THE MOVEMENTS AND HABITS OF CLIMBING PLANTS(攀爬植物的行为和习性)

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THE MOVEMENTS AND HABITS OF CLIMBING PLANTS

THE MOVEMENTS

AND HABITS OF

CLIMBING PLANTS

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).

THE MOVEMENTS AND HABITS OF CLIMBING PLANTS

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

THE MOVEMENTS AND HABITS OF CLIMBING PLANTS

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

THE MOVEMENTS AND HABITS OF CLIMBING PLANTS

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.

THE MOVEMENTS AND HABITS OF CLIMBING PLANTS

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;

THE MOVEMENTS AND HABITS OF CLIMBING PLANTS

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

THE MOVEMENTS AND HABITS OF CLIMBING PLANTS

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

THE MOVEMENTS AND HABITS OF CLIMBING PLANTS

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

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THEMOVEMENTSANDHABITSOFCLIMBINGPLANTS1THEMOVEMENTSANDHABITSOFCLIMBINGPLANTSTHEMOVEMENTSANDHABITSOFCLIMBINGPLANTS2PREFACEThisEssayfirstappearedintheninthvolumeofthe'JournaloftheLinneanSociety,'publishedin1865.Itisherereproducedinacorrectedand,Ihope,clearerform,withsomeadditionalfacts.Theillustrations...

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THE MOVEMENTS AND HABITS OF CLIMBING PLANTS(攀爬植物的行为和习性)

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