ke
those of twining stems. Those of some Passion-flowers, in sultry
weather, are apt to move fast enough for the movement actually to be
seen for a part of the circuit, as plainly as that of the second-hand of
a watch. Two herbaceous species, Passiflora gracilis and P. sicyoides
(the first an annual, the second a strong-rooted perennial of the
easiest cultivation), are admirable for illustration both of revolving
movements and of sensitive coiling.

[Illustration: Fig. 490. Piece of stem of Sensitive Plant (Mimosa
pudica), with two leaves, the lower open, the upper in the closed
state.]

472. =Movements under Irritation.= The most familiar case is that of the
Sensitive Plant (Fig. 490). The leaves suddenly take their nocturnal
position when roughly touched or when shocked by a jar. The leaflets
close in pairs, the four outspread partial petioles come closer
together, and the common petiole is depressed. The seat of the movements
is at the base of the leaf-stalk and stalklets. Schrankia, a near
relative of the Sensitive Plant, acts in the same way, but is slower.
These are not anomalous actions, but only extreme manifestations of a
faculty more or less common in foliage. In Locust and Honey-Locusts for
example, repeated jars will slowly produce similar effects.

473. Leaf-stalks and tendrils are adapted to their uses in climbing by
a similar sensitiveness. The coiling of the leaf-stalk is in response to
a kind of irritation produced by contact with the supporting body. This
may be shown by gentle rubbing or prolonged pressure upon the upper face
of the leaf-stalk, which is soon followed by a curvature. Tendrils are
still more sensitive to contact or light friction. This causes the free
end of the tendril to coil round the support, and the sensitiveness,
propagated downward along the tendril, causes that side of it to become
less turgescent or the opposite side more so, thus throwing the tendril
into coils. This shortening draws the plant up to the support. Tendrils
which have not laid hold will at length commonly coil spontaneously, in
a simple coil, from the free apex downward. In Sicyos, Echinocystis, and
the above mentioned Passion-flowers (471), the tendril is so sensitive,
under a high summer temperature, that it will curve and coil promptly
after one or two light strokes by the hand.

[Illustration: Fig. 491. Portion of stem and leaves of Telegraph-plant
(Desmodium gyrans), almost of natural size.]

474. Amon