is line of turgescence travels
round the shoot continually, from right to left or from left to right
according to the species: thus the shoot bends to all points of the
compass in succession. Commonly this nutation is slight or hardly
observable. It is most marked in

468. =Twining Stems= (Fig. 90). The growing upper end of such stems, as
is familiar in the Hop, Pole Beans, and Morning-Glory, turns over in an
inclined or horizontal direction, thus stretching out to reach a
neighboring support, and by the continual change in the direction of the
nodding, sweeps the whole circle, the sweeps being the longer as the
stem lengthens. When it strikes against a support, such as a stem or
branch of a neighboring plant, the motion is arrested at the contact,
but continues at the growing apex beyond, and this apex is thus made to
wind spirally around the supporting body.

469. =Leaf-movements= are all but universal. The presentation by most
leaves of their upper surface to the light, from whatever direction that
may come, is an instance; for when turned upside down they twist or bend
round on the stalk to recover this normal position. Leaves, and the
leaflets of compound leaves, change this position at nightfall, or when
the light is withdrawn; they then take what is called their sleeping
posture, resuming the diurnal position when daylight returns. This is
very striking in Locust-trees, in the Sensitive Plant (Fig. 490), and
in Woodsorrel. Young seedlings droop or close their leaves at night in
plants which are not thus affected in the adult foliage. All this is
thought to be a protection against the cold by nocturnal radiation.

470. Various plants climb by a coiling movement of their leaves or their
leaf-stalks. Familiar examples are seen in Clematis, Maurandia,
Tropaeolum, and in a Solanum which is much cultivated in greenhouses
(Fig. 172). In the latter, and in other woody plants which climb in this
way, the petioles thicken and harden after they have grasped their
support, thus securing a very firm hold.

471. =Tendril movements.= Tendrils are either leaves or stems (98, 168),
specially developed for climbing purposes. Cobaea is a good example of
partial transformation; some of the leaflets are normal, some of the
same leaf are little tendrils, and some intermediate in character. The
Passion-flowers give good examples of simple stem-tendrils (Fig. 92);
Grape-Vines, of branched ones. Most tendrils make revolving sweeps, li