The Movements and Habits of Climbing Plants
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Charles Darwin >> The Movements and Habits of Climbing Plants
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13 This etext was prepared by David Price, email ccx074@coventry.ac.uk
from the 1906 John Murray edition.
THE MOVEMENTS AND HABITS OF CLIMBING PLANTS
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 Muller,
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 Wurzburg,'
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, {1} 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, Cobaea, 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 uber
Schlingpflanzen' (Arbeiten des botanischen Instituts in Wurzburg, 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. {2} 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, {3} and had
subsequently been the subject of two memoirs by Dutrochet. {4}
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, 1.75 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 3.5 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 1.75 to 6 inches in length,
and carried an internode 1.875 inch long, which was just perceptibly
moving; and this carried a very minute ultimate internode. After the
twenty-first revolution, the penultimate internode was 2.5 inches
long, and probably revolved in a period of about three hours. At the
twenty-seventh revolution the lower and still moving internode was
8.375, the penultimate 3.5, and the ultimate 2.5 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 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.,
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. {9}
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 Asclepiadaceae.
The hooked tip, in all the cases observed by me, viz, in Ceropegia,
Sphaerostemma, 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. {10} 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, 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}
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