Movements and Habits of Climbing Plants | Page 6

at first, as could be seen by the ridges on its surface, not
in the least twisted; but when, after the 37th revolution, it had grown 9
inches long, and its revolving movement had ceased, it had become
twisted three times round its own axis, in the line of the course of the
sun; on the other hand, the common Convolvulus, which revolves in an
opposite course to the Hop, becomes twisted in an opposite direction.
Hence it is not surprising that Hugo von Mohl (p. 105, 108, &c.)
thought that the twisting of the axis caused the revolving movement;
but it is not possible that the twisting of the axis of the Hop three times
should have caused thirty-seven revolutions. Moreover, the revolving
movement commenced in the young internode before any twisting of
its axis could be detected. The internodes of a young Siphomeris and
Lecontea revolved during several days, but became twisted only once
round their own axes. The best evidence, however, that the twisting
does not cause the revolving movement is afforded by many
leaf-climbing and tendril-bearing plants (as Pisum sativum,
Echinocystis lobata, Bignonia capreolata, Eccremocarpus scaber, and
with the leaf-climbers, Solanum jasminoides and various species of

Clematis), of which the internodes are not twisted, but which, as we
shall hereafter see, regularly perform revolving movements like those
of true twining-plants. Moreover, according to Palm (pp. 30, 95) and
Mohl (p. 149), and Leon, {5} internodes may occasionally, and even
not very rarely, be found which are twisted in an opposite direction to
the other internodes on the same plant, and to the course of their
revolutions; and this, according to Leon (p. 356), is the case with all the
internodes of a certain variety of Phaseolus multiflorus. Internodes
which have become twisted round their own axes, if they have not
ceased to revolve, are still capable of twining round a support, as I have
several times observed.
Mohl has remarked (p. 111) that when a stem twines round a smooth
cylindrical stick, it does not become twisted. {6} Accordingly I allowed
kidney-beans to run up stretched string, and up smooth rods of iron and
glass, one-third of an inch in diameter, and they became twisted only in
that degree which follows as a mechanical necessity from the spiral
winding. The stems, on the other hand, which had ascended ordinary
rough sticks were all more or less and generally much twisted. The
influence of the roughness of the support in causing axial twisting was
well seen in the stems which had twined up the glass rods; for these
rods were fixed into split sticks below, and were secured above to cross
sticks, and the stems in passing these places became much twisted. As
soon as the stems which had ascended the iron rods reached the summit
and became free, they also became twisted; and this apparently
occurred more quickly during windy than during calm weather. Several
other facts could be given, showing that the axial twisting stands in
some relation to inequalities in the support, and likewise to the shoot
revolving freely without any support. Many plants, which are not
twiners, become in some degree twisted round their own axes; {7} but
this occurs so much more generally and strongly with twining-plants
than with other plants, that there must be some connexion between the
capacity for twining and axial twisting. The stem probably gains
rigidity by being twisted (on the same principle that a much twisted
rope is stiffer than a slackly twisted one), and is thus indirectly
benefited so as to be enabled to pass over inequalities in its spiral
ascent, and to carry its own weight when allowed to revolve freely. {8}
I have alluded to the twisting which necessarily follows on mechanical

principles from the spiral ascent of a stem, namely, one twist for each
spire completed. This was well shown by painting straight lines on
living stems, and then allowing them to twine; but, as I shall have to
recur to this subject under Tendrils, it may be here passed over.
The revolving movement of a twining plant has been compared with
that of the tip of a sapling, moved round and round by the hand held
some way down the stem; but there is one important difference. The
upper part of the sapling when thus moved remains straight; but with
twining plants every part of the revolving shoot has its own separate
and independent movement. This is easily proved; for when the lower
half or two-thirds of a long revolving shoot is tied to a stick, the upper
free part continues steadily revolving. Even if the whole shoot, except
an inch or two of the extremity, be tied up, this part, as I have seen in
the case of the Hop, Ceropegia, Convolvulus, &c.,