Movements and Habits of Climbing Plants | Page 8

this seems to be much
more important) it causes the extremity of the shoot to embrace the
support much more closely than it could otherwise have done, and thus
aids in preventing the stem from being blown away during windy
weather, as I have many times observed. In Lonicera brachypoda the
hook only straightens itself periodically, and never becomes reversed. I
will not assert that the tips of all twining plants when hooked, either
reverse themselves or become periodically straight, in the manner just
described; for the hooked form may in some cases be permanent, and
be due to the manner of growth of the species, as with the tips of the
shoots of the common vine, and more plainly with those of Cissus
discolor--plants which are not spiral twiners.
The first purpose of the spontaneous revolving movement, or, more
strictly speaking, of the continuous bowing movement directed
successively to all points of the compass, is, as Mohl has remarked, to
favour the shoot finding a support. This is admirably effected by the
revolutions carried on night and day, a wider and wider circle being
swept as the shoot increases in length. This movement likewise
explains how the plants twine; for when a revolving shoot meets with a
support, its motion is necessarily arrested at the point of contact, but the
free projecting part goes on revolving. As this continues, higher and
higher points are brought into contact with the support and are arrested;
and so onwards to the extremity; and thus the shoot winds round its
support. When the shoot follows the sun in its revolving course, it
winds round the support from right to left, the support being supposed
to stand in front of the beholder; when the shoot revolves in an opposite
direction, the line of winding is reversed. As each internode loses from
age its power of revolving, it likewise loses its power of spirally
twining. If a man swings a rope round his head, and the end hits a stick,
it will coil round the stick according to the direction of the swinging
movement; so it is with a twining plant, a line of growth travelling
round the free part of the shoot causing it to bend towards the opposite

side, and this replaces the momentum of the free end of the rope.
All the authors, except Palm and Mohl, who have discussed the spiral
twining of plants, maintain that such plants have a natural tendency to
grow spirally. Mohl believes (p. 112) that twining stems have a dull
kind of irritability, so that they bend towards any object which they
touch; but this is denied by Palm. Even before reading Mohl's
interesting treatise, this view seemed to me so probable that I tested it
in every way that I could, but always with a negative result. I rubbed
many shoots much harder than is necessary to excite movement in any
tendril or in the foot-stalk of any leaf climber, but without any effect. I
then tied a light forked twig to a shoot of a Hop, a Ceropegia,
Sphaerostemma, and Adhatoda, so that the fork pressed on one side
alone of the shoot and revolved with it; I purposely selected some very
slow revolvers, as it seemed most likely that these would profit most
from possessing irritability; but in no case was any effect produced.
{11} Moreover, when a shoot winds round a support, the winding
movement is always slower, as we shall immediately see, than whilst it
revolves freely and touches nothing. Hence I conclude that twining
stems are not irritable; and indeed it is not probable that they should be
so, as nature always economizes her means, and irritability would have
been superfluous. Nevertheless I do not wish to assert that they are
never irritable; for the growing axis of the leaf-climbing, but not
spirally twining, Lophospermum scandens is, certainly irritable; but
this case gives me confidence that ordinary twiners do not possess any
such quality, for directly after putting a stick to the Lophopermum, I
saw that it behaved differently from a true twiner or any other
leaf-climber. {12}
The belief that twiners have a natural tendency to grow spirally,
probably arose from their assuming a spiral form when wound round a
support, and from the extremity, even whilst remaining free, sometimes
assuming this form. The free internodes of vigorously growing plants,
when they cease to revolve, become straight, and show no tendency to
be spiral; but when a shoot has nearly ceased to grow, or when the plant
is unhealthy, the extremity does occasionally become spiral. I have
seen this in a remarkable manner with the ends of the shoots of the
Stauntonia and of the allied Akebia, which became wound up into a
close spire, just like a tendril; and