Movements and Habits of Climbing Plants | Page 9

this was apt to occur after some

small, ill-formed leaves had perished. The explanation, I believe, is,
that in such cases the lower parts of the terminal internodes very
gradually and successively lose their power of movement, whilst the
portions just above move onwards and in their turn become motionless;
and this ends in forming an irregular spire.
When a revolving shoot strikes a stick, it winds round it rather more
slowly than it revolves. For instance, a shoot of the Ceropegia, revolved
in 6 hrs., but took 9 hrs. 30 m. to make one complete spire round a stick;
Aristolochia gigas revolved in about 5 hrs., but took 9 hrs. 15 m. to
complete its spire. This, I presume, is due to the continued disturbance
of the impelling force by the arrestment of the movement at successive
points; and we shall hereafter see that even shaking a plant retards the
revolving movement. The terminal internodes of a long, much-inclined,
revolving shoot of the Ceropegia, after they had wound round a stick,
always slipped up it, so as to render the spire more open than it was at
first; and this was probably in part due to the force which caused the
revolutions, being now almost freed from the constraint of gravity and
allowed to act freely. With the Wistaria, on the other hand, a long
horizontal shoot wound itself at first into a very close spire, which
remained unchanged; but subsequently, as the shoot twined spirally up
its support, it made a much more open spire. With all the many plants
which were allowed freely to ascend a support, the terminal internodes
made at first a close spire; and this, during windy weather, served to
keep the shoots in close contact with their support; but as the
penultimate internodes grew in length, they pushed themselves up for a
considerable space (ascertained by coloured marks on the shoot and on
the support) round the stick, and the spire became more open. {13}
It follows from this latter fact that the position occupied by each leaf
with respect to the support depends on the growth of the internodes
after they have become spirally wound round it. I mention this on
account of an observation by Palm (p. 34), who states that the opposite
leaves of the Hop always stand in a row, exactly over one another, on
the same side of the supporting stick, whatever its thickness may be.
My sons visited a hop-field for me, and reported that though they
generally found the points of insertion of the leaves standing over each
other for a space of two or three feet in height, yet this never occurred
up the whole length of the pole; the points of insertion forming, as

might have been expected, an irregular spire. Any irregularity in the
pole entirely destroyed the regularity of position of the leaves. From
casual inspection, it appeared to me that the opposite leaves of
Thunbergia alata were arranged in lines up the sticks round which they
had twined; accordingly, I raised a dozen plants, and gave them sticks
of various thicknesses, as well as string, to twine round; and in this case
one alone out of the dozen had its leaves arranged in a perpendicular
line: I conclude, therefore, Palm's statement is not quite accurate.
The leaves of different twining-plants are arranged on the stem (before
it has twined) alternately, or oppositely, or in a spire. In the latter case
the line of insertion of the leaves and the course of the revolutions
coincide. This fact has been well shown by Dutrochet, {14} who found
different individuals of Solanum dulcamara twining in opposite
directions, and these had their leaves in each case spirally arranged in
the same direction. A dense whorl of many leaves would apparently be
incommodious for a twining plant, and some authors assert that none
have their leaves thus arranged; but a twining Siphomeris has whorls of
three leaves.
If a stick which has arrested a revolving shoot, but has not as yet been
encircled, be suddenly taken away, the shoot generally springs forward,
showing that it was pressing with some force against the stick. After a
shoot has wound round a stick, if this be withdrawn, it retains for a time
its spiral form; it then straightens itself, and again commences to
revolve. The long, much-inclined shoot of the Ceropegia previously
alluded to offered some curious peculiarities. The lower and older
internodes, which continued to revolve, were incapable, on repeated
trials, of twining round a thin stick; showing that, although the power
of movement was retained, this was not sufficient to enable the plant to
twine. I then moved the stick to a greater distance, so that it was struck
by a point 2.5 inches from the extremity of