable kingdom, being led up to by
degrees--we reach, through numerous gradations, the highest style of