[Picture: Book cover]
THE MOVEMENTS
AND HABITS OF
CLIMBING PLANTS.
BY CHARLES DARWIN, F.R.S.
* * * * *
POPULAR EDITION
* * * * *
LONDON:
JOHN MURRAY, ALBEMARLE STREET.
1906.
PREFACE
THIS Essay first appeared in the ninth volume of the ‘Journal of the
Linnean Society,’ published in 1865. It is here reproduced in a
corrected and, I hope, clearer form, with some additional facts. The
illustrations were drawn by my son, George Darwin. Fritz Müller, after
the publication of my paper, sent to the Linnean Society (Journal, vol.
ix., p. 344) some interesting observations on the climbing plants of
South Brazil, to which I shall frequently refer. Recently two important
memoirs, chiefly on the difference in growth between the upper and lower
sides of tendrils, and on the mechanism of the movements of
twining-plants, by Dr. Hugo de Vries, have appeared in the ‘Arbeiten des
Botanischen Instituts in Würzburg,’ Heft. iii., 1873. These memoirs
ought to be carefully studied by every one interested in the subject, as
I can here give only references to the more important points. This
excellent observer, as well as Professor Sachs, {iv} attributes all the
movements of tendrils to rapid growth along one side; but, from reasons
assigned towards the close of my fourth chapter, I cannot persuade myself
that this holds good with respect to those due to a touch. In order that
the reader may know what points have interested me most, I may call his
attention to certain tendril-bearing plants; for instance, Bignonia
capreolata, Cobæa, Echinocystis, and Hanburya, which display as beautiful
adaptations as can be found in any part of the kingdom of nature. It is,
also, an interesting fact that intermediate states between organs fitted
for widely different functions, may be observed on the same individual
plant of Corydalis claviculata and the common vine; and these cases
illustrate in a striking manner the principle of the gradual evolution of
species.
APPENDIX TO PREFACE (1882).
SINCE the publication of this Edition two papers by eminent botanists
have appeared; Schwendener, ‘Das Winden der Pflanzen’ (Monatsberichte der
Berliner Akademie, Dec. 1881), and J. Sachs, ‘Notiz über Schlingpflanzen’
(Arbeiten des botanischen Instituts in Würzburg, Bd. ii. p. 719, 1882).
The view “that the capacity of revolving, on which most climbers depend,
is inherent, though undeveloped, in almost every plant in the vegetable
kingdom” (‘Climbing Plants,’ p. 205), has been confirmed by the
observations on circumnutation since given in ‘The Power of Movement in
Plants.’
ERRATA.
On pp. 28, 32, 40, 53, statements are made with reference to the supposed
acceleration of the revolving movement towards the light. It appears
from the observations given in ‘The Power of Movement in Plants,’ p. 451,
that these conclusions were drawn from insufficient observations, and are
erroneous.
THE MOVEMENTS AND HABITS OF CLIMBING PLANTS.
CHAPTER I.
TWINING PLANTS.
Introductory remarks—Description of the twining of the Hop—Torsion of the
stems—Nature of the revolving movement, and manner of ascent—Stems not
irritable—Rate of revolution in various plants—Thickness of the support
round which plants can twine—Species which revolve in an anomalous
manner.
I WAS led to this subject by an interesting, but short paper by Professor
Asa Gray on the movements of the tendrils of some Cucurbitaceous plants.
{1a} My observations were more than half completed before I learnt that
the surprising phenomenon of the spontaneous revolutions of the stems and
tendrils of climbing plants had been long ago observed by Palm and by
Hugo von Mohl, {1b} and had subsequently been the subject of two memoirs
by Dutrochet. {1c} Nevertheless, I believe that my observations, founded
on the examination of above a hundred widely distinct living species,
contain sufficient novelty to justify me in publishing them.
Climbing plants may be divided into four classes. First, those which
twine spirally round a support, and are not aided by any other movement.
Secondly, those endowed with irritable organs, which when they touch any
object clasp it; such organs consisting of modified leaves, branches, or
flower-peduncles. But these two classes sometimes graduate to a certain
extent into one another. Plants of the third class ascend merely by the
aid of hooks; and those of the fourth by rootlets; but as in neither
class do the plants exhibit any special movements, they present little
interest, and generally when I speak of climbing plants I refer to the
two first great classes.
TWINING PLANTS.
This is the largest subdivision, and is apparently the primordial and
simplest condition of the class. My observations will be best given by
taking a few special cases. When the shoot of a Hop (_Humulus lupulus_)
rises from the ground, the two or three first-formed joints or internodes
are straight and remain stationary; but the next-formed, whilst very
young, may be seen to bend to one side and to travel slowly round towards
all points of the compass, moving, like the hands of a watch, with the
sun. The movement very soon acquires its full ordinary velocity. From
seven observations made during August on shoots proceeding from a plant
which had been cut down, and on another plant during April, the average
rate during hot weather and during the day is 2 hrs. 8 m. for each
revolution; and none of the revolutions varied much from this rate. The
revolving movement continues as long as the plant continues to grow; but
each separate internode, as it becomes old, ceases to move.
To ascertain more precisely what amount of movement each internode
underwent, I kept a potted plant, during the night and day, in a
well-warmed room to which I was confined by illness. A long shoot
projected beyond the upper end of the supporting stick, and was steadily
revolving. I then took a longer stick and tied up the shoot, so that
only a very young internode, 1¾ of an inch in length, was left free.
This was so nearly upright that its revolution could not be easily
observed; but it certainly moved, and the side of the internode which was
at one time convex became concave, which, as we shall hereafter see, is a
sure sign of the revolving movement. I will assume that it made at least
one revolution during the first twenty-four hours. Early the next
morning its position was marked, and it made a second revolution in 9
hrs.; during the latter part of this revolution it moved much quicker,
and the third circle was performed in the evening in a little over 3 hrs.
As on the succeeding morning I found that the shoot revolved in 2 hrs. 45
m., it must have made during the night four revolutions, each at the
average rate of a little over 3 hrs. I should add that the temperature
of the room varied only a little. The shoot had now grown 3½ inches in
length, and carried at its extremity a young internode 1 inch in length,
which showed slight changes in its curvature. The next or ninth
revolution was effected in 2 hrs. 30 m. From this time forward, the
revolutions were easily observed. The thirty-sixth revolution was
performed at the usual rate; so was the last or thirty-seventh, but it
was not completed; for the internode suddenly became upright, and after
moving to the centre, remained motionless. I tied a weight to its upper
end, so as to bow it slightly and thus detect any movement; but there was
none. Some time before the last revolution was half performed, the lower
part of the internode ceased to move.
A few more remarks will complete all that need be said about this
internode. It moved during five days; but the more rapid movements,
after the performance of the third revolution, lasted during three days
and twenty hours. The regular revolutions, from the ninth to
thirty-sixth inclusive, were effected at the average rate of 2 hrs. 31
m.; but the weather was cold, and this affected the temperature of the
room, especially during the night, and consequently retarded the rate of
movement a little. There was only one irregular movement, which
consisted in the stem rapidly making, after an unusually slow revolution,
only the segment of a circle. After the seventeenth revolution the
internode had grown from 1¾ to 6 inches in length, and carried an
internode 1⅞ inch long, which was just perceptibly moving; and this
carried a very minute ultimate internode. After the twenty-first
revolution, the penultimate internode was 2½ inches long, and probably
revolved in a period of about three hours. At the twenty-seventh
revolution the lower and still moving internode was 8⅜, the penultimate
3½, and the ultimate 2½ inches in length; and the inclination of the
whole shoot was such, that a circle 19 inches in diameter was swept by
it. When the movement ceased, the lower internode was 9 inches, and the
penultimate 6 inches in length; so that, from the twenty-seventh to
thirty-seventh revolutions inclusive, three internodes were at the same
time revolving.
The lower internode, when it ceased revolving, became upright and rigid;
but as the whole shoot was left to grow unsupported, it became after a
time bent into a nearly horizontal position, the uppermost and growing
internodes still revolving at the extremity, but of course no longer
round the old central point of the supporting stick. From the changed
position of the centre of gravity of the extremity, as it revolved, a
slight and slow swaying movement was given to the long horizontally
projecting shoot; and this movement I at first thought was a spontaneous
one. As the shoot grew, it hung down more and more, whilst the growing
and revolving extremity turned itself up more and more.
With the Hop we have seen that three internodes were at the same time
revolving; and this was the case with most of the plants observed by me.
With all, if in full health, two internodes revolved; so that by the time
the lower one ceased to revolve, the one above was in full action, with a
terminal internode just commencing to move. With _Hoya carnosa_, on the
other hand, a depending shoot, without any developed leaves, 32 inches in
length, and consisting of seven internodes (a minute terminal one, an
inch in length, being counted), continually, but slowly, swayed from side
to side in a semicircular course, with the extreme internodes making
complete revolutions. This swaying movement was certainly due to the
movement of the lower internodes, which, however, had not force
sufficient to swing the whole shoot round the central supporting stick.
The case of another Asclepiadaceous plant, viz., _Ceropegia Gardnerii_,
is worth briefly giving. I allowed the top to grow out almost
horizontally to the length of 31 inches; this now consisted of three long
internodes, terminated by two short ones. The whole revolved in a course
opposed to the sun (the reverse of that of the Hop), at rates between 5
hrs. 15 m. and 6 hrs. 45 m. for each revolution. The extreme tip thus
made a circle of above 5 feet (or 62 inches) in diameter and 16 feet in
circumference, travelling at the rate of 32 or 33 inches per hour. The
weather being hot, the plant was allowed to stand on my study-table; and
it was an interesting spectacle to watch the long shoot sweeping this
grand circle, night and day, in search of some object round which to
twine.
If we take hold of a growing sapling, we can of course bend it to all
sides in succession, so as to make the tip describe a circle, like that
performed by the summit of a spontaneously revolving plant. By this
movement the sapling is not in the least twisted round its own axis. I
mention this because if a black point be painted on the bark, on the side
which is uppermost when the sapling is bent towards the holder’s body, as
the circle is described, the black point gradually turns round and sinks
to the lower side, and comes up again when the circle is completed; and
this gives the false appearance of twisting, which, in the case of
spontaneously revolving plants, deceived me for a time. The appearance
is the more deceitful because the axes of nearly all twining-plants are
really twisted; and they are twisted in the same direction with the
spontaneous revolving movement. To give an instance, the internode of
the Hop of which the history has been recorded, was 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 Léon, {8}
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
Léon (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. {9a} 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; {9b} 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. {10}
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., goes on revolving, but much more
slowly; for the internodes, until they have grown to some little length,
always move slowly. If we look to the one, two, or several internodes of
a revolving shoot, they will be all seen to be more or less bowed, either
during the whole or during a large part of each revolution. Now if a
coloured streak be painted (this was done with a large number of twining
plants) along, we will say, the convex surface, the streak will after a
time (depending on the rate of revolution) be found to be running
laterally along one side of the bow, then along the concave side, then
laterally on the opposite side, and, lastly, again on the originally
convex surface. This clearly proves that during the revolving movement
the internodes become bowed in every direction. The movement is, in
fact, a continuous self-bowing of the whole shoot, successively directed
to all points of the compass; and has been well designated by Sachs as a
revolving nutation.
As this movement is rather difficult to understand, it will be well to
give an illustration. Take a sapling and bend it to the south, and paint
a black line on the convex surface; let the sapling spring up and bend it
to the east, and the black line will be seen to run along the lateral
face fronting the north; bend it to the north, the black line will be on
the concave surface; bend it to the west, the line will again be on the
lateral face; and when again bent to the south, the line will be on the
original convex surface. Now, instead of bending the sapling, let us
suppose that the cells along its northern surface from the base to the
tip were to grow much more rapidly than on the three other sides, the
whole shoot would then necessarily be bowed to the south; and let the
longitudinal growing surface creep round the shoot, deserting by slow
degrees the northern side and encroaching on the western side, and so
round by the south, by the east, again to the north. In this case the
shoot would remain always bowed with the painted line appearing on the
several above specified surfaces, and with the point of the shoot
successively directed to each point of the compass. In fact, we should
have the exact kind of movement performed by the revolving shoots of
twining plants. {12}
It must not be supposed that the revolving movement is as regular as that
given in the above illustration; in very many cases the tip describes an
ellipse, even a very narrow ellipse. To recur once again to our
illustration, if we suppose only the northern and southern surfaces of
the sapling alternately to grow rapidly, the summit would describe a
simple arc; if the growth first travelled a very little to the western
face, and during the return a very little to the eastern face, a narrow
ellipse would be described; and the sapling would be straight as it
passed to and fro through the intermediate space; and a complete
straightening of the shoot may often be observed in revolving plants.
The movement is frequently such that three of the sides of the shoot seem
to be growing in due order more rapidly than the remaining side; so that
a semi-circle instead of a circle is described, the shoot becoming
straight and upright during half of its course.
When a revolving shoot consists of several internodes, the lower ones
bend together at the same rate, but one or two of the terminal ones bend
at a slower rate; hence, though at times all the internodes are in the
same direction, at other times the shoot is rendered slightly serpentine.
The rate of revolution of the whole shoot, if judged by the movement of
the extreme tip, is thus at times accelerated or retarded. One other
point must be noticed. Authors have observed that the end of the shoot
in many twining plants is completely hooked; this is very general, for
instance, with the Asclepiadaceæ. The hooked tip, in all the cases
observed by me, viz. in _Ceropegia_, _Sphærostemma_, _Clerodendron_,
_Wistaria_, _Stephania_, _Akebia_, and _Siphomeris_, has exactly the same
kind of movement as the other internodes; for a line painted on the
convex surface first becomes lateral and then concave; but, owing to the
youth of these terminal internodes, the reversal of the hook is a slower
process than that of the revolving movement. {14} This strongly marked
tendency in the young, terminal and flexible internodes, to bend in a
greater degree or more abruptly than the other internodes, is of service
to the plant; for not only does the hook thus formed sometimes serve to
catch a support, but (and 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_,
_Sphærostemma_, 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.
{16} 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.
{17}
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 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. {18}
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, {19}
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 2½ inches from the extremity of the penultimate
internode; and it was then neatly encircled by this part of the
penultimate and by the ultimate internode. After leaving the spirally
wound shoot for eleven hours, I quietly withdrew the stick, and in the
course of the day the curled portion straightened itself and recommenced
revolving; but the lower and not curled portion of the penultimate
internode did not move, a sort of hinge separating the moving and the
motionless part of the same internode. After a few days, however, I
found that this lower part had likewise recovered its revolving power.
These several facts show that the power of movement is not immediately
lost in the arrested portion of a revolving shoot; and that after being
temporarily lost it can be recovered. When a shoot has remained for a
considerable time round a support, it permanently retains its spiral form
even when the support is removed.
When a tall stick was placed so as to arrest the lower and rigid
internodes of the _Ceropegia_, at the distance at first of 15 and then of
21 inches from the centre of revolution, the straight shoot slowly and
gradually slid up the stick, so as to become more and more highly
inclined, but did not pass over the summit. Then, after an interval
sufficient to have allowed of a semi-revolution, the shoot suddenly
bounded from the stick and fell over to the opposite side or point of the
compass, and reassumed its previous slight inclination. It now
recommenced revolving in its usual course, so that after a
semi-revolution it again came into contact with the stick, again slid up
it, and again bounded from it and fell over to the opposite side. This
movement of the shoot had a very odd appearance, as if it were disgusted
with its failure but was resolved to try again. We shall, I think,
understand this movement by considering the former illustration of the
sapling, in which the growing surface was supposed to creep round from
the northern by the western to the southern face; and thence back again
by the eastern to the northern face, successively bowing the sapling in
all directions. Now with the _Ceropegia_, the stick being placed to the
south of the shoot and in contact with it, as soon as the circulatory
growth reached the western surface, no effect would be produced, except
that the shoot would be pressed firmly against the stick. But as soon as
growth on the southern surface began, the shoot would be slowly dragged
with a sliding movement up the stick; and then, as soon as the eastern
growth commenced, the shoot would be drawn from the stick, and its weight
coinciding with the effects of the changed surface of growth, would cause
it suddenly to fall to the opposite side, reassuming its previous slight
inclination; and the ordinary revolving movement would then go on as
before. I have described this curious case with some care, because it
first led me to understand the order in which, as I then thought, the
surfaces contracted; but in which, as we now know from Sachs and II. de
Vries, they grow for a time rapidly, thus causing the shoot to bow
towards the opposite side.
The view just given further explains, as I believe, a fact observed by
Mohl (p. 135), namely, that a revolving shoot, though it will twine round
an object as thin as a thread, cannot do so round a thick support. I
placed some long revolving shoots of a _Wistaria_ close to a post between
5 and 6 inches in diameter, but, though aided by me in many ways, they
could not wind round it. This apparently was due to the flexure of the
shoot, whilst winding round an object so gently curved as this post, not
being sufficient to hold the shoot to its place when the growing surface
crept round to the opposite surface of the shoot; so that it was
withdrawn at each revolution from its support.
When a free shoot has grown far beyond its support, it sinks downwards
from its weight, as already explained in the case of the Hop, with the
revolving extremity turned upwards. If the support be not lofty, the
shoot falls to the ground, and resting there, the extremity rises up.
Sometimes several shoots, when flexible, twine together into a cable, and
thus support one another. Single thin depending shoots, such as those of
the _Sollya Drummondii_, will turn abruptly backwards and wind up on
themselves. The greater number of the depending shoots, however, of one
twining plant, the _Hibbertia dentata_, showed but little tendency to
turn upwards. In other cases, as with the _Cryptostegia grandiflora_,
several internodes which were at first flexible and revolved, if they did
not succeed in twining round a support, become quite rigid, and
supporting themselves upright, carried on their summits the younger
revolving internodes.
Here will be a convenient place to give a Table showing the direction and
rate of movement of several twining plants, with a few appended remarks.
These plants are arranged according to Lindley’s ‘Vegetable Kingdom’ of
1853; and they have been selected from all parts of the series so as to
show that all kinds behave in a nearly uniform manner. {24}
The Rate of Revolution of various Twining Plants.
(ACOTYLEDONS.)
_Lygodium scandens_ (Polypodiaceæ) moves against the sun.
H. M.
June 18, 1st circle was made in 6 0
18, 2nd 6 15 (late in evening)
19, 3rd 5 32 (very hot day)
19, 4th 5 0 (very hot day)
20, 5th 6 0
_Lygodium articulatum_ moves against the sun.
H. M.
July 19, 1st circle was made in 16 30 (shoot very young)
20, 2nd 15 0
21, 3rd 8 0
22, 4th 10 30
(MONOCOTYLEDONS.)
_Ruscus androgynus_ (Liliaceæ), placed in the hot-house, moves against
the sun.
H. M.
May 24, 1st circle was made in 6 14 (shoot very young)
25, 2nd 2 21
25, 3rd 3 37
25, 4th 3 22
26, 5th 2 50
27, 6th 3 52
27, 7th 4 11
_Asparagus_ (unnamed species from Kew) (Liliaceæ) moves against the sun,
placed in hothouse.
H. M.
Dec. 26, 1st circle was made in 5 0
27, 2nd 5 40
_Tamus communis_ (Dioscoreaceæ). A young shoot from a tuber in a pot
placed in the greenhouse: follows the sun.
H. M.
July, 7, 1st circle was made in 3 10
7, 2nd 2 38
8, 3rd 3 5
8, 4th 2 56
8, 5th 2 30
8, 6th 2 30
_Lapagerea rosea_ (Philesiaceæ), in greenhouse, follows the sun.
H. M.
March 9, 1st circle was made in 26 15 (shoot young)
10, semicircle 8 15
11, 2nd circle 11 0
12, 3rd 15 30
13, 4th 14 15
16, 5th 8 40 when placed in the
hothouse; but the
next day the shoot
remained
stationary.
_Roxburghia viridiflora_ (Roxburghiaceæ) moves against the sun; it
completed a circle in about 24 hours.
(DICOTYLEDONS.)
_Humulus Lupulus_ (Urticaceæ) follows the sun. The plant was kept in a
room during warm weather.
H. M.
April 9, 2 circles were made in 4 16
Aug. 13, 3rd circle was 2 0
14, 4th 2 20
14, 5th 2 16
14, 6th 2 2
14, 7th 2 0
14, 8th 2 4
With the Hop a semicircle was performed, in travelling from the light, in
1 hr. 33 m.; in travelling to the light, in 1 hr. 13 m.; difference of
rate, 20 m.
_Akebia quinata_ (Lardizabalaceæ), placed in hothouse, moves against the
sun.
H. M.
March 17, 1st circle was made in 4 0 (shoot young)
18, 2nd 1 40
18, 3rd 1 30
19, 4th 1 45
_Stauntonia latifolia_ (Lardizabalaceæ), placed in hothouse, moves
against the sun.
H. M.
March 28, 1st circle was made in 3 30
29, 2nd 3 45
_Sphærostemma marmoratum_ (Schizandraceæ) follows the sun.
H. M.
August 5th, 1st circle was made in about 24 0
5th, 2nd circle was made in 18 30
_Stephania rotunda_ (Menispermaceæ) moves against the sun.
H. M.
May 27, 1st circle was made in 5 5
30, 2nd 7 6
June 2, 3rd 5 15
3, 4th 6 28
Thryallis brachystachys (Malpighiaceæ) moves against the sun: one shoot
made a circle in 12 hrs., and another in 10 hrs. 30 m.; but the next day,
which was much colder, the first shoot took 10 hrs. to perform only a
semicircle.
Hibbertia dentata (Dilleniaceæ), placed in the hothouse, followed the
sun, and made (May 18th) a circle in 7 hrs. 20 m.; on the 19th, reversed
its course, and moved against the sun, and made a circle in 7 hrs.; on
the 20th, moved against the sun one-third of a circle, and then stood
still; on the 26th, followed the sun for two-thirds of a circle, and then
returned to its starting-point, taking for this double course 11 hrs. 46
m.
_Sollya Drummondii_ (Pittosporaceæ) moves against the sun kept in
greenhouse.
H. M.
April 4, 1st circle was made in 4 25
5, 2nd 8 0 (very cold day)
6, 3rd 6 25
7, 4th 7 5
_Polygonum dumetorum_ (Polygonaceæ). This case is taken from Dutrochet
(p. 299), as I observed, no allied plant: follows the sun. Three
shoots, cut off a plant, and placed in water made circles in 3 hrs. 10
m., 5 hrs. 20 m., and 7 hrs. 15 m.
_Wistaria Chinensis_ (Leguminosæ), in greenhouse, moves against the sun.
H. M.
May 13, 1st circle was made in 3 5
13, 2nd 3 20
16, 3rd 2 5
24, 4th 3 21
25, 5th 2 37
25, 6th 2 35
_Phaseolus vulgaris_ (Leguminosæ), in greenhouse, moves against the sun.
H. M.
May, 1st circle was made in 2 0
2nd 1 55
3rd 1 55
_Dipladenia urophylla_ (Apocynaceæ) moves against the sun.
H. M.
April 18, 1st circle was made in 8 0
19, 2nd 9 15
30, 3rd 9 40
_Dipladenia crassinoda_ moves against the sun.
H. M.
May 16, 1st circle was made in 9 5
July 20, 2nd 8 0
21, 3rd 8 5
_Ceropegia Gardnerii_ (Asclepiadaceæ) moves against the sun.
H. M.
Shoot very young, 2 inches in 1st circle was 7 55
length performed in
Shoot still young 2nd 7 0
Long shoot 3rd 6 33
Long shoot 4th 5 15
Long shoot 5th 6 45
_Stephanotis floribunda_ (Asclepiadaceæ) moves against the sun and made a
circle in 6 hrs. 40 m., a second circle in about 9 hrs.
_Hoya carnosa_ (Asclepiadaceæ) made several circles in from 16 hrs. to 22
hrs. or 24 hrs.
_Ipomæa purpurea_ (Convolvulaceæ) moves against the sun. Plant placed in
room with lateral light.
1st circle was made in 2 hrs. 42 Semicircle, from the light in 1
m. hr. 14 m., to the light 1 hr. 28
m.: difference 14 m.
2nd circle was made in 2 hrs. 47 Semicircle, from the light in 1
m. hr. 17 m., to the light 1 hr. 30
m.: difference 13 m.
_Ipomæa jucunda_ (Convolvulaceæ) moves against the sun, placed in my
study, with windows facing the north-east. Weather hot.
1st circle was made in 5 hrs. 30 Semicircle, from the light in 4
m. hrs. 30 m., to the light 1 hr. 0
m.: difference 3 hrs. 30 m.
2nd circle was made in 5 hrs. 20 Semicircle, from the light in 3
m. (Late in afternoon: circle hrs. 50 m., to the light 1 hr. 30
completed at 6 hrs. 40 m. P.M.) m.: difference 2 hrs. 20 m.
We have here a remarkable instance of the power of light in retarding and
hastening the revolving movement. (_See_ ERRATA.)
_Convolvulus sepium_ (large-flowered cultivated var.) moves against the
sun. Two circles, were made each in 1 hr. 42 m.: difference in
semicircle from and to the light 14 m.
_Rivea tiliæfolia_ (Convolvulaceæ) moves against the sun, made four
revolutions in 9 hrs.; so that, on an average, each was performed in 2
hrs. 15 m.
_Plumbago rosea_ (Plumbaginaceæ) follows the sun. The shoot did not
begin to revolve until nearly a yard in height; it then made a fine
circle in 10 hrs. 45 m. During the next few days it continued to move,
but irregularly. On August 15th the shoot followed, during a period of
10 hrs. 40 m., a long and deeply zigzag course and then made a broad
ellipse. The figure apparently represented three ellipses, each of which
averaged 3 hrs. 38 m. for its completion.
_Jasminum pauciflorum_, Bentham (Jasminaceæ), moves against the sun. A
circle was made in 7 hrs. 15 m., and a second rather more quickly.
_Clerodendrum Thomsonii_ (Verbenaceæ) follows the sun.
H. M.
April 12, 1st circle was made in 5 45 (shoot very young)
14, 2nd 3 30
18, a semicircle 5 0 (directly after
the plant was
shaken on being
moved)
19, 3rd circle 3 0
20, 4th 4 20
_Tecoma jasminoides_ (Bignoniaceæ) moves against the sun.
H. M.
March 17, 1st circle was made in 6 30
19, 2nd 7 0
22, 3rd 8 30 (very cold day)
24, 4th 6 45
_Thunbergia alata_ (Acanthaceæ) moves against sun.
H. M.
April 14, 1st circle was made in 3 20
18, 2nd 2 50
18, 3rd 2 55
18, 4th 3 55 (late in
afternoon)
_Adhadota cydonæfolia_ (Acanthaceæ) follows the sun. A young shoot made
a semicircle in 24 hrs.; subsequently it made a circle in between 40 hrs.
and 48 hrs. Another shoot, however, made a circle in 26 hrs. 30 m.
_Mikania scandens_ (Compositæ) moves against the sun.
H. M.
March 14, 1st circle was made in 3 10
15, 2nd 3 0
16, 3rd 3 0
17, 4th 3 33
April 7, 5th 2 50
7, 6th 2 40 This circle was
made after a
copious watering
with cold water at
47° Fahr.
_Combretum argenteum_ (Combretaceæ) moves against the sun. Kept in
hothouse.
H. M.
Jan. 24, 1st circle was made in 2 55 Early in
morning, when
the temperature
of the house
had fallen a
little.
24, 2 circles each at an 2 20
average of
25, 4th circle was made in 2 25
_Combretum purpureum_ revolves not quite so quickly as _C. argenteum_.
_Loasa aurantiaca_ (Loasaceæ). Revolutions variable in their course: a
plant which moved against the sun.
H. M.
June 20, 1st circle was made in 2 37
20, 2nd 2 13
20, 3rd 4 0
21, 4th 2 35
22, 5th 3 26
23, 6th 3 5
Another plant which followed the sun in its revolutions.
H. M.
July 11, 1st circle was made in 1 51 Very hot day.
11, 2nd 1 46
11, 3rd 1 41
11, 4th 1 48
12, 5th 2 35
_Scyphanthus elegans_ (Loasaceæ) follows the sun.
H. M.
June 13, 1st circle was made in 1 45
13, 2nd 1 17
14, 3rd 1 36
14, 4th 1 59
14, 5th 2 3
_Siphomeris_ or _Lecontea_ (unnamed sp.) (Cinchonaceæ) follows the sun.
H. M.
May 25, semicircle was made in 10 27 (shoot extremely
young)
26, 1st circle 10 15 (shoot still young)
30, 2nd 8 55
June 2, 3rd 8 11
6, 4th 6 8
8, 5th 7 20 Taken from the
hothouse, and placed
in a room in my
house.
9, 6th 8 36
_Manettia bicolor_ (Cinchonaceæ), young plant, follows the sun.
H. M.
July 7, 1st circle was made in 6 18
8, 2nd 6 53
9, 3rd 6 30
_Lonicera brachypoda_ (Caprifoliaceæ) follows the sun, kept in a warm
room in the house.
H. M.
April, 1st circle was made in 9 10 (about)
April, 2nd circle was made in 12 20 (a distinct shoot,
very young, on same
plant)
3rd 7 30
4th 8 0 In this latter
circle, the
semicircle from the
light took 5 hrs. 23
m., and to the light
2 hrs. 37 min.:
difference 2 hrs 46
m.
_Aristolochia gigas_ (Aristolochiaceæ) moves against the sun.
H. M.
July 22, 1st circle was made in 8 0 (rather young
shoot)
23, 2nd 7 15
24, 3rd 5 0 (about)
In the foregoing Table, which includes twining plants belonging to widely
different orders, we see that the rate at which growth travels or
circulates round the axis (on which the revolving movement depends),
differs much. As long as a plant remains under the same conditions, the
rate is often remarkably uniform, as with the Hop, _Mikania_,
_Phaseolus_, &c. The Scyphanthus made one revolution in 1 hr. 17 m., and
this is the quickest rate observed by me; but we shall hereafter see a
tendril-bearing Passiflora revolving more rapidly. A shoot of the
_Akebia quinata_ made a revolution in 1 hr. 30 m., and three revolutions
at the average rate of 1 hr. 38 m.; a Convolvulus made two revolutions at
the average of 1 hr. 42 m., and _Phaseolus vulgaris_ three at the average
of 1 hr. 57 m. On the other hand, some plants take 24 hrs. for a single
revolution, and the _Adhadota_ sometimes required 48 hrs.; yet this
latter plant is an efficient twiner. Species of the same genus move at
different rates. The rate does not seem governed by the thickness of the
shoots: those of the _Sollya_ are as thin and flexible as string, but
move more slowly than the thick and fleshy shoots of the _Ruscus_, which
seem little fitted for movement of any kind. The shoots of the
_Wistaria_, which become woody, move faster than those of the herbaceous
_Ipomoea_ or _Thunbergia_.
We know that the internodes, whilst still very young, do not acquire
their proper rate of movement; hence the several shoots on the same plant
may sometimes be seen revolving at different rates. The two or three, or
even more, internodes which are first formed above the cotyledons, or
above the root-stock of a perennial plant, do not move; they can support
themselves, and nothing superfluous is granted.
A greater number of twiners revolve in a course opposed to that of the
sun, or to the hands of a watch, than in the reversed course, and,
consequently, the majority, as is well known, ascend their supports from
left to right. Occasionally, though rarely, plants of the same order
twine in opposite directions, of which Mohl (p. 125) gives a case in the
Leguminosæ, and we have in the table another in the Acanthaceæ. I have
seen no instance of two species of the same genus twining in opposite
directions, and such cases must be rare; but Fritz Müller {33} states
that although _Mikania scandens_ twines, as I have described, from left
to right, another species in South Brazil twines in an opposite
direction. It would have been an anomalous circumstance if no such cases
had occurred, for different individuals of the same species, namely, of
_Solanum dulcamara_ (Dutrochet, tom. xix. p. 299), revolve and twine in
two directions: this plant, however; is a most feeble twiner. _Loasa
aurantiaca_ (Léon, p. 351) offers a much more curious case: I raised
seventeen plants: of these eight revolved in opposition to the sun and
ascended from left to right; five followed the sun and ascended from
right to left; and four revolved and twined first in one direction, and
then reversed their course, {34} the petioles of the opposite leaves
affording a _point d’appui_ for the reversal of the spire. One of these
four plants made seven spiral turns from right to left, and five turns
from left to right. Another plant in the same family, the _Scyphanthus
elegans_, habitually twines in this same manner. I raised many plants of
it, and the stems of all took one turn, or occasionally two or even three
turns in one direction, and then, ascending for a short space straight,
reversed their course and took one or two turns in an opposite direction.
The reversal of the curvature occurred at any point in the stem, even in
the middle of an internode. Had I not seen this case, I should have
thought its occurrence most improbable. It would be hardly possible with
any plant which ascended above a few feet in height, or which lived in an
exposed situation; for the stem could be pulled away easily from its
support, with but little unwinding; nor could it have adhered at all, had
not the internodes soon become moderately rigid. With leaf-climbers, as
we shall soon see, analogous cases frequently occur; but these present no
difficulty, as the stem is secured by the clasping petioles.
In the many other revolving and twining plants observed by me, I never
but twice saw the movement reversed; once, and only for a short space, in
_Ipomoea jucunda_; but frequently with _Hibbertia dentata_. This plant
at first perplexed me much, for I continually observed its long and
flexible shoots, evidently well fitted for twining, make a whole, or
half, or quarter circle in one direction and then in an opposite
direction; consequently, when I placed the shoots near thin or thick
sticks, or perpendicularly stretched string, they seemed as if constantly
trying to ascend, but always failed. I then surrounded the plant with a
mass of branched twigs; the shoots ascended, and passed through them, but
several came out laterally, and their depending extremities seldom turned
upwards as is usual with twining plants. Finally, I surrounded a second
plant with many thin upright sticks, and placed it near the first one
with twigs; and now both had got what they liked, for they twined up the
parallel sticks, sometimes winding round one and sometimes round several;
and the shoots travelled laterally from one to the other pot; but as the
plants grew older, some of the shoots twined regularly up thin upright
sticks. Though the revolving movement was sometimes in one direction and
sometimes in the other, the twining was invariably from left to right;
{36} so that the more potent or persistent movement of revolution must
have been in opposition to the course of the sun. It would appear that
this _Hibbertia_ is adapted both to ascend by twining, and to ramble
laterally through the thick Australian scrub.
I have described the above case in some detail, because, as far as I have
seen, it is rare to find any special adaptations with twining plants, in
which respect they differ much from the more highly organized
tendril-bearers. The _Solanum dulcamara_, as we shall presently see, can
twine only round stems which are both thin and flexible. Most twining
plants are adapted to ascend supports of moderate though of different
thicknesses. Our English twiners, as far as I have seen, never twine
round trees, excepting the honeysuckle (_Lonicera periclymenum_), which I
have observed twining up a young beech-tree nearly 4½ inches in diameter.
Mohl (p. 134) found that the _Phaseolus multiflorus_ and _Ipomoea
purpurea_ could not, when placed in a room with the light entering on one
side, twine round sticks between 3 and 4 inches in diameter; for this
interfered, in a manner presently to be explained, with the revolving
movement. In the open air, however, the _Phaseolus_ twined round a
support of the above thickness, but failed in twining round one 9 inches
in diameter. Nevertheless, some twiners of the warmer temperate regions
can manage this latter degree of thickness; for I hear from Dr. Hooker
that at Kew the _Ruscus androgynus_ has ascended a column 9 inches in
diameter; and although a _Wistaria_ grown by me in a small pot tried in
vain for weeks to get round a post between 5 and 6 inches in thickness,
yet at Kew a plant ascended a trunk above 6 inches in diameter. The
tropical twiners, on the other hand, can ascend thicker trees; I hear
from Drs. Thomson and Hooker that this is the case with the _Butea
parviflora_, one of the Menispermaceæ, and with some Dalbergias and other
Leguminosæ. {37} This power would be necessary for any species which had
to ascend by twining the large trees of a tropical forest; otherwise they
would hardly ever be able to reach the light. In our temperate countries
it would be injurious to the twining plants which die down every year if
they were enabled to twine round trunks of trees, for they could not grow
tall enough in a single season to reach the summit and gain the light.
By what means certain twining plants are adapted to ascend only thin
stems, whilst others can twine round thicker ones, I do not know. It
appeared to me probable that twining plants with very long revolving
shoots would be able to ascend thick supports; accordingly I placed
_Ceropegia Gardnerii_ near a post 6 inches in diameter, but the shoots
entirely failed to wind round it; their great length and power of
movement merely aid them in finding a distant stem round which to twine.
The _Sphærostemma marmoratum_ is a vigorous tropical twiner; and as it is
a very slow revolver, I thought that this latter circumstance might help
it in ascending a thick support; but though it was able to wind round a
6-inch post, it could do this only on the same level or plane, and did
not form a spire and thus ascend.
As ferns differ so much in structure from phanerogamic plants, it may be
worth while here to show that twining ferns do not differ in their habits
from other twining plants. In _Lygodium articulatum_ the two internodes
of the stem (properly the rachis) which are first formed above the
root-stock do not move; the third from the ground revolves, but at first
very slowly. This species is a slow revolver: but _L. scandens_ made
five revolutions, each at the average rate of 5 hrs. 45 m.; and this
represents fairly well the usual rate, taking quick and slow movers,
amongst phanerogamic plants. The rate was accelerated by increased
temperature. At each stage of growth only the two upper internodes
revolved. A line painted along the convex surface of a revolving
internode becomes first lateral, then concave, then lateral and
ultimately again convex. Neither the internodes nor the petioles are
irritable when rubbed. The movement is in the usual direction, namely,
in opposition to the course of the sun; and when the stem twines round a
thin stick, it becomes twisted on its own axis in the same direction.
After the young internodes have twined round a stick, their continued
growth causes them to slip a little upwards. If the stick be soon
removed, they straighten themselves, and recommence revolving. The
extremities of the depending shoots turn upwards, and twine on
themselves. In all these respects we have complete identity with twining
phanerogamic plants; and the above enumeration may serve as a summary of
the leading characteristics of all twining plants.
The power of revolving depends on the general health and vigour of the
plant, as has been laboriously shown by Palm. But the movement of each
separate internode is so independent of the others, that cutting off an
upper one does not affect the revolutions of a lower one. When, however,
Dutrochet cut off two whole shoots of the Hop, and placed them in water,
the movement was greatly retarded; for one revolved in 20 hrs. and the
other in 23 hrs., whereas they ought to have revolved in between 2 hrs.
and 2 hrs. 30 m. Shoots of the Kidney-bean, cut off and placed in water,
were similarly retarded, but in a less degree. I have repeatedly
observed that carrying a plant from the greenhouse to my room, or from
one part to another of the greenhouse, always stopped the movement for a
time; hence I conclude that plants in a state of nature and growing in
exposed situations, would not make their revolutions during very stormy
weather. A decrease in temperature always caused a considerable
retardation in the rate of revolution; but Dutrochet (tom. xvii. pp. 994,
996) has given such precise observations on this head with respect to the
common pea that I need say nothing more. When twining plants are placed
near a window in a room, the light in some cases has a remarkable power
(as was likewise observed by Dutrochet, p. 998, with the pea) on the
revolving movement, but this differs in degree with different plants;
thus _Ipomoea jucunda_ made a complete circle in 5 hrs. 30 m.; the
semicircle from the light taking 4 hrs. 80 m., and that towards the light
only 1 hr. _Lonicera brachypoda_ revolved, in a reversed direction to
the _Ipomoea_, in 8 hrs.; the semicircle from the light taking 5 hrs. 23
m., and that to the light only 2 hrs. 37 m. From the rate of revolution
in all the plants observed by me, being nearly the same during the night
and the day, I infer that the action of the light is confined to
retarding one semicircle and accelerating the other, so as not to modify
greatly the rate of the whole revolution. This action of the light is
remarkable, when we reflect how little the leaves are developed on the
young and thin revolving internodes. It is all the more remarkable, as
botanists believe (Mohl, p. 119) that twining plants are but little
sensitive to the action of light.
I will conclude my account of twining plants by giving a few
miscellaneous and curious cases. With most twining plants all the
branches, however many there may be, go on revolving together; but,
according to Mohl (p. 4), only the lateral branches of _Tamus
elephantipes_ twine, and not the main stem. On the other hand, with a
climbing species of Asparagus, the leading shoot alone, and not the
branches, revolved and twined; but it should be stated that the plant was
not growing vigorously. My plants of _Combretum argenteum_ and _C.
purpureum_ made numerous short healthy shoots; but they showed no signs
of revolving, and I could not conceive how these plants could be
climbers; but at last _C. argenteum_ put forth from the lower part of one
of its main branches a thin shoot, 5 or 6 feet in length, differing
greatly in appearance from the previous shoots, owing to its leaves being
little developed, and this shoot revolved vigorously and twined. So that
this plant produces shoots of two kinds. With _Periploca Græca_ (Palm,
p. 43) the uppermost shoots alone twine. Polygonum convolvulus twines
only during the middle of the summer (Palm, p. 43, 94); and plants
growing vigorously in the autumn show no inclination to climb. The
majority of Asclepiadaceæ are twiners; but _Asclepias nigra_ only “in
fertiliori solo incipit scandere subvolubili caule” (Willdenow, quoted
and confirmed by Palm, p. 41). _Asclepias vincetoxicum_ does not
regularly twine, but occasionally does so (Palm, p. 42; Mohl, p. 112)
when growing under certain conditions. So it is with two species of
_Ceropegia_, as I hear from Prof. Harvey, for these plants in their
native dry South African home generally grow erect, from 6 inches to 2
feet in height,—a very few taller specimens showing some inclination to
curve; but when cultivated near Dublin, they regularly twined up sticks 5
or 6 feet in height. Most Convolvulaceæ are excellent twiners; but in
South Africa _Ipomoea argyræoides_ almost always grows erect and compact,
from about 12 to 18 inches in height, one specimen alone in Prof.
Harvey’s collection showing an evident disposition to twine. On the
other hand, seedlings raised near Dublin twined up sticks above 8 feet in
height. These facts are remarkable; for there can hardly be a doubt that
in the dryer provinces of South Africa these plants have propagated
themselves for thousands of generations in an erect condition; and yet
they have retained during this whole period the innate power of
spontaneously revolving and twining, whenever their shoots become
elongated under proper conditions of life. Most of the species of
_Phaseolus_ are twiners; but certain varieties of the _P. multiflorus_
produce (Léon, p. 681) two kinds of shoots, some upright and thick, and
others thin and twining. I have seen striking instances of this curious
case of variability in “Fulmer’s dwarf forcing-bean,” which occasionally
produced a single long twining shoot.
_Solanum dulcamara_ is one of the feeblest and poorest of twiners: it may
often be seen growing as an upright bush, and when growing in the midst
of a thicket merely scrambles up between the branches without twining;
but when, according to Dutrochet (tom. xix. p. 299), it grows near a thin
and flexible support, such as the stem of a nettle, it twines round it.
I placed sticks round several plants, and vertically stretched strings
close to others, and the strings alone were ascended by twining. The
stem twines indifferently to the right or left. Some others species of
Solanum, and of another genus, viz. _Habrothamnus_, belonging to the same
family, are described in horticultural works as twining plants, but they
seem to possess this faculty in a very feeble degree. We may suspect
that the species of these two genera have as yet only partially acquired
the habit of twining. On the other hand with _Tecoma radicans_, a member
of a family abounding with twiners and tendril-bearers, but which climbs,
like the ivy, by the aid of rootlets, we may suspect that a former habit
of twining has been lost, for the stem exhibited slight irregular
movements which could hardly be accounted for by changes in the action of
the light. There is no difficulty in understanding how a spirally
twining plant could graduate into a simple root-climber; for the young
internodes of _Bignonia Tweedyana_ and of _Hoya carnosa_ revolve and
twine, but likewise emit rootlets which adhere to any fitting surface, so
that the loss of twining would be no great disadvantage and in some
respects an advantage to these species, as they would then ascend their
supports in a more direct line. {44}