On the movements and habits of climbing plants/Part 1

by Charles Darwin

Part I.—Spirally 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 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 was 2 h. 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 grows old, ceases to move.

To ascertain more precisely what amount of movement each internode underwent, I kept a potted plant in a well-warmed room to which I was confined during the night and day. A long inclined 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 he 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 the second revolution in 9 h.; 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 h. As on the succeeding morning I found that the shoot revolved in 2 h. 45 m., it must have made during the night four revolutions, each at the average rate of a little over 3 h. 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 h. 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 quite completed; for the internode abruptly became upright, and, after moving to the centre, remained motionless. I tied a weight to its upper end, so as to slightly bow it, and thus to detect any movement; but there was none. Some time before the last revolution the lower part of the internode had ceased to move.

A few more remarks will complete all that need be said on this one internode. It moved during five days; but the more rapid movement after the third revolution lasted during three days and twenty hours. The regular revolutions, from the ninth to thirty-sixth inclusive, were performed at the average rate of 2 h. 31 m.: the weather was cold; and this affected the temperature of the room, especially during the night, and consequently retarded a little the rate of movement. There was only one irregular movement, when a segment of a circle was rapidly performed (not counted in the above enumeration); and this occurred after an unusually slow revolution of 2 h. 49 m. 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 our lower internode was 8⅜, the penultimate 3½, and 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 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 continued to grow unsupported, it became nearly horizontal, 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 change in the position of the centre of gravity of the revolving extremity, a slight and slow swaying movement was given to the long and horizontally projecting shoot, which I mistook at first for a spontaneous movement. As the shoot grew, it depended 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 revolved; so that by the time one had ceased, that above it was in full action, with a terminal internode just commencing to revolve. With Hoya carnosa, on the other hand, a depending shoot, 32 inches in length, without any developed leaves, 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 h. 15 m. and 6 h. 45 m. for each revolution. Hence, as the extreme tip made a circle of above 5 feet (or 62 inches) in diameter and 16 feet in circumference, the tip travelled at the rate (assuming the circuit to have been completed in six hours) 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, night and day, this grand circle in search of some object round which to twine.

If we take hold of a growing sapling, we can of course bend it so as to make its tip describe a circle, like that performed by the tip 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 (S. 105, 108, &c.) thought that the twisting of the axis caused the revolving movement. I cannot fully understand how the one movement is supposed to cause the other; but it is scarcely possible that the twisting of the axis of the Hop three times could have caused thirty-seven revolutions. Moreover, the revolving movement commenced in the young internode before any twisting of the axis could be detected; and the internode of a young Siphomeris or Lecontea revolved during several days, and became twisted only once on its own axis. But the best evidence 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 regularly twisted, but which regularly perform, as we shall hereafter see, revolving movements like those of true twining-plants. Moreover, according to Palm (S. 30, 95) and Mohl (S. 149), and Léon^[1], 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 revolution; and this, according to Léon (p. 356), is the case with all the internodes of a variety of the Phaseolus multiflorus. Internodes which have become twisted round their own axes, if they have not ceased revolving, are still capable of twining, as I have several times observed.

Mohl has remarked (S. 111) that when a stem twines round a smooth cylindrical stick, it does not become twisted. 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 the 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 were fixed in split sticks below, and were secured above to cross sticks, and the stems in passing these places became very 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 weather. Several other facts could be given, showing that the axial twisting stands in 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^[2]; but this occurs so much more generally and strongly with training-plants than with other plants, that there must be some connexion between the capacity for twining and axial twisting. The most probable view, as it seems to me, is that the stem twists itself to gain rigidity (on the same principle that a much twisted rope is stiffer than a slackly twisted one), so as to be enabled either to pass over inequalities in its spiral ascent, or to carry its own weight when allowed to revolve freely^[3].

I have just alluded to the twisting which necessarily follows from the spiral ascent of the stem, namely, one twist for each spire completed. This was well shown by painting straight lines on 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.

I have already compared the revolving movement of a twining plant to that of the tip of a sapling, moved round and round by the hand held some way down the stem; but there is a most important difference. The upper part of the sapling moves as a rigid body, and remains straight; but with twining plants every inch 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 quietly tied to a stick, the upper free part steadily continues revolving: even if the whole shoot, except the terminal tip of an inch or two in length, be tied up, this tip, 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 line of surface, this coloured streak will after a time (depending on the rate of revolution) be found to lie along one side of the bow, then along the concave side, then on the opposite side, and, lastly, again on the original convex surface. This clearly proves that the internodes, during the revolving movement, 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.

As this movement is rather difficult to understand, it will be well to give an illustration. Let us take the tip of a sapling and bend it to the south, and paint a black line on the convex surface; then let the sapling spring up and bend it to the east, the black line will then be seen on the lateral face (fronting the north) of the shoot; bend it to the north, the black line will be on the concave surface; bend it to the west, the line will be on the southern lateral face; and when again bent to the south, the line will again be on the original convex surface. Now, instead of bending the sapling, let us suppose that the cells on its whole southern surface were to contract from the base to the tip, the whole shoot would be bowed to the south; and let the longitudinal contracting surface slowly creep round the shoot, deserting by slow degrees the southern side and encroaching on the eastern side, and so round by the north, by the west, again to the south; in this case the shoot would remain always bowed with the painted line appearing on the convex, on the lateral, and concave surfaces, and with the point of the shoot successively directed to all points of the compass. In fact, we should then have the exact kind of movement seen in the revolving shoots of twining plants. I have spoken in the illustration, for brevity's sake, of the cells along each face successively contracting; of course turgescence of the cells on the opposite face, or both forces combined, would do equally well.

It must not be supposed that the revolving movement of twining plants is as regular as that given in this 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 the southern and then the northern face of the sapling to contract, the summit would describe a simple arc; if the contraction first travelled a very little to the eastern face, and during the return a very little to the western face, a narrow ellipse would be described; and the sapling would become straight as it passed to and fro by the central point. A complete straightening of the shoot may often be observed in revolving plants; but the weight of the shoot apparently interferes with the regularity of the movement, and with the place of straitening. The movement is often (in appearance at least) as if the southern, eastern, and northern faces had contracted, but not the western face; so that a semicircle is described, and the shoot becomes straight and upright in one part of its course.

When a revolving shoot consists of several internodes, the several lower ones bend together at the same rate, but the one or two terminal internodes bend at a slower rate; hence, though at times all the internodes may be bowed in the same line, at other times the shoot is rendered slightly serpentine, as I have often observed. The rate of revolution of the whole shoot, if judged by the movement of the extreme tip, is thus at times accelerated and 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 which I observed, viz. in Ceropegia, Spharostema, Clerodendron, Wistaria, Stephania, Akebia, and Siphomeris, has exactly the same kind of movement as the other revolving internodes; for a line painted on the convex surface becomes lateral and then concave; but, owing to the youth of these terminal internodes, the reversal of the hook is a slower process than the revolving movement. This strongly marked tendency in the young terminal and flexible internodes to bend 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 much more closely its support than it otherwise could have done, and thus aids in preventing the stem from being blown away from it during windy weather, as I have many times observed. In Lonicera brachypoda the hook only straightened itself periodically, and never became reversed. I will not assert that the tips of all twining plants, when hooked, move as above described; for this position may in some cases be due to the manner of growth, as with the bent tips of the shoots of the common vine, and more plainly with those of Cissus discolor; these plants, however, are not spiral twiners.

The purpose of the spontaneous revolving movement, or, more strictly speaking, of the continuous bending movement successively directed to all points of the compass, is, as Mohl has remarked, obviously in part to favour the shoot finding a support. This is admirably effected by the revolutions carried on night and day, with a wider and wider circle swept as the shoot increases in length. But as we now understand the nature of the movement, we can see that, when at last the shoot meets with a support, the motion at the point of contact is necessarily arrested, but the free projecting part goes on revolving. Almost immediately another and upper point of the shoot is brought into contact with the support and is arrested; and so onwards to the extremity of the shoot; and thus it winds round its support. When the shoot follows the sun in its revolving course, it winds itself 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 loses its power of spirally twining round a support. 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 rope; so it is with twining plants, the continued contraction or turgescence of the cells along the free part of the shoot replacing the momentum of each atom of the free end of the rope.

All the authors, except Von Mohl, who have discussed the spiral twining of plants maintain that such plants have a natural tendency to grow spirally. Mohl believes (S. 112) that twining stems have a dull kind of irritability, so that they bend towards any object which they touch. 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 negative results. I rubbed many shoots much harder than is necessary to excite movement in any tendril or in any foot-stalk of a leaf-climber, but without result. I then tied a very light forked twig to a shoot of a Hop, a Ceropegia, Sphærostema, 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 from possessing irritability; but in no case was any effect produced. Moreover, when a shoot winds round a support, the movement is always slower, as we shall immediately see, than whilst its 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 be 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, as we shall hereafter see, certainly irritable; but this case gives me confidence that ordinary twiners do not possess this quality, for directly after putting a stick to the Lophospermum, I saw that it behaved differently from any true twiner or any other leaf-climber.

The belief that twiners have a natural tendency to grow spirally probably arose from their assuming this form when wound round a support, and from the extremity, even whilst remaining free, sometimes assuming this same form. The free internodes of vigorously growing plants, when they cease to revolve, become straight, and show no tendency to be spiral; but when any 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 degree with the ends of the shoots of the Stauntonia and of the allied Akebia, which became closely wound up spirally, just like a tendril, especially after the small, ill-formed leaves had perished. The explanation of this fact is, I believe, that the lower parts of such 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 took 9 h. 30 m. to make one complete spire round a stick, whilst it revolved in 6 h.; Aristolochia gigas revolved in about 5 h., but took 9 h. 15 m. to complete its spire. This, I presume, is due to the continued disturbance of the moving force by its arrestment at each successive point; 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 evidently 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 in a very close spire, which remained unchanged; but subsequently, as the shoot grew, 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, well served to keep the shoots in 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.

It follows from this latter fact that the position occupied by each leaf with respect to the support, in fact, 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 (S. 84), who states that the opposite leaves of the Hop always stand exactly over each other, in a row, on the same side of the supporting stick, though this may differ in thickness. My sons visited a hop-field for me, and reported that though they generally found the points of insertion of the leaves over each other for a space of two or three feet in height, yet this never occurred up the whole length of a pole, the point 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 a line up the sticks round which they had twined; accordingly I raised a dozen plants, and gave them sticks of various thicknesses and string to twine round; and in this case one alone out of the dozen had its leaves arranged in a perpendicular line: so I conclude that there is nothing remarkable in Palm's statement.

The leaves of twining-plants rise from the stem (before it has twined) either alternately, or oppositely, or in a spire; in this latter case the line of insertion of the leaves and the course of revolution or of twining coincide. This fact has been well shown by Dutrochet^[4], who found different individuals of Solanum Dulcamara twining in opposite directions, and these had their leaves spirally arranged in opposite directions. A dense whorl of many leaves would apparently be incommodious for a twining plant, and some authors have supposed that none have their leaves thus arranged; but a twining Siphomeris has whorls of three.

If a stick which has arrested a revolving shoot, but has not as yet been wound round, be suddenly taken away, the shoot generally springs forward, showing that it has continued to press against the stick. If the stick, shortly after having been wound round, be withdrawn, the shoot retains for a time its spiral form, 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, had become so stiff that they were incapable, on repeated trials, of twining round a thin stick, showing that the power of movement was retained after flexibility had been lost. 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 wound round by this part 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 part 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 the lower part of this internode had likewise recovered its revolving power. These several facts show that, in the arrested portion of a revolving shoot, the power of movement is not immediately lost, and that when temporarily lost it can be recovered. When a shoot has remained for a considerable time wound round its support, it permanently retains its spiral form even when the support is removed.

When a 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 shoot slowly and gradually slid up the stick, so as to become more and more highly inclined; and then, after an interval sufficient to have allowed of a semirevolution, it suddenly bounded from the stick and fell over to the opposite side, to its ordinary slight inclination. It now recommenced revolving in its usual course, so that after a semirevolution it again came into contact with the stick, again slid up it, and again bounded from it. This movement of the shoot had a very odd appearance, as if it were disgusted with its failure but resolved to try again. We shall, I think, understand this movement by considering the former illustration of the sapling, in which the contracting surface was supposed to creep from the southern, by the eastern, to the northern, and thence back again by the western side to the southern face, successively bowing the sapling in all directions. Now with the Ceropegia, the stick being placed a very little to the east of due south of the plant, the eastern contraction could produce no effect beyond pressing the rigid internode against the stick; but as soon as the contraction on the northern face began, it would slowly drag the shoot up the stick; and then, as soon as the western contraction had well begun, the shoot would be drawn from the stick, and its weight, coinciding with the north-western contraction, would cause it suddenly to fall to the opposite side with its proper slightly inclined positions; and the ordinary revolving movement would go on. I have described this case because it first made me understand the order in which the contracting or turgescent cells of revolving shoots must act.

The view just given further explains, as I believe, a fact observed by Von Mohl (S. 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 they could not, though aided by me in many ways, wind round it. This apparently is owing to the flexure of the shoot, when winding round an object so gently curved as this post, not being sufficient to hold the shoot to its place when the contracting force creeps round to the opposite surface of the shoot; so that it is at each revolution withdrawn from its support.

When a 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 end always turning upwards. If the support be not lofty, it falls to the ground, and, resting there, the extremity rises again. Sometimes several shoots, when flexible, twine together into a cable, and thus support each other. Single thin depending shoots, such as those of the Sollya Drummondii, will turn abruptly back and wind upwards 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 at first were flexible and revolved, if they did not succeed in twining round a support, became quite rigid, and, supporting themselves upright, carried on their summit 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 to show that all kinds behave in a nearly uniform manner^[5].

Twining plants not aided by tendrils or by irritable leaf-stalks.

(Acotyledons.)

Lygodium scandens (Polypodiaceæ) moves against the sun.

		h.	m.				h.	m.
June 18,	1st circle	6	0		June 19,	4th circle	5	0	(very hot day).
June„ 18,	2nd circle„	6	15	(late in evening).	June„ 20,	5th circle„	6	0
June„ 19,	3rd circle„	5	32	(very hot day).

Lygodium articulatum moves against the sun.

		h.	m.				h.	m.
July 19,	1st circle	16	30	(shoot very young).	July 21,	3rd circle	8	0
July„ 20,	2nd circle„	15	0		July„ 22,	4th circle„	10	30

(Monocotyledons.)

Ruscus androgynus (Liliaceæ), placed in the hot-house, moves against the sun.

		h.	m.				h.	m.
May 24,	1st circle	6	14	(shoot very young).	May 26,	5th circle	2	50
May„ 25,	2nd circle„	2	21		May„ 27,	6th circle„	3	52
May„ 25,	3rd circle„	3	37		May„ 27,	7th circle„	4	11
May„ 25,	4th circle„	3	22

Asparagus (unnamed species from Kew) (Liliaceæ) moves against the sun, placed in hothouse.

		h.	m.
Dec. 26,	1st circle . . . . . . . . . . . . . . .	5	0
Dec.„ 27,	2nd circle„ . . . . . . . . . . . . . . .	5	40

Tamus communis (Dioscoreaceæ). A young shoot from a potted tuber placed in the greenhouse; follows the sun.

		h.	m.			h.	m.
July 7,	1st circle . . . . . .	3	10	July 8,	4th circle . . . . . .	2	56
July„ 7,	2nd circle„ . . . . . .	2	38	July„ 8,	5th circle„ . . . . . .	2	30
July„ 8,	3rd circle„ . . . . . .	3	5	July„ 8,	6th circle„ . . . . . .	2	30

Lapagerea rosea (Philesiaceæ), in greenhouse, follows the sun.

		h.	m.
March  9,	1st circle . . . . . .	26	15	(shot young).
March„ 10,	semicircle . . . . . .	8	15
March„ 11,	2nd circle„ . . . . . .	11	0
March„ 12,	3rd circle„ . . . . . .	15	30
March„ 13,	4th circle„ . . . . . .	14	15
March„ 16,	5th circle„ . . . . . .	8	40	when placed in the hothouse; but the
	next day the shoot remained stationary.

Roxhurghia viridiflora (Roxburghiaceæ) moves against the sun; it travelled a circle in about 24 hours.

(Dicotyledons.)

Humulus Lupulus (Urticaceæ) follows the sun.

		h.	m.			h.	m.
April  9,	2 circles . . . . . .	4	16	August 14,	6th circle . . . . . .	2	2
Aug. 13,	3rd circle . . . . . .	2	0	August„ 14,	7th circle„ . . . . . .	2	0
Aug.„ 14,	4th circle . . . . . .	2	20	August„ 14,	8th circle„ . . . . . .	2	4
Aug.„ 14,	5th circle . . . . . .	2	16

A plant placed in a room; a semicircle was performed in travelling from the light in 1 h. 33 m., in travelling to the light in 1 h. 13 m.: difference of rale 20 m.

Akebia quinata (Lardizabalaceæ), placed in hothouse, moves against the sun.

Lygodium scandens (Polypodiaceæ) moves against the sun.

		h.	m.				h.	m.
March 17,	1st circle	4	0	(shot young).	March 18,	3rd circle	1	30
March„ 18,	2nd circle„	1	40		March„ 19,	4th circle„	1	45

Stauntonia lalifolia (Lardizabalaceæ), placed in hothouse, moves against the sun.

		h.	m.
March 28,	1st circle . . . . . . . . . . . . . . .	3	30
March„ 29,	2nd circle„ . . . . . . . . . . . . . . .	3	45

Sphærostema marmoratum (Schizandraceæ) follows the sun.

August 5th, 1st circle in about 24 h.; 2nd circle in 18 h. 30 m.

Stephania rotunda (Menispermaceæ) moves against the sun.

		h.	m.			h.	m.
May 27,	1st circle . . . . . .	5	5	June 2,	3rd circle . . . . . .	5	15
May„ 30,	2nd circle„ . . . . . .	7	6	June„ 3,	4th circle„ . . . . . .	6	28

Thryallis brachystachya (Malpighiaceæ) moves against the sun: one shoot made a circle in 12 h., and another in 10 h. 30 m.; but the next day, which was much colder, the first shoot in my study took 10 h. to perform only a semicircle.

Hibbertia dentata (Dilleniaceæ), placed in the hothouse, followed the sun, and made (May 18th) a circle in 7 h. 20 m.; on the 19th, reversed its course and moved against the sun, and made a circle in 7 h.; on the 20th, moved against the sun one-third of circle, and then stood still; on the 26th, followed the sun for two-thirds of circle, and then returned to its starting-point, taking for this double course 11 h. 46 m.

Sollya Drummondii (Pittosporaceæ) moves against the sun; in greenhouse.

		h.	m.				h.	m.
April 4,	1st circle	4	25		April 19,	3rd circle	6	25
April„ 5,	2nd circle„	8	0	(very cold day).	April„ 7,	4th circle„	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 and placed in water made circles in 3 h. 10 m., 5 h. 20 m., and 7 h. 15 m.

Wistaria Chinensis (Leguminosæ), in greenhouse, moves against the sun.

		h.	m.			h.	m.
May 13,	1st circle . . . . . .	3	5	May 24,	4th circle . . . . . .	3	21
May„ 13,	2nd circle„ . . . . . .	3	20	May„ 25,	5th circle„ . . . . . .	2	37
May„ 16,	3rd circle„ . . . . . .	2	5	May„ 25,	6th circle„ . . . . . .	2	35

Phaseolus vulgaris (Leguminosæ), in greenhouse, moves against the sun.

		h.	m.
May,	1st circle . . . . . . . . . . . . . . .	2	0
May,„	2nd circle„ . . . . . . . . . . . . . . .	1	55
May,„	3rd circle„ . . . . . . . . . . . . . . .	1	55

Dipladenia urophylla (Apocynaceæ) moves against the sun.

		h.	m.
April 18,	1st circle . . . . . . . . . . . . . . .	8	0
April„ 19,	2nd circle„ . . . . . . . . . . . . . . .	9	15
April„ 30,	3rd circle„ . . . . . . . . . . . . . . .	9	40

Dipladenia crassinoda moves against the sun.

		h.	m.
May 16,	1st circle . . . . . . . . . . . . . . .	9	5
July 20,	2nd circle„ . . . . . . . . . . . . . . .	8	0
July„ 21,	3rd circle„ . . . . . . . . . . . . . . .	8	5

Ceropegia Gardnerii (Asclepiadaceæ) moves against the sun.

		h.	m.
Shoot very young, 2 inches in length.	1st circle in	7	55
Shoot still young	2nd circle in„	7	0
Long shoot	3rd circle in„	6	33
Long shoot	4th circle in„	5	15
Long shoot	5th circle in„	6	45

Slephanotis floribunda (Asclepiadaceæ) moves against the sun, and made a circle in 6 h. 40 in., a second circle in about 9 hours.

Hoya carnosa (Asclepiadaceæ) made several circles in from 16 h. to 22 h. or 24 h.

Convolvulus major (Convolvulaceæ) moves against the sun. Plant placed in room with lateral light.

1st circle … 2 h. 42 m.	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Semicircle, from light in 1 h. 14 m., to light 1 h. 28 m.: difference 14 m.
2nd circle … 2 h. 47 m.	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Semicircle, from light in 1 h. 17 m., to light 1 h. 30 m.: difference 13 m.

Convolvulus sepium (large-flowered cultivated var.) moves against the sun. Two circles, each in 1 h. 42 m.: difference in semicircle from and to light 14 m.

Iponuea jucunda (Convolvulaceæ) moves against the sun, placed in my study, with windows facing the north-east. Weather hot.

1st circle 5 h. 30 m.	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Semicircle, from light in 4 h. 30 m., to light 1 h. 0 m.: difference 3 h. 30 m.
2nd circle 5 h. 20 m. (Late in afternoon: circle completed at 6 m. 40 h. p.m.	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,}$	Semicircle, from light in 3 h. 50 m., to light 1 h. 30 m.: difference 2 h. 20 m.

We have here a remarkable instance of the power of light in retarding and hastening the revolving movement.

Rivea tiliæfolia (Convolvulaceæ) moves against the sun, and made four revolutions in 9 h.; so that each, on average, was performed in 2 h. 15 m.

Plumbago rosea (Plumbaginaecæ) follows the sun. The shoot did not begin to revolve until nearly a yard in height; it then made a fine circle in 10 h. 45 m. During the next few days it continued to move, but irregularly. On August 15th the shoot followed, during a period of 10 h. 40 m., a long and deeply zigzag course and then made a broad ellipse. The figure thus traced altogether apparently represented three ellipses, each of which averaged 3 h. 33 m. for its completion.

Jasminum pauciflorum, Bentham (Jasminaceæ), moves against the sun. First circle in 7 h. 15 m., second circle rather more quickly.

Clerodendrum Thomsonii (Verbenaceæ) follows the sun.

		h.	m.
April 12,	1st circle . . . . . .	5	45	(shot very young).
April„ 14,	2nd circle„ . . . . . .	3	30
April„ 18,	semicircle . . . . . .	5	0	(directly after the plant was shaken in
April„ 19,	3rd circle„ . . . . . .	3	0	[being moved).
April„ 20,	4th circle„ . . . . . .	4	20

Tecoma jasminoides (Bignoniaceæ) moves against sun.

		h.	m.				h.	m.
March 17,	1st circle	6	30		March 28,	3rd circle	8	30	(very cold day).
March„ 19,	2nd circle„	7	0		March„ 24,	4th circle„	6	45

Thunbergia alata (Acanthaceæ) moves against sun.

		h.	m.				h.	m.
April 14,	1st circle	3	20		April 18,	3rd circle	2	55
April„ 18,	2nd circle„	2	50		April„ 18,	4th circle„	3	55	(late in afternoon).

Adhadota cydoaæfolia (Acanthaceæ) follows the sun. A young shoot made a semicircle in 24 h.; subsequently made a circle in between 40 h. and 48 h.; subsequently did not complete a circle in 50 h. Another shoot, however, made a circle in 26 h. 30 m.

Mikania scandens (Compositæ) moves against the sun.

		h.	m.
March 14,	1st circle	3	10
March„ 15,	2nd circle„	3	0
March„ 16,	3rd circle„	3	0
March„ 17,	4th circle„	3	33
April  7,	5th circle„	2	50
April„  7,	6th circle„	2	40	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	This circle was made after a copious intentional watering with cold water at 47° Fahr.

Combretum argenteum (Combretaceæ) moves against the sun.

			h.	m.
Jun. 24,	1st circle . . . . . . . . .		2	55	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Early in morning, when the temperature of the house had fallen a little.
Jun.„ 24,	2 circles, each at an	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$	2	20
	average of		2	20
Jun.„ 24,	4th circle. . . . . . . . .		2	25

Combretum purpureum revolves not quite so quickly as C. argenteum.

Loasa aurantiaca (Loasaceæ). First plant moved against the sun.

		h.	m.			h.	m.
June 20,	1st circle . . . . . .	2	37	June 20,	4th circle . . . . . .	2	35
June„ 20,	2nd circle„ . . . . . .	2	13	June„ 22,	5th circle„ . . . . . .	3	26
June„ 20,	3rd circle„ . . . . . .	4	0	June„ 23,	6th circle„ . . . . . .	3	5

Second plant followed the sun.

		h.	m.
July 11,	1st circle . . . . . . . . .	1	51	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right\}\,}$	Very hot day.
July„ 11,	2nd circle„ . . . . . . . . .	1	46
July„ 11,	3rd circle„ . . . . . . . . .	1	41
July„ 11,	4th circle„ . . . . . . . . .	1	48
July„ 12,	5th circle„ . . . . . . . . .	2	35		Cool morning.

Scyphanthus elegans (Loasaceæ) follows the sun.

		h.	m.			h.	m.
June 13,	1st circle . . . . . .	1	45	June 14,	4th circle . . . . . .	1	59
June„ 13,	2nd circle„ . . . . . .	1	17	June„ 14,	5th circle„ . . . . . .	2	3
June„ 14,	3rd circle„ . . . . . .	1	36

Siphomeris or Lecontea (unnamed sp.) (Cinchonaceæ) follows the sun.

		h.	m.
May 25,	semicircle . . . . . . . . .	10	27	(shoot extremely young).
May„ 26,	1st circle . . . . . . . . .	10	15	(shoot still young).
May„ 30,	2nd circle„ . . . . . . . . .	8	55
June  2,	3rd circle„ . . . . . . . . .	8	11
June„  6,	4th circle„ . . . . . . . . .	6	8
June„  8,	5th circle„ . . . . . . . . .	7	20	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$	Taken from the hothouse and placed in a room in my house.
June„  9,	6th circle„ . . . . . .	8	36

Mænettia bicolor (Cinchonaceæ), young plant, follows the sun.

		h.	m.
July 7,	1st circle . . . . . . . . . . . . . . .	6	18
July„ 8,	2nd circle„ . . . . . . . . . . . . . . .	6	53
July„ 9,	3rd circle„ . . . . . . . . . . . . . . .	6	30

Lonicera brachypoda (Caprifoliaceæ) follows the sun, in a warm room in the house.

		h.	m.
April,	1st circle about	9	10
April,„	2nd circle„ about„	12	20	(another shoot very young).
April,„	3rd circle„ about„	7	30
April,„	4th circle„ about„	8	0	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	In this latter circle, the semicircle from the light took 5 h. 23 m., and to the light 2 h. 37 m.: difference 2 h. 46 m.

Aristolochia gigas (Aristolochiaceæ) moves against the sun.

		h.	m.
July 22,	1st circle . . . . . .	8	0	(rather young shoot).
July„ 23,	2nd circle„ . . . . . .	7	15
July„ 24,	3rd circle„ . . . . . .	5	0	(about).

In the foregoing table, which includes twining plants belonging to as widely different orders as is possible, we see that the contraction or turgescence of the cells circulating round the axis, on which the revolving movement depends, differs much in rate. As long as a plant remains under the same conditions, the rate is often remarkably uniform, as we see with the Hop, Mikania, Phaseolus, &c. The Scyphanthus made one revolution in 1 h. 17 m., and this is the quickest rate observed; but we shall afterwards see a tendril-bearing Passiflora revolving even more rapidly. A shoot of the Akebia quinata made a revolution in 1 h. 30 m., and three revolutions at the average rate of 1 h. 38 m.; a Convolvulus made two revolutions at the average of 1 h. 42 m., and Phaseolus vulgaris three at the average of 1 h. 57 m. On the other hand, some plants take 24 h. for a single revolution, and the Adhadota sometimes required 48 h.; 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 slower than the thick and fleshy shoots of the Ruscus, which seems so little fitted for movement of any kind; the shoots of the Wistaria, which become woody, move faster than those of the Ipomæa or Thunbergia.

We know that the internodes, whilst very young, do not acquire their proper rate of movement; hence 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 perennial root-stock, do not move; these first-formed shoots can support themselves, and nothing superfluous is granted them.

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 (S. 125) gives a case in the Leguminosæ, and we have in the table another in the Acanthaceæ. At present no instance is known of two species of the same genus twining in opposite directions; and this is a singular fact, because different individuals of Solanum dulcamara (Dutrochet, tom. xix. p. 299) revolve and twine in both directions: this plant, however, is a most feeble twiner. Loasa aurantiaca (Léon, p. 351) offers a much more striking 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^[6], 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. These individuals of the Loasa are interesting, as showing how almost every change is effected most gradually. For another plant in the same family, the Scyphanthus elegans, habitually twines in this 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 could hardly occur with any plant which ascended above a few feet in height, or which lived in an exposed situation; for the stem could be easily pulled from its support with 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 Ipomæa jucunda; but frequently with Hibbertia dentata. This plant at first much perplexed me, 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 the opposite direction; consequently, when I placed the shoots near thin or thick sticks, or stretched string, they seemed perpetually to be trying to ascend these supports, but 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 another plant with many thin upright sticks, and placed this plant near the other plant with the twigs; and now the Hibbertia had got what it liked, for it twined up the parallel sticks, sometimes winding round one and sometimes round several; and the shoots travelled laterally from one to the other plant; but as the plants grew older, some of the shoots twined regularly up a thin upright stick. Though the revolving movement was sometimes in one direction and sometimes in the other, the twining was invariably from left to right; 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 to ascend by twining, and to ramble laterally over the thick Australian scrub.

I have described this case in some detail, because, as far as I have seen, it is rare to find with twining plants any especial adaptations, 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 such stems as are both thin and flexible. Most twining plants apparently are adapted to ascend supports 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 (S. 131) found that the Phascolus multiflorus and Ipomæa 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 ascends 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 6 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 thick 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. This power would evidently be almost necessary for twining plants inhabiting tropical forests, as otherwise they could hardly ever reach the light. In our temperate countries twining plants which die down every year to the root would suffer 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 some twining plants are adapted to ascend only thin stems, whilst others can twine round thick trees, I do not know. It appeared to me probable that twining plants with very long revolving shoots might 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 length and power of movement apparently serving merely to find some distant but thin 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 aid it in ascending a thick support; but though it was able to wind round the 6-inch post, it could do this only on the same level or plane, and could not ascend in a spire. We can, however, see, in accordance with the views previously explained, that a revolving shoot, which, after coming into contact with any support, quickly lost its power of movement, would not again be drawn away from its support by the returning or opposite movement, and therefore remaining in contact with it, might thus ascend a thick support. But whether this slight difference in retaining for some time or in quickly losing the power of movement after coming into contact with a support alone determines how thick an object the stem can ascend I do not know.

As ferns differ so much from phanerogamic plants, it may be worth while here to show that twining ferns act in no respect differently from other twining plants. In Lygodium articulatum the two internodes first formed above the root-stock did not move; the third from the ground revolved, and at first very slowly. This species is a slow revolver: but L. scandens made five revolutions at an average rate of 5 h. 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. The two young upper internodes alone moved. A line painted along the surface of a revolving internode which was at the time convex, became first lateral, then concave, and ultimately convex again. Neither the internodes nor petioles are irritable when rubbed. The movement is in the more usual direction, namely in opposition to the course of the sun; and when the stem has twined 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 and onwards. If the stick be soon removed, the internodes 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 phanerogamic twining plants; and the above enumeration may serve as a summary of the leading characteristics of common twining plants.

The power of revolving depends on the general health and vigour of the plant, as has laboriously been shown to be the case 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 h. and the other in 23 h., whereas they ought to have revolved in between 2 h. and 2 h. 30 m. Cut shoots of the Kidney-bean were similarly retarded, but in a less degree. I have repeatedly observed that carrying a plant from the greenhouse to my house, or from one to another part of the greenhouse, always stopped the movement for a time; hence I conclude that naturally exposed plants would not make their revolutions during 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 tendril-bearing 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 different in degree with different plants: thus Ipomæa jucunda (as maybe seen in the table) revolved in 5 h. 20 m., the semicircle from the light taking 4 h. 30 m., and that towards the light only 1 h.; Lonicera brachypoda revolved, in a reversed direction to the Ipomæa, in 8 h., the semicircle from the light taking 5 h. 23 m., and that to the light only 2 h. 37 m. From the rate of revolution in all the plants which I have observed 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 greatly modify the whole rate. This action is remarkable when we reflect how little the leaves are developed on the young and very thin revolving internodes. It is the more remarkable, as botanists have thought (Mohl, S. 119) that twining plants are but little sensitive to the action of light.

I will conclude my account of twining plants by collecting 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 (S. 4), the main stem of Tamus elephantipes does not twine—only the branches. On the other hand, with the Asparagus, given in the table, 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 from its leaves being little developed, and this shoot revolved vigorously and twined. So that this plant produces shoots of two sorts. With Periploca Græca (Palm, S. 43) the uppermost shoots alone twine. Polygonum convolvulus twines only during the middle of the summer (Palm. S. 43, 94): plants growing vigorously in the autumn show no inclination to twine. The majority of Asclepiadaceæ are twiners; but Asclepias nigra only "in fertiliori solo incipit scandere sub volubili caule" (Willdenow, quoted and confirmed by Palm, S. 41). Asclepias vincetoxicum does not regularly twine, but only occasionally (Palm, S. 42; Mohl, S. 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 Ipomæa argyræoides in South Africa 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. Seedlings, on the other hand, raised near Dublin twined up sticks above 8 feet in height. These facts are highly remarkable; for there can hardly be a doubt that in the dryer provinces of South Africa these plants must have propagated themselves for thousands of generations in an erect condition; and yet during this whole period they have retained 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 with "Fulmer's dwarf forcing-bean," on which occasionally a long twining shoot appeared.

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 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 vertical stretched strings close to others, and the strings alone were ascended by twining. We here, perhaps, see the first stage in the habit of twining; and the stem twines indifferently to the right or the left. Some other species of the genus, and of another genus, viz. Habrothamnus, of the same family of Solanaceæ, which are described in horticultural works as twining plants, seemed to possess this faculty in a very feeble manner. On the other hand, I suspect that with Tecoma radicans we have the last vestige of a lost habit: this plant belongs to a group abounding with twining and with tendril-bearing species, but it ascends by rootlets like those of the Ivy; yet I observed that the young internodes seldom remained quite stationary, but performed slight irregular movements which could hardly be accounted for by changes in the action of the light. Anyhow it need not be supposed that there would be any difficulty in the passage from a spirally twining plant to a simple root-climber; for the young internodes of Bignonia Tweedyana and of Hoya carnosa revolve and twine, and likewise emit rootlets which adhere to any fitting surface.

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1. Bull. Bot. Soc. de France, tom. v. 1858, p. 336.
2. Professor Asa Gray has remarked to me, in a letter, that in Thuja occidentalis the twisting of the bark is very conspicuous. The twist is generally to the right of the observer; but, in noticing about a hundred trunks, four or five were observed to be twisted in an opposite direction.
3. It is well known that stems of many plants occasionally become spirally twisted in a monstrous manner; and since the reading of this paper, Dr. Maxwell Masters has remarked to me in a letter that "some of these cases, if not all, are dependent upon some obstacle or resistance to their upward growth." This conclusion agrees with, and perhaps explains, the normal axial twisting of twining-plants; but does not preclude the twisting being of service to the plant and giving greater rigidity to the stem.
4. Comptes Rendus, 1844, tom. xix. p. 295, and Annales des Soc. Nat. 3rd series, Bot., tom. ii. p. 163.
5. I am much indebted to Dr. Hooker for having sent me many plants from Kew; and to Mr. Veitch, of the Royal Exotic Nursery, for having generously given me a large collection of fine specimens of climbing plants. Professor Asa Gray, Prof. Oliver, and Dr. Hooker have afforded me, as on many previous occasions, much information and many valuable references.
6. I raised nine plants of the hybrid Loasa Herbertii, and six of these reversed their spire in ascending their supports.