CHAPTER IV.

TENDRIL-BEARERS—(_continued_).

CUCURBITACEÆ.—Homologous nature of the tendrils—_Echinocystis lobata_,

remarkable movements of the tendrils to avoid seizing the terminal

shoot—Tendrils not excited by contact with another tendril or by drops of

water—Undulatory movement of the extremity of the tendril—_Hanburya_,

adherent discs—VITACÆ—Gradation between the flower-peduncles and tendrils

of the vine—Tendrils of the Virginian Creeper turn from the light, and,

after contact, develop adhesive

discs—SAPINDACEÆ—PASSIFLORACEÆ—_Passiflora gracilis_—Rapid revolving

movement and sensitiveness of the tendrils—Not sensitive to the contact

of other tendrils or of drops of water—Spiral contraction of

tendrils—Summary on the nature and action of tendrils.

CUCURBITACEÆ.—The tendrils in this family have been ranked by competent

judges as modified leaves, stipules, or branches; or as partly a leaf and

partly a branch. De Candolle believes that the tendrils differ in their

homological nature in two of the tribes. {127a} From facts recently

adduced, Mr. Berkeley thinks that Payer’s view is the most probable,

namely, that the tendril is “a separate portion of the leaf itself;” but

much may be said in favour of the belief that it is a modified

flower-peduncle. {127b}

_Echinocystis lobata_.—Numerous observations were made on this plant

(raised from seed sent me by Prof. Asa Gray), for the spontaneous

revolving movements of the internodes and tendrils were first observed by

me in this case, and greatly perplexed me. My observations may now be

much condensed. I observed thirty-five revolutions of the internodes and

tendrils; the slowest rate was 2 hrs. and the average rate, with no great

fluctuations, 1 hr. 40 m. Sometimes I tied the internodes, so that the

tendrils alone moved; at other times I cut off the tendrils whilst very

young, so that the internodes revolved by themselves; but the rate was

not thus affected. The course generally pursued was with the sun, but

often in an opposite direction. Sometimes the movement during a short

time would either stop or be reversed; and this apparently was due to

interference from the light, as, for instance, when I placed a plant

close to a window. In one instance, an old tendril, which had nearly

ceased revolving, moved in one direction, whilst a young tendril above

moved in an opposite course. The two uppermost internodes alone revolve;

and as soon as the lower one grows old, only its upper part continues to

move. The ellipses or circles swept by the summits of the internodes are

about three inches in diameter; whilst those swept by the tips of the

tendrils, are from 15 to 16 inches in diameter. During the revolving

movement, the internodes become successively curved to all points of the

compass; in one part of their course they are often inclined, together

with the tendrils, at about 45° to the horizon, and in another part stand

vertically up. There was something in the appearance of the revolving

internodes which continually gave the false impression that their

movement was due to the weight of the long and spontaneously revolving

tendril; but, on cutting off the latter with sharp scissors, the top of

the shoot rose only a little, and went on revolving. This false

appearance is apparently due to the internodes and tendrils all curving

and moving harmoniously together.

A revolving tendril, though inclined during the greater part of its

course at an angle of about 45° (in one case of only 37°) above the

horizon, stiffened and straightened itself from tip to base in a certain

part of its course, thus becoming nearly or quite vertical. I witnessed

this repeatedly; and it occurred both when the supporting internodes were

free and when they were tied up; but was perhaps most conspicuous in the

latter case, or when the whole shoot happened to be much inclined. The

tendril forms a very acute angle with the projecting extremity of the

stem or shoot; and the stiffening always occurred as the tendril

approached, and had to pass over the shoot in its circular course. If it

had not possessed and exercised this curious power, it would infallibly

have struck against the extremity of the shoot and been arrested. As

soon as the tendril with its three branches begins to stiffen itself in

this manner and to rise from an inclined into a vertical position, the

revolving motion becomes more rapid; and as soon as the tendril has

succeeded in passing over the extremity of the shoot or point of

difficulty, its motion, coinciding with that from its weight, often

causes it to fall into its previously inclined position so quickly, that

the apex could be seen travelling like the minute hand of a gigantic

clock.

The tendrils are thin, from 7 to 9 inches in length, with a pair of short

lateral branches rising not far from the base. The tip is slightly and

permanently curved, so as to act to a limited extent as a hook. The

concave side of the tip is highly sensitive to a touch; but not so the

convex side, as was likewise observed to be the case with other species

of the family by Mohl (p. 65). I repeatedly proved this difference by

lightly rubbing four or five times the convex side of one tendril, and

only once or twice the concave side of another tendril, and the latter

alone curled inwards. In a few hours afterwards, when the tendrils which

had been rubbed on the concave side had straightened themselves, I

reversed the process of rubbing, and always with the same result. After

touching the concave side, the tip becomes sensibly curved in one or two

minutes; and subsequently, if the touch has been at all rough, it coils

itself into a helix. But the helix will, after a time, straighten

itself, and be again ready to act. A loop of thin thread only

one-sixteenth of a grain in weight caused a temporary flexure. The lower

part was repeatedly rubbed rather roughly, but no curvature ensued; yet

this part is sensitive to prolonged pressure, for when it came into

contact with a stick, it would slowly wind round it.

One of my plants bore two shoots near together, and the tendrils were

repeatedly drawn across one another, but it is a singular fact that they

did not once catch each other. It would appear as if they had become

habituated to contact of this kind, for the pressure thus caused must

have been much greater than that caused by a loop of soft thread weighing

only the one-sixteenth of a grain. I have, however, seen several

tendrils of _Bryonia dioica_ interlocked, but they subsequently released

one another. The tendrils of the Echinocystis are also habituated to

drops of water or to rain; for artificial rain made by violently flirting

a wet brush over them produced not the least effect.

The revolving movement of a tendril is not stopped by the curving of its

extremity after it has been touched. When one of the lateral branches

has firmly clasped an object, the middle branch continues to revolve.

When a stem is bent down and secured, so that the tendril depends but is

left free to move, its previous revolving movement is nearly or quite

stopped; but it soon begins to bend upwards, and as soon as it has become

horizontal the revolving movement recommences. I tried this four times;

the tendril generally rose to a horizontal position in an hour or an hour

and a half; but in one case, in which a tendril depended at an angle of

45° beneath the horizon, the uprising took two hours; in half an hour

afterwards it rose to 23° above the horizon and then recommenced

revolving. This upward movement is independent of the action of light,

for it occurred twice in the dark, and on another occasion the light came

in on one side alone. The movement no doubt is guided by opposition to

the force of gravity, as in the case of the ascent of the plumules of

germinating seeds.

A tendril does not long retain its revolving power; and as soon as this

is lost, it bends downwards and contracts spirally. After the revolving

movement has ceased, the tip still retains for a short time its

sensitiveness to contact, but this can be of little or no use to the

plant.

Though the tendril is highly flexible, and though the extremity travels,

under favourable circumstances, at about the rate of an inch in two

minutes and a quarter, yet its sensitiveness to contact is so great that

it hardly ever fails to seize a thin stick placed in its path. The

following case surprised me much: I placed a thin, smooth, cylindrical

stick (and I repeated the experiment seven times) so far from a tendril,

that its extremity could only curl half or three-quarters round the

stick; but I always found that the tip managed in the course of a few

hours to curl twice or even thrice round the stick. I at first thought

that this was due to rapid growth on the outside; but by coloured points

and measurements I proved that there had been no sensible increase of

length within the time. When a stick, flat on one side, was similarly

placed, the tip of the tendril could not curl beyond the flat surface,

but coiled itself into a helix, which, turning to one side, lay flat on

the little flat surface of wood. In one instance a portion of tendril

three-quarters of an inch in length was thus dragged on to the flat

surface by the coiling in of the helix. But the tendril thus acquires a

very insecure hold, and generally after a time slips off. In one case

alone the helix subsequently uncoiled itself, and the tip then passed

round and clasped the stick. The formation of the helix on the flat side

of the stick apparently shows us that the continued striving of the tip

to curl itself closely inwards gives the force which drags the tendril

round a smooth cylindrical stick. In this latter case, whilst the

tendril was slowly and quite insensibly crawling onwards, I observed

several times through a lens that the whole surface was not in close

contact with the stick; and I can understand the onward progress only by

supposing that the movement is slightly undulatory or vermicular, and

that the tip alternately straightens itself a little and then again curls

inwards. It thus drags itself onwards by an insensibly slow, alternate

movement, which may be compared to that of a strong man suspended by the

ends of his fingers to a horizontal pole, who works his fingers onwards

until he can grasp the pole with the palm of his hand. However this may

be, the fact is certain that a tendril which has caught a round stick

with its extreme point, can work itself onwards until it has passed twice

or even thrice round the stick, and has permanently grasped it.

_Hanburya Mexicana_.—The young internodes and tendrils of this anomalous

member of the family, revolve in the same manner and at about the same

rate as those of the _Echinocystis_. The stem does not twine, but can

ascend an upright stick by the aid of its tendrils. The concave tip of

the tendril is very sensitive; after it had become rapidly coiled into a

ring owing to a single touch, it straightened itself in 50 m. The

tendril, when in full action, stands vertically up, with the projecting

extremity of the young stem thrown a little on one side, so as to be out

of the way; but the tendril bears on the inner side, near its base, a

short rigid branch, which projects out at right angles like a spur, with

the terminal half bowed a little downwards. Hence, as the main vertical

branch revolves, the spur, from its position and rigidity, cannot pass

over the extremity of the shoot, in the same curious manner as do the

three branches of the tendril of the _Echinocystis_, namely, by

stiffening themselves at the proper point. The spur is therefore pressed

laterally against the young stem in one part of the revolving course, and

thus the sweep of the lower part of the main branch is much restricted.

A nice case of co-adaptation here comes into play: in all the other

tendrils observed by me, the several branches become sensitive at the

same period: had this been the case with the _Hanburya_, the inwardly

directed, spur-like branch, from being pressed, during the revolving

movement, against the projecting end of the shoot, would infallibly have

seized it in a useless or injurious manner. But the main branch of the

tendril, after revolving for a time in a vertical position, spontaneously

bends downwards; and in doing so, raises the spur-like branch, which

itself also curves upwards; so that by these combined movements it rises

above the projecting end of the shoot, and can now move freely without

touching the shoot; and now it first becomes sensitive.

The tips of both branches, when they come into contact with a stick,

grasp it like any ordinary tendril. But in the course of a few days, the

lower surface swells and becomes developed into a cellular layer, which

adapts itself closely to the wood, and firmly adheres to it. This layer

is analogous to the adhesive discs formed by the extremities of the

tendrils of some species of _Bignonia_ and of _Ampelopsis_; but in the

_Hanburya_ the layer is developed along the terminal inner surface,

sometimes for a length of 1¾ inches, and not at the extreme tip. The

layer is white, whilst the tendril is green, and near the tip it is

sometimes thicker than the tendril itself; it generally spreads a little

beyond the sides of the tendril, and is fringed with free elongated

cells, which have enlarged globular or retort-shaped heads. This

cellular layer apparently secretes some resinous cement; for its adhesion

to the wood was not lessened by an immersion of 24 hrs. in alcohol or

water, but was quite loosened by a similar immersion in ether or

turpentine. After a tendril has once firmly coiled itself round a stick,

it is difficult to imagine of what use the adhesive cellular layer can

be. Owing to the spiral contraction which soon ensues, the tendrils were

never able to remain, excepting in one instance, in contact with a thick

post or a nearly flat surface; if they had quickly become attached by

means of the adhesive layer, this would evidently have been of service to

the plant.

The tendrils of _Bryonia dioica_, _Cucurbita ovifera_, and _Cucumis

sativa_ are sensitive and revolve. Whether the internodes likewise

revolve I did not observe. In _Anguria Warscewiczii_, the internodes,

though thick and stiff, revolve: in this plant the lower surface of the

tendril, some time after clasping a stick, produces a coarsely cellular

layer or cushion, which adapts itself closely to the wood, like that

formed by the tendril of the _Hanburya_; but it is not in the least

adhesive. In _Zanonia Indica_, which belongs to a different tribe of the

family, the forked tendrils and the internodes revolve in periods between

2 hrs. 8 m. and 3 hrs. 35 m., moving against the sun.

VITACEÆ.—In this family and in the two following, namely, the Sapindacæ

and Passifloraceæ, the tendrils are modified flower-peduncles; and are

therefore axial in their nature. In this respect they differ from all

those previously described, with the exception, perhaps, of the

Cucurbitaceæ. The homological nature, however, of a tendril seems to

make no difference in its action.

[Picture: Fig. 9. Tendril of the Vine. A. Peduncle of tendril. B.

Longer Branch, with a scale at its base. C. Shorter branch. D.

Petiole of the opposite leaf]

_Vitis vinifera_.—The tendril is thick and of great length; one from a

vine growing out of doors and not vigorously, was 16 inches long. It

consists of a peduncle (A), bearing two branches which diverge equally

from it. One of the branches (B) has a scale at its base; it is always,

as far as I have seen, longer than the other and often bifurcates. The

branches when rubbed become curved, and subsequently straighten

themselves. After a tendril has clasped any object with its extremity,

it contracts spirally; but this does not occur (Palm, p. 56) when no

object has been seized. The tendrils move spontaneously from side to

side; and on a very hot day, one made two elliptical revolutions, at an

average rate of 2 hrs. 15 m. During these movements a coloured line,

painted along the convex surface, appeared after a time on one side, then

on the concave side, then on the opposite side, and lastly again on the

convex side. The two branches of the same tendril have independent

movements. After a tendril has spontaneously revolved for a time, it

bends from the light towards the dark: I do not state this on my own

authority, but on that of Mohl and Dutrochet. Mohl (p. 77) says that in

a vine planted against a wall the tendrils point towards it, and in a

vineyard generally more or less to the north.

The young internodes revolve spontaneously; but the movement is unusually

slight. A shoot faced a window, and I traced its course on the glass

during two perfectly calm and hot days. On one of these days it

described, in the course of ten hours, a spire, representing two and a

half ellipses. I also placed a bell-glass over a young Muscat grape in

the hot-house, and it made each day three or four very small oval

revolutions; the shoot moving less than half an inch from side to side.

Had it not made at least three revolutions whilst the sky was uniformly

overcast, I should have attributed this slight degree of movement to the

varying action of the light. The extremity of the stem is more or less

bent downwards, but it never reverses its curvature, as so generally

occurs with twining plants.

[Picture: Fig. 10. Flower-stalk of the Vine. A. Common Peduncle. B.

Flower-tendril. C. Sub-Peduncle, bearing the flower-buds. D. Petiole

of the opposite leaf]

Various authors (Palm, p. 55; Mohl, p. 45; Lindley, &c.) believe that the

tendrils of the vine are modified flower-peduncles. I here give a

drawing (fig. 10) of the ordinary state of a young flower-stalk: it

consists of the “common peduncle” (A); of the “flower-tendril” (B), which

is represented as having caught a twig; and of the “sub-peduncle” (C)

bearing the flower-buds. The whole moves spontaneously, like a true

tendril, but in a less degree; the movement, however, is greater when the

sub-peduncle (C) does not bear many flower-buds. The common peduncle (A)

has not the power of clasping a support, nor has the corresponding part

of a true tendril. The flower-tendril (B) is always longer than the

sub-peduncle (C) and has a scale at its base; it sometimes bifurcates,

and therefore corresponds in every detail with the longer scale-bearing

branch (B, fig. 9) of the true tendril. It is, however, inclined

backwards from the sub-peduncle (C), or stands at right angles with it,

and is thus adapted to aid in carrying the future bunch of grapes. When

rubbed, it curves and subsequently straightens itself; and it can, as is

shown in the drawing, securely clasp a support. I have seen an object as

soft as a young vine-leaf caught by one.

The lower and naked part of the sub-peduncle (C) is likewise slightly

sensitive to a rub, and I have seen it bent round a stick and even partly

round a leaf with which it had come into contact. That the sub-peduncle

has the same nature as the corresponding branch of an ordinary tendril,

is well shown when it bears only a few flowers; for in this case it

becomes less branched, increases in length, and gains both in

sensitiveness and in the power of spontaneous movement. I have twice

seen sub-peduncles which bore from thirty to forty flower-buds, and which

had become considerably elongated and were completely wound round sticks,

exactly like true tendrils. The whole length of another sub-peduncle,

bearing only eleven flower-buds, quickly became curved when slightly

rubbed; but even this scanty number of flowers rendered the stalk less

sensitive than the other branch, that is, the flower-tendril; for the

latter after a lighter rub became curved more quickly and in a greater

degree. I have seen a sub-peduncle thickly covered with flower-buds,

with one of its higher lateral branchlets bearing from some cause only

two buds; and this one branchlet had become much elongated and had

spontaneously caught hold of an adjoining twig; in fact, it formed a

little sub-tendril. The increasing length of the sub-peduncle (C) with

the decreasing number of the flower-buds is a good instance of the law of

compensation. In accordance with this same principle, the true tendril

as a whole is always longer than the flower-stalk; for instance, on the

same plant, the longest flower-stalk (measured from the base of the

common peduncle to the tip of the flower-tendril) was 8½ inches in

length, whilst the longest tendril was nearly double this length, namely

16 inches.

The gradations from the ordinary state of a flower-stalk, as represented

in the drawing (fig. 10), to that of a true tendril (fig. 9) are

complete. We have seen that the sub-peduncle (C), whilst still bearing

from thirty to forty flower-buds, sometimes becomes a little elongated

and partially assumes all the characters of the corresponding branch of a

true tendril. From this state we can trace every stage till we come to a

full-sized perfect tendril, bearing on the branch which corresponds with

the sub-peduncle one single flower-bud! Hence there can be no doubt that

the tendril is a modified flower-peduncle.

Another kind of gradation well deserves notice. Flower-tendrils (B, fig.

10) sometimes produce a few flower-buds. For instance, on a vine growing

against my house, there were thirteen and twenty-two flower-buds

respectively on two flower-tendrils, which still retained their

characteristic qualities of sensitiveness and spontaneous movement, but

in a somewhat lessened degree. On vines in hothouses, so many flowers

are occasionally produced on the flower-tendrils that a double bunch of

grapes is the result; and this is technically called by gardeners a

“cluster.” In this state the whole bunch of flowers presents scarcely

any resemblance to a tendril; and, judging from the facts already given,

it would probably possess little power of clasping a support, or of

spontaneous movement. Such flower-stalks closely resemble in structure

those borne by _Cissus_. This genus, belonging to the same family of the

Vitaceæ, produces well-developed tendrils and ordinary bunches of

flowers; but there are no gradations between the two states. If the

genus _Vitis_ had been unknown, the boldest believer in the modification

of species would never have surmised that the same individual plant, at

the same period of growth, would have yielded every possible gradation

between ordinary flower-stalks for the support of the flowers and fruit,

and tendrils used exclusively for climbing. But the vine clearly gives

us such a case; and it seems to me as striking and curious an instance of

transition as can well be conceived.

_Cissus discolor_.—The young shoots show no more movement than can be

accounted for by daily variations in the action of the light. The

tendrils, however, revolve with much regularity, following the sun; and,

in the plants observed by me, swept circles of about 5 inches in

diameter. Five circles were completed in the following times:—4 hrs. 45

m., 4 hrs. 50 m., 4 hrs. 45 m., 4 hrs. 30 m., and 5 hrs. The same tendril

continues to revolve during three or four days. The tendrils are from 3½

to 5 inches in length. They are formed of a long foot-stalk, bearing two

short branches, which in old plants again bifurcate. The two branches

are not of quite equal length; and as with the vine, the longer one has a

scale at its base. The tendril stands vertically upwards; the extremity

of the shoot being bent abruptly downwards, and this position is probably

of service to the plant by allowing the tendril to revolve freely and

vertically.

Both branches of the tendril, whilst young, are highly sensitive. A

touch with a pencil, so gentle as only just to move a tendril borne at

the end of a long flexible shoot, sufficed to cause it to become

perceptibly curved in four or five minutes. It became straight again in

rather above one hour. A loop of soft thread weighing one-seventh of a

grain (9.25 mg.) was thrice tried, and each time caused the tendril to

become curved in 30 or 40 m. Half this weight produced no effect. The

long foot-stalk is much less sensitive, for a slight rubbing produced no

effect, although prolonged contact with a stick caused it to bend. The

two branches are sensitive on all sides, so that they converge if touched

on their inner sides, and diverge if touched on their outer sides. If a

branch be touched at the same time with equal force on opposite sides,

both sides are equally stimulated and there is no movement. Before

examining this plant, I had observed only tendrils which are sensitive on

one side alone, and these when lightly pressed between the finger and

thumb become curved; but on thus pinching many times the tendrils of the

_Cissus_ no curvature ensued, and I falsely inferred at first that they

were not at all sensitive.

_Cissus antarcticus_.—The tendrils on a young plant were thick and

straight, with the tips a little curved. When their concave surfaces

were rubbed, and it was necessary to do this with some force, they very

slowly became curved, and subsequently straight again. They are

therefore much less sensitive than those of the last species; but they

made two revolutions, following the sun, rather more rapidly, viz., in 3

hrs. 30 m. and 4 hrs. The internodes do not revolve.

_Ampelopsis hederacea_ (_Virginian Creeper_).—The internodes apparently

do not move more than can be accounted for by the varying action of the

light. The tendrils are from 4 to 5 inches in length, with the main stem

sending off several lateral branches, which have their tips curved, as

may be seen in the upper figure (fig. 11). They exhibit no true

spontaneous revolving movement, but turn, as was long ago observed by

Andrew Knight, {145} from the light to the dark. I have seen several

tendrils move in less than 24 hours, through an angle of 180° to the dark

side of a case in which a plant was placed, but the movement is sometimes

much slower. The several lateral branches often move independently of

one another, and sometimes irregularly, without any apparent cause.

These tendrils are less sensitive to a touch than any others observed by

me. By gentle but repeated rubbing with a twig, the lateral branches,

but not the main stem, became in the course of three or four hours

slightly curved; but they seemed to have hardly any power of again

straightening themselves. The tendrils of a plant which had crawled over

a large box-tree clasped several of the branches; but I have repeatedly

seen that they will withdraw themselves after seizing a stick. When they

meet with a flat surface of wood or a wall (and this is evidently what

they are adapted for), they turn all their branches towards it, and,

spreading them widely apart, bring their hooked tips laterally into

contact with it. In effecting this, the several branches, after touching

the surface, often rise up, place themselves in a new position, and again

come down into contact with it.

In the course of about two days after a tendril has arranged its branches

so as to press on any surface, the curved tips swell, become bright red,

and form on their under-sides the well-known little discs or cushions

with which they adhere firmly. In one case the tips were slightly

swollen in 38 hrs. after coming into contact with a brick; in another

case they were considerably swollen in 48 hrs., and in an additional 24

hrs. were firmly attached to a smooth board; and lastly, the tips of a

younger tendril not only swelled but became attached to a stuccoed wall

in 42 hrs. These adhesive discs resemble, except in colour and in being

larger, those of _Bignonia capreolata_. When they were developed in

contact with a ball of tow, the fibres were separately enveloped, but not

in so effective a manner as by _B. capreolata_. Discs are never

developed, as far as I have seen, without the stimulus of at least

temporary contact with some object. {146} They are generally first

formed on one side of the curved tip, the whole of which often becomes so

much changed in appearance, that a line of the original green tissue can

be traced only along the concave surface. When, however, a tendril has

clasped a cylindrical stick, an irregular rim or disc is sometimes formed

along the inner surface at some little distance from the curved tip; this

was also observed (p. 71) by Mohl. The discs consist of enlarged cells,

with smooth projecting hemispherical surfaces, coloured red; they are at

first gorged with fluid (see section given by Mohl, p. 70), but

ultimately become woody.

As the discs soon adhere firmly to such smooth surfaces as planed or

painted wood, or to the polished leaf of the ivy, this alone renders it

probable that some cement is secreted, as has been asserted to be the

case (quoted by Mohl, p. 71) by Malpighi. I removed a number of discs

formed during the previous year from a stuccoed wall, and left them

during many hours, in warm water, diluted acetic acid and alcohol; but

the attached grains of silex were not loosened. Immersion in sulphuric

ether for 24 hrs. loosened them much, but warmed essential oils (I tried

oil of thyme and peppermint) completely released every particle of stone

in the course of a few hours. This seems to prove that some resinous

cement is secreted. The quantity, however, must be small; for when a

plant ascended a thinly whitewashed wall, the discs adhered firmly to the

whitewash; but as the cement never penetrated the thin layer, they were

easily withdrawn, together with little scales of the whitewash. It must

not be supposed that the attachment is effected exclusively by the

cement; for the cellular outgrowth completely envelopes every minute and

irregular projection, and insinuates itself into every crevice.

[Picture: Fig. 11. Ampelopsis hederacea. A. Tendril fully developed,

with a young leaf on the opposite side of the stem. B. Older tendril,

several weeks after its attachment to a wall, with the branches thickened

and spirally contracted, and with the extremities developed into discs.

The unattached branches of this tendril have withered and dropped off]

A tendril which has not become attached to any body, does not contract

spirally; and in course of a week or two shrinks into the finest thread,

withers and drops off. An attached tendril, on the other hand, contracts

spirally, and thus becomes highly elastic, so that when the main

foot-stalk is pulled the strain is distributed equally between all the

attached discs. For a few days after the attachment of the discs, the

tendril remains weak and brittle, but it rapidly increases in thickness

and acquires great strength. During the following winter it ceases to

live, but adheres firmly in a dead state both to its own stem and to the

surface of attachment. In the accompanying diagram (fig. 11.) we see the

difference between a tendril (B) some weeks after its attachment to a

wall, with one (A) from the same plant fully grown but unattached. That

the change in the nature of the tissues, as well as the spiral

contraction, are consequent on the formation of the discs, is well shown

by any lateral branches which have not become attached; for these in a

week or two wither and drop off, in the same manner as does the whole

tendril if unattached. The gain in strength and durability in a tendril

after its attachment is something wonderful. There are tendrils now

adhering to my house which are still strong, and have been exposed to the

weather in a dead state for fourteen or fifteen years. One single

lateral branchlet of a tendril, estimated to be at least ten years old,

was still elastic and supported a weight of exactly two pounds. The

whole tendril had five disc-bearing branches of equal thickness and

apparently of equal strength; so that after having been exposed during

ten years to the weather, it would probably have resisted a strain of ten

pounds!

SAPINDACEÆ.—_Cardiospermum halicacabum_.—In this family, as in the last,

the tendrils are modified flower-peduncles. In the present plant the two

lateral branches of the main flower-peduncle have been converted into a

pair of tendrils, corresponding with the single “flower-tendril” of the

common vine. The main peduncle is thin, stiff, and from 3 to 4½ inches

in length. Near the summit, above two little bracts, it divides into

three branches. The middle one divides and re-divides, and bears the

flowers; ultimately it grows half as long again as the two other modified

branches. These latter are the tendrils; they are at first thicker and

longer than the middle branch, but never become more than an inch in

length. They taper to a point and are flattened, with the lower clasping

surface destitute of hairs. At first they project straight up; but soon

diverging, spontaneously curl downwards so as to become symmetrically and

elegantly hooked, as represented in the diagram. They are now, whilst

the flower-buds are still small, ready for action.

[Picture: Fig. 12. Cardiospermum halicacabum. Upper part of the

flower-peduncle with its two tendrils]

The two or three upper internodes, whilst young, steadily revolve; those

on one plant made two circles, against the course of the sun, in 3 hrs.

12 m.; in a second plant the same course was followed, and the two

circles were completed in 3 hrs. 41 m.; in a third plant, the internodes

followed the sun and made two circles in 3 hrs. 47 m. The average rate

of these six revolutions was 1 hr. 46 m. The stem shows no tendency to

twine spirally round a support; but the allied tendril-bearing genus

_Paullinia_ is said (Mohl, p. 4) to be a twiner. The flower-peduncles,

which stand up above the end of the shoot, are carried round and round by

the revolving movement of the internodes; and when the stem is securely

tied, the long and thin flower-peduncles themselves are seen to be in

continued and sometimes rapid movement from side to side. They sweep a

wide space, but only occasionally revolve in a regular elliptical course.

By the combined movements of the internodes and peduncles, one of the two

short hooked tendrils, sooner or later, catches hold of some twig or

branch, and then it curls round and securely grasps it. These tendrils

are, however, but slightly sensitive; for by rubbing their under surface

only a slight movement is slowly produced. I hooked a tendril on to a

twig; and in 1 hr. 45 m. it was curved considerably inwards; in 2 hrs. 30

m. it formed a ring; and in from 5 to 6 hours from being first hooked, it

closely grasped the stick. A second tendril acted at nearly the same

rate; but I observed one that took 24 hours before it curled twice round

a thin twig. Tendrils which have caught nothing, spontaneously curl up

to a close helix after the interval of several days. Those which have

curled round some object, soon become a little thicker and tougher. The

long and thin main peduncle, though spontaneously moving, is not

sensitive and never clasps a support. Nor does it ever contract

spirally, {152} although a contraction of this kind apparently would have

been of service to the plant in climbing. Nevertheless it climbs pretty

well without this aid. The seed-capsules though light, are of enormous

size (hence its English name of balloon-vine), and as two or three are

carried on the same peduncle, the tendrils rising close to them may be of

service in preventing their being dashed to pieces by the wind. In the

hothouse the tendrils served simply for climbing.

The position of the tendrils alone suffices to show their homological

nature. In two instances one of two tendrils produced a flower at its

tip; this, however, did not prevent its acting properly and curling round

a twig. In a third case both lateral branches which ought to have been

modified into tendrils, produced flowers like the central branch, and had

quite lost their tendril-structure.

I have seen, but was not enabled carefully to observe, only one other

climbing Sapindaceous plant, namely, _Paullinia_. It was not in flower,

yet bore long forked tendrils. So that, _Paullinia_, with respect to its

tendrils, appears to bear the same relation to _Cardiospermum_ that

_Cissus_ does to _Vitis_.

PASSIFLORACEÆ.—After reading the discussion and facts given by Mohl (p.

47) on the nature of the tendrils in this family, no one can doubt that

they are modified flower-peduncles. The tendrils and the

flower-peduncles rise close side by side; and my son, William E. Darwin,

made sketches for me of their earliest state of development in the hybrid

_P. floribunda_. The two organs appear at first as a single papilla

which gradually divides; so that the tendril appears to be a modified

branch of the flower-peduncle. My son found one very young tendril

surmounted by traces of floral organs, exactly like those on the summit

of the true flower-peduncle at the same early age.

_Passiflora gracilis_.—This well-named, elegant, annual species differs

from the other members of the group observed by me, in the young

internodes having the power of revolving. It exceeds all the other

climbing plants which I have examined, in the rapidity of its movements,

and all tendril-bearers in the sensitiveness of the tendrils. The

internode which carries the upper active tendril and which likewise

carries one or two younger immature internodes, made three revolutions,

following the sun, at an average rate of 1 hr. 4 m.; it then made, the

day becoming very hot, three other revolutions at an average rate of

between 57 and 58 m.; so that the average of all six revolutions was 1

hr. 1 m. The apex of the tendril describes elongated ellipses, sometimes

narrow and sometimes broad, with their longer axes inclined in slightly

different directions. The plant can ascend a thin upright stick by the

aid of its tendrils; but the stem is too stiff for it to twine spirally

round it, even when not interfered with by the tendrils, these having

been successively pinched off at an early age.

When the stem is secured, the tendrils are seen to revolve in nearly the

same manner and at the same rate as the internodes. {154} The tendrils

are very thin, delicate, and straight, with the exception of the tips,

which are a little curved; they are from 7 to 9 inches in length. A

half-grown tendril is not sensitive; but when nearly full-grown they are

extremely sensitive. A single delicate touch on the concave surface of

the tip soon caused one to curve; and in 2 minutes it formed an open

helix. A loop of soft thread weighing one thirty-second of a grain (2.02

mg.) placed most gently on the tip, thrice caused distinct curvature. A

bent bit of thin platina wire weighing only fiftieth of a grain (1.23

mg.) twice produced the same effect; but this latter weight, when left

suspended, did not suffice to cause a permanent curvature. These trials

were made under a bell-glass, so that the loops of thread and wire were

not agitated by the wind. The movement after a touch is very rapid: I

took hold of the lower part of several tendrils, and then touched their

concave tips with a thin twig and watched them carefully through a lens;

the tips evidently began to bend after the following intervals—31, 25,

32, 31, 28, 39, 31, and 30 seconds; so that the movement was generally

perceptible in half a minute after a touch; but on one occasion it was

distinctly visible in 25 seconds. One of the tendrils which thus became

bent in 31 seconds, had been touched two hours previously and had coiled

into a helix; so that in this interval it had straightened itself and had

perfectly recovered its irritability.

To ascertain how often the same tendril would become curved when touched,

I kept a plant in my study, which from being cooler than the hot-house

was not very favourable for the experiment. The extremity was gently

rubbed four or five times with a thin stick, and this was done as often

as it was observed to have become nearly straight again after having been

in action; and in the course of 54 hrs. it answered to the stimulus 21

times, becoming each time hooked or spiral. On the last occasion,

however, the movement was very slight, and soon afterwards permanent

spiral contraction commenced. No trials were made during the night, so

that the tendril would perhaps have answered a greater number of times to

the stimulus; though, on the other hand, from having no rest it might

have become exhausted from so many quickly repeated efforts.

I repeated the experiment made on the _Echinocystis_, and placed several

plants of this _Passiflora_ so close together, that their tendrils were

repeatedly dragged over each other; but no curvature ensued. I likewise

repeatedly flirted small drops of water from a brush on many tendrils,

and syringed others so violently that the whole tendril was dashed about,

but they never became curved. The impact from the drops of water was

felt far more distinctly on my hand than that from the loops of thread

(weighing one thirty-second of a grain) when allowed to fall on it from a

height, and these loops, which caused the tendrils to become curved, had

been placed most gently on them. Hence it is clear, that the tendrils

either have become habituated to the touch of other tendrils and drops of

rain, or that they were from the first rendered sensitive only to

prolonged though excessively slight pressure of solid objects, with the

exclusion of that from other tendrils. To show the difference in the

kind of sensitiveness in different plants and likewise to show the force

of the syringe used, I may add that the lightest jet from it instantly

caused the leaves of a _Mimosa_ to close; whereas the loop of thread

weighing one thirty-second of a grain, when rolled into a ball and placed

gently on the glands at the bases of the leaflets of the _Mimosa_, caused

no action.

_Passiflora punctata_.—The internodes do not move, but the tendrils

revolve regularly. A half-grown and very sensitive tendril made three

revolutions, opposed to the course of the sun, in 3 hrs. 5 m., 2 hrs. 40

m. and 2 hrs. 50 m.; perhaps it might have travelled more quickly when

nearly full-grown. A plant was placed in front of a window, and, as with

twining stems, the light accelerated the movement of the tendril in one

direction and retarded it in the other; the semicircle towards the light

being performed in one instance in 15 m. less time and in a second

instance in 20 m. less time than that required by the semicircle towards

the dark end of the room. Considering the extreme tenuity of these

tendrils, the action of the light on them is remarkable. The tendrils

are long, and, as just stated, very thin, with the tip slightly curved or

hooked. The concave side is extremely sensitive to a touch—even a single

touch causing it to curl inwards; it subsequently straightened itself,

and was again ready to act. A loop of soft thread weighing one

fourteenth of a grain (4.625 mg.) caused the extreme tip to bend; another

time I tried to hang the same little loop on an inclined tendril, but

three times it slid off; yet this extraordinarily slight degree of

friction sufficed to make the tip curl. The tendril, though so

sensitive, does not move very quickly after a touch, no conspicuous

movement being observable until 5 or 10 m. had elapsed. The convex side

of the tip is not sensitive to a touch or to a suspended loop of thread.

On one occasion I observed a tendril revolving with the convex side of

the tip forwards, and in consequence it was not able to clasp a stick,

against which it scraped; whereas tendrils revolving with the concave

side forward, promptly seize any object in their path.

_Passiflora quadrangularis_.—This is a very distinct species. The

tendrils are thick, long, and stiff; they are sensitive to a touch only

on the concave surface towards the extremity. When a stick was placed so

that the middle of the tendril came into contact with it, no curvature

ensued. In the hothouse a tendril made two revolutions, each in 2 hrs.

22 m.; in a cool room one was completed in 3 hrs., and a second in 4 hrs.

The internodes do not revolve; nor do those of the hybrid _P.

floribunda_.

_Tacsonia manicata_.—Here again the internodes do not revolve. The

tendrils are moderately thin and long; one made a narrow ellipse in 5

hrs. 20 m., and the next day a broad ellipse in 5 hrs. 7 m. The

extremity being lightly rubbed on the concave surface, became just

perceptibly curved in 7 m., distinctly in 10 m., and hooked in 20 m.

We have seen that the tendrils in the last three families, namely, the

Vitaceæ, Sapindaceæ and Passifloraceæ, are modified flower-peduncles.

This is likewise the case, according to De Candolle (as quoted by Mohl),

with the tendrils of _Brunnichia_, one of the Polygonaceæ. In two or

three species of _Modecca_, one of the Papayaceæ, the tendrils, as I hear

from Prof. Oliver, occasionally bear flowers and fruit; so that they are

axial in their nature.

_The Spiral Contraction of Tendrils_.

This movement, which shortens the tendrils and renders them elastic,

commences in half a day, or in a day or two after their extremities have

caught some object. There is no such movement in any leaf-climber, with

the exception of an occasional trace of it in the petioles of _Tropæolum

tricolorum_. On the other hand, the tendrils of all tendril-bearing

plants, contract spirally after they have caught an object with the

following exceptions. Firstly, _Corydalis claviculata_, but then this

plant might be called a leaf-climber. Secondly and thirdly, _Bignonia

unguis_ with its close allies, and _Cardiospermum_; but their tendrils

are so short that their contraction could hardly occur, and would be

quite superfluous. Fourthly, _Smilaæ aspera_ offers a more marked

exception, as its tendrils are moderately long. The tendrils of

_Dicentra_, whilst the plant is young, are short and after attachment

only become slightly flexuous; in older plants they are longer and then

they contract spirally. I have seen no other exceptions to the rule that

tendrils, after clasping with their extremities a support, undergo spiral

contraction. When, however, the tendril of a plant of which the stem is

immovably fixed, catches some fixed object, it does not contract, simply

because it cannot; this, however, rarely occurs. In the common Pea the

lateral branches alone contract, and not the central stem; and with most

plants, such as the Vine, Passiflora, Bryony, the basal portion never

forms a spire.

I have said that in _Corydalis claviculata_ the end of the leaf or

tendril (for this part may be indifferently so called) does not contract

into a spire. The branchlets, however, after they have wound round thin

twigs, become deeply sinuous or zigzag. Moreover the whole end of the

petiole or tendril, if it seizes nothing, bends after a time abruptly

downwards and inwards, showing that its outer surface has gone on growing

after the inner surface has ceased to grow. That growth is the chief

cause of the spiral contraction of tendrils may be safely admitted, as

shown by the recent researches of H. de Vries. I will, however, add one

little fact in support of this conclusion.

If the short, nearly straight portion of an attached tendril of

_Passiflora gracilis_, (and, as I believe, of other tendrils,) between

the opposed spires, be examined, it will be found to be transversely

wrinkled in a conspicuous manner on the outside; and this would naturally

follow if the outer side had grown more than the inner side, this part

being at the same time forcibly prevented from becoming curved. So again

the whole outer surface of a spirally wound tendril becomes wrinkled if

it be pulled straight. Nevertheless, as the contraction travels from the

extremity of a tendril, after it has been stimulated by contact with a

support, down to the base, I cannot avoid doubting, from reasons

presently to be given, whether the whole effect ought to be attributed to

growth. An unattached tendril rolls itself up into a flat helix, as in

the case of Cardiospermum, if the contraction commences at the extremity

and is quite regular; but if the continued growth of the outer surface is

a little lateral, or if the process begins near the base, the terminal

portion cannot be rolled up within the basal portion, and the tendril

then forms a more or less open spire. A similar result follows if the

extremity has caught some object, and is thus held fast.

The tendrils of many kinds of plants, if they catch nothing, contract

after an interval of several days or weeks into a spire; but in these

cases the movement takes place after the tendril has lost its revolving

power and hangs down; it has also then partly or wholly lost its

sensibility; so that this movement can be of no use. The spiral

contraction of unattached tendrils is a much slower process than that of

attached ones. Young tendrils which have caught a support and are

spirally contracted, may constantly be seen on the same stem with the

much older unattached and uncontracted tendrils. In the _Echinocystis_ I

have seen a tendril with the two lateral branches encircling twigs and

contracted into beautiful spires, whilst the main branch which had caught

nothing remained for many days straight. In this plant I once observed a

main branch after it had caught a stick become spirally flexuous in 7

hrs., and spirally contracted in 18 hrs. Generally the tendrils of the

_Echinocystis_ begin to contract in from 12 hrs. to 24 hrs. after

catching some object; whilst unattached tendrils do not begin to contract

until two or three or even more days after all revolving movement has

ceased. A full-grown tendril of _Passiflora quadrangularis_ which had

caught a stick began in 8 hrs. to contract, and in 24 hrs. formed several

spires; a younger tendril, only two-thirds grown, showed the first trace

of contraction in two days after clasping a stick, and in two more days

formed several spires. It appears, therefore, that the contraction does

not begin until the tendril is grown to nearly its full length. Another

young tendril of about the same age and length as the last did not catch

any object; it acquired its full length in four days; in six additional

days it first became flexuous, and in two more days formed one complete

spire. This first spire was formed towards the basal end, and the

contraction steadily but slowly progressed towards the apex; but the

whole was not closely wound up into a spire until 21 days had elapsed

from the first observation, that is, until 17 days after the tendril had

grown to its full length.

The spiral contraction of tendrils is quite independent of their power of

spontaneously revolving, for it occurs in tendrils, such as those of

_Lathyrus grandiflorus_ and _Ampelopsis hederacea_, which do not revolve.

It is not necessarily related to the curling of the tips round a support,

as we see with the Ampelopsis and _Bignonia capreolata_, in which the

development of adherent discs suffices to cause spiral contraction. Yet

in some cases this contraction seems connected with the curling or

clasping movement, due to contact with a support; for not only does it

soon follow this act, but the contraction generally begins close to the

curled extremity, and travels downwards to the base. If, however, a

tendril be very slack, the whole length almost simultaneously becomes at

first flexuous and then spiral. Again, the tendrils of some few plants

never contract spirally unless they have first seized hold of some

object; if they catch nothing they hang down, remaining straight, until

they wither and drop off: this is the case with the tendrils of Bignonia,

which consist of modified leaves, and with those of three genera of the

Vitaceæ, which are modified flower-peduncles. But in the great majority

of cases, tendrils which have never come in contact with any object,

after a time contract spirally. All these facts taken together, show

that the act of clasping a support and the spiral contraction of the

whole length of the tendril, are phenomena not necessarily connected.

The spiral contraction which ensues after a tendril has caught a support

is of high service to the plant; hence its almost universal occurrence

with species belonging to widely different orders. When a shoot is

inclined and its tendril has caught an object above, the spiral

contraction drags up the shoot. When the shoot is upright, the growth of

the stem, after the tendrils have seized some object above, would leave

it slack, were it not for the spiral contraction which draws up the stem

as it increases in length. Thus there is no waste of growth, and the

stretched stem ascends by the shortest course. When a terminal branchlet

of the tendril of Cobæa catches a stick, we have seen how well the spiral

contraction successively brings the other branchlets, one after the

other, into contact with the stick, until the whole tendril grasps it in

an inextricable knot. When a tendril has caught a yielding object, this

is sometimes enveloped and still further secured by the spiral folds, as

I have seen with _Passiflora quadrangularis_; but this action is of

little importance.

A far more important service rendered by the spiral contraction of the

tendrils is that they are thus made highly elastic. As before remarked

under Ampelopsis, the strain is thus distributed equally between the

several attached branches; and this renders the whole far stronger than

it otherwise would be, as the branches cannot break separately. It is

this elasticity which protects both branched and simple tendrils from

being torn away from their supports during stormy weather. I have more

than once gone on purpose during a gale to watch a Bryony growing in an

exposed hedge, with its tendrils attached to the surrounding bushes; and

as the thick and thin branches were tossed to and fro by the wind, the

tendrils, had they not been excessively elastic, would instantly have

been torn off and the plant thrown prostrate. But as it was, the Bryony

safely rode out the gale, like a ship with two anchors down, and with a

long range of cable ahead to serve as a spring as she surges to the

storm.

When an unattached tendril contracts spirally, the spire always runs in

the same direction from tip to base. A tendril, on the other hand, which

has caught a support by its extremity, although the same side is concave

from end to end, invariably becomes twisted in one part in one direction,

and in another part in the opposite direction; the oppositely turned

spires being separated by a short straight portion. This curious and

symmetrical structure has been noticed by several botanists, but has not

been sufficiently explained. {165} It occurs without exception with all

tendrils which after catching an object contract spirally, but is of

course most conspicuous in the longer tendrils. It never occurs with

uncaught tendrils; and when this appears to have occurred, it will be

found that the tendril had originally seized some object and had

afterwards been torn free. Commonly, all the spires at one end of an

attached tendril run in one direction, and all those at the other end in

the opposite direction, with a single short straight portion in the

middle; but I have seen a tendril with the spires alternately turning

five times in opposite directions, with straight pieces between them; and

M. Léon has seen seven or eight such alternations. Whether the spires

turn once or more than once in opposite directions, there are as many

turns in the one direction as in the other. For instance, I gathered ten

attached tendrils of the Bryony, the longest with 33, and the shortest

with only 8 spiral turns; and the number of turns in the one direction

was in every case the same (within one) as in the opposite direction.

[Picture: Fig. 13. A caught tendril of Bryonia dioica, spirally

contracted in reserved directions]

The explanation of this curious little fact is not difficult. I will not

attempt any geometrical reasoning, but will give only a practical

illustration. In doing this, I shall first have to allude to a point

which was almost passed over when treating of Twining-plants. If we hold

in our left hand a bundle of parallel strings, we can with our right hand

turn these round and round, thus imitating the revolving movement of a

twining plant, and the strings do not become twisted. But if we hold at

the same time a stick in our left hand, in such a position that the

strings become spirally turned round it, they will inevitably become

twisted. Hence a straight coloured line, painted along the internodes of

a twining plant before it has wound round a support, becomes twisted or

spiral after it has wound round. I painted a red line on the straight

internodes of a _Humulus_, _Mikania_, _Ceropegia_, _Convolvulus_, and

_Phaseolus_, and saw it become twisted as the plant wound round a stick.

It is possible that the stems of some plants by spontaneously turning on

their own axes, at the proper rate and in the proper direction, might

avoid becoming twisted; but I have seen no such case.

In the above illustration, the parallel strings were wound round a stick;

but this is by no means necessary, for if wound into a hollow coil (as

can be done with a narrow slip of elastic paper) there is the same

inevitable twisting of the axis. When, therefore, a free tendril coils

itself into a spire, it must either become twisted along its whole length

(and this never occurs), or the free extremity must turn round as many

times as there are spires formed. It was hardly necessary to observe

this fact; but I did so by affixing little paper vanes to the extreme

points of the tendrils of _Echinocystis_ and _Passiflora quadrangularis_;

and as the tendril contracted itself into successive spires, the vane

slowly revolved.

We can now understand the meaning of the spires being invariably turned

in opposite directions, in tendrils which from having caught some object

are fixed at both ends. Let us suppose a caught tendril to make thirty

spiral turns all in the same direction; the inevitable result would be

that it would become twisted thirty times on its own axis. This twisting

would not only require considerable force, but, as I know by trial, would

burst the tendril before the thirty turns were completed. Such cases

never really occur; for, as already stated, when a tendril has caught a

support and is spirally contracted, there are always as many turns in one

direction as in the other; so that the twisting of the axis in the one

direction is exactly compensated by the twisting in the opposite

direction. We can further see how the tendency is given to make the

later formed coils opposite to those, whether turned to the right or to

the left, which are first made. Take a piece of string, and let it hang

down with the lower end fixed to the floor; then wind the upper end

(holding the string quite loosely) spirally round a perpendicular pencil,

and this will twist the lower part of the string; and after it has been

sufficiently twisted, it will be seen to curve itself into an open spire,

with the curves running in an opposite direction to those round the

pencil, and consequently with a straight piece of string between the

opposed spires. In short, we have given to the string the regular spiral

arrangement of a tendril caught at both ends. The spiral contraction

generally begins at the extremity which has clasped a support; and these

first-formed spires give a twist to the axis of the tendril, which

necessarily inclines the basal part into an opposite spiral curvature. I

cannot resist giving one other illustration, though superfluous: when a

haberdasher winds up ribbon for a customer, he does not wind it into a

single coil; for, if he did, the ribbon would twist itself as many times

as there were coils; but he winds it into a figure of eight on his thumb

and little finger, so that he alternately takes turns in opposite

directions, and thus the ribbon is not twisted. So it is with tendrils,

with this sole difference, that they take several consecutive turns in

one direction and then the same number in an opposite direction; but in

both cases the self-twisting is avoided.

_Summary on the Nature and Action of Tendrils_.

With the majority of tendril-bearing plants the young internodes revolve

in more or less broad ellipses, like those made by twining plants; but

the figures described, when carefully traced, generally form irregular

ellipsoidal spires. The rate of revolution varies from one to five hours

in different species, and consequently is in some cases more rapid than

with any twining plant, and is never so slow as with those many twiners

which take more than five hours for each revolution. The direction is

variable even in the same individual plant. In _Passiflora_, the

internodes of only one species have the power of revolving. The Vine is

the weakest revolver observed by me, apparently exhibiting only a trace

of a former power. In the _Eccremocarpus_ the movement is interrupted by

many long pauses. Very few tendril-bearing plants can spirally twine up

an upright stick. Although the power of twining has generally been lost,

either from the stiffness or shortness of the internodes, from the size

of the leaves, or from some other unknown cause, the revolving movement

of the stem serves to bring the tendrils into contact with surrounding

objects.

The tendrils themselves also spontaneously revolve. The movement begins

whilst the tendril is young, and is at first slow. The mature tendrils

of _Bignonia littoralis_ move much slower than the internodes.

Generally, the internodes and tendrils revolve together at the same rate;

in Cissus, Cobæa, and most Passifloræ, the tendrils alone revolve; in

other cases, as with _Lathyrus aphaca_, only the internodes move,

carrying with them the motionless tendrils; and, lastly (and this is the

fourth possible case), neither internodes nor tendrils spontaneously

revolve, as with _Lathyrus grandiflorus_ and _Ampelopsis_. In most

Bignonias, Eccremocarpus Mutisia, and the Fumariaceæ, the internodes,

petioles and tendrils all move harmoniously together. In every case the

conditions of life must be favourable in order that the different parts

should act in a perfect manner.

Tendrils revolve by the curvature of their whole length, excepting the

sensitive extremity and the base, which parts do not move, or move but

little. The movement is of the same nature as that of the revolving

internodes, and, from the observations of Sachs and H. de Vries, no doubt

is due to the same cause, namely, the rapid growth of a longitudinal

band, which travels round the tendril and successively bows each part to

the opposite side. Hence, if a line be painted along that surface which

happens at the time to be convex, the line becomes first lateral, then

concave, then lateral, and ultimately again convex. This experiment can

be tried only on the thicker tendrils, which are not affected by a thin

crust of dried paint. The extremities are often slightly curved or

hooked, and the curvature of this part is never reversed; in this respect

they differ from the extremities of twining shoots, which not only

reverse their curvature, or at least become periodically straight, but

curve themselves in a greater degree than the lower part. In most other

respects a tendril acts as if it were one of several revolving

internodes, which all move together by successively bending to each point

of the compass. There is, however, in many cases this unimportant

difference, that the curving tendril is separated from the curving

internode by a rigid petiole. With most tendril-bearers the summit of

the stem or shoot projects above the point from which the tendril arises;

and it is generally bent to one side, so as to be out of the way of the

revolutions swept by the tendril. In those plants in which the terminal

shoot is not sufficiently out of the way, as we have seen with the

Echinocystis, as soon as the tendril comes in its revolving course to

this point, it stiffens and straightens itself, and thus rising

vertically up passes over the obstacle in an admirable manner.

All tendrils are sensitive, but in various degrees, to contact with an

object, and curve towards the touched side. With several plants a single

touch, so slight as only just to move the highly flexible tendril, is

enough to induce curvature. _Passiflora gracilis_ possesses the most

sensitive tendrils which I have observed: a bit of platina wire 0.02 of a

grain (1.23 mg.) in weight, gently placed on the concave point, caused a

tendril to become hooked, as did a loop of soft, thin cotton thread

weighing one thirty-second of a grain (2.02 mg.) With the tendrils of

several other plants, loops weighing one sixteenth of a grain (4.05 mg.)

sufficed. The point of a tendril of _Passiflora gracilis_ began to move

distinctly in 25 seconds after a touch, and in many cases after 30

seconds. Asa Gray also saw movement in the tendrils of the

Cucurbitaceous genus, _Sicyos_, in 30 seconds. The tendrils of some

other plants, when lightly rubbed, moved in a few minutes; with Dicentra

in half-an-hour; with Smilax in an hour and a quarter or half; and with

Ampelopsis still more slowly. The curling movement consequent on a

single touch continues to increase for a considerable time, then ceases;

after a few hours the tendril uncurls itself, and is again ready to act.

When the tendrils of several kinds of plants were caused to bend by

extremely light weights suspended on them, they seemed to grow accustomed

to so slight a stimulus, and straightened themselves, as if the loops had

been removed. It makes no difference what sort of object a tendril

touches, with the remarkable exception of other tendrils and drops of

water, as was observed with the extremely sensitive-tendrils of

_Passiflora gracilis_ and of the _Echinocystis_. I have, however, seen

tendrils of the Bryony which had temporarily caught other tendrils, and

often in the case of the vine.

Tendrils of which the extremities are permanently and slightly curved,

are sensitive only on the concave surface; other tendrils, such as those

of the Cobæa (though furnished with horny hooks directed to one side) and

those of _Cissus discolor_, are sensitive on all sides. Hence the

tendrils of this latter plant, when stimulated by a touch of equal force

on opposite sides, did not bend. The inferior and lateral surfaces of

the tendrils of _Mutisia_ are sensitive, but not the upper surface. With

branched tendrils, the several branches act alike; but in the _Hanburya_

the lateral spur-like branch does not acquire (for excellent reasons

which have been explained) its sensitiveness nearly so soon as the main

branch. With most tendrils the lower or basal part is either not at all

sensitive, or sensitive only to prolonged contact. We thus see that the

sensitiveness of tendrils is a special and localized capacity. It is

quite independent of the power of spontaneously revolving; for the

curling of the terminal portion from touch does not in the least

interrupt the former movement. In _Bignonia unguis_ and its close

allies, the petioles of the leaves, as well as the tendrils, are

sensitive to a touch.

Twining plants when they come into contact with a stick, curl round it

invariably in the direction of their revolving movement; but tendrils

curl indifferently to either side, in accordance with the position of the

stick and the side which is first touched. The clasping movement of the

extremity is apparently not steady, but undulatory or vermicular in its

nature, as may be inferred from the curious manner in which the tendrils

of the Echinocystis slowly crawled round a smooth stick.

As with a few exceptions tendrils spontaneously revolve, it may be

asked,—why have they been endowed with sensitiveness?—why, when they come

into contact with a stick, do they not, like twining plants, spirally

wind round it? One reason may be that they are in most cases so flexible

and thin, that when brought into contact with any object, they would

almost certainly yield and be dragged onwards by the revolving movement.

Moreover, the sensitive extremities have no revolving power as far as I

have observed, and could not by this means curl round a support. With

twining plants, on the other hand, the extremity spontaneously bends more

than any other part; and this is of high importance for the ascent of the

plant, as may be seen on a windy day. It is, however, possible that the

slow movement of the basal and stiffer parts of certain tendrils, which

wind round sticks placed in their path, may be analogous to that of

twining plants. But I hardly attended sufficiently to this point, and it

would have been difficult to distinguish between a movement due to

extremely dull irritability, from the arrestment of the lower part,

whilst the upper part continued to move onwards.

Tendrils which are only three-fourths grown, and perhaps even at an

earlier age, but not whilst extremely young, have the power of revolving

and of grasping any object which they touch. These two capacities are

generally acquired at about the same period, and both fail when the

tendril is full grown. But in _Cobæa_ and _Passiflora punctata_ the

tendrils begin to revolve in a useless manner, before they have become

sensitive. In the Echinocystis they retain their sensitiveness for some

time after they have ceased to revolve and after they have sunk

downwards; in this position, even if they were able to seize an object,

such power would be of no service in supporting the stem. It is a rare

circumstance thus to detect any superfluity or imperfection in the action

of tendrils—organs which are so excellently adapted for the functions

which they have to perform; but we see that they are not always perfect,

and it would be rash to assume that any existing tendril has reached the

utmost limit of perfection.

Some tendrils have their revolving motion accelerated or retarded, in

moving to or from the light; others, as with the Pea, seem indifferent to

its action; others move steadily from the light to the dark, and this

aids them in an important manner in finding a support. For instance, the

tendrils of _Bignonia capreolata_ bend from the light to the dark as

truly as a wind-vane from the wind. In the Eccremocarpus the extremities

alone twist and turn about so as to bring their finer branches and hooks

into close contact with any dark surface, or into crevices and holes.

A short time after a tendril has caught a support, it contracts with some

rare exceptions into a spire; but the manner of contraction and the

several important advantages thus gained have been discussed so lately,

that nothing need here be repeated on the subject. Tendrils soon after

catching a support grow much stronger and thicker, and sometimes more

durable to a wonderful degree; and this shows how much their internal

tissues must be changed. Occasionally it is the part which is wound

round a support which chiefly becomes thicker and stronger; I have seen,

for instance, this part of a tendril of _Bignonia æquinoctialis_ twice as

thick and rigid as the free basal part. Tendrils which have caught

nothing soon shrink and wither; but in some species of Bignonia they

disarticulate and fall off like leaves in autumn.

* * * * *

Any one who had not closely observed tendrils of many kinds would

probably infer that their action was uniform. This is the case with the

simpler kinds, which simply curl round an object of moderate thickness,

whatever its nature may be. {176} But the genus Bignonia shows us what

diversity of action there may be between the tendrils of closely allied

species. In all the nine species observed by me, the young internodes

revolve vigorously; the tendrils also revolve, but in some of the species

in a very feeble manner; and lastly the petioles of nearly all revolve,

though with unequal power. The petioles of three of the species, and the

tendrils of all are sensitive to contact. In the first-described

species, the tendrils resemble in shape a bird’s foot, and they are of no

service to the stem in spirally ascending a thin upright stick, but they

can seize firm hold of a twig or branch. When the stem twines round a

somewhat thick stick, a slight degree of sensitiveness possessed by the

petioles is brought into play, and the whole leaf together with the

tendril winds round it. In _B. unguis_ the petioles are more sensitive,

and have greater power of movement than those of the last species; they

are able, together with the tendrils, to wind inextricably round a thin

upright stick; but the stem does not twine so well. _B. Tweedyana_ has

similar powers, but in addition, emits aërial roots which adhere to the

wood. In _B. venusta_ the tendrils are converted into elongated

three-pronged grapnels, which move spontaneously in a conspicuous manner;

the petioles, however, have lost their sensitiveness. The stem of this

species can twine round an upright stick, and is aided in its ascent by

the tendrils seizing the stick alternately some way above and then

contracting spirally. In _B. littoralis_ the tendrils, petioles, and

internodes, all revolve spontaneously. The stem, however, cannot twine,

but ascends an upright stick by seizing it above with both tendrils

together, which then contract into a spire. The tips of these tendrils

become developed into adhesive discs. _B. speciosa_ possesses similar

powers of movement as the last species, but it cannot twine round a

stick, though it can ascend by clasping the stick horizontally with one

or both of its unbranched tendrils. These tendrils continually insert

their pointed ends into minute crevices or holes, but as they are always

withdrawn by the subsequent spiral contraction, the habit seems to us in

our ignorance useless. Lastly, the stem of _B. capreolata_ twines

imperfectly; the much-branched tendrils revolve in a capricious manner,

and bend from the light to the dark; their hooked extremities, even

whilst immature, crawl into crevices, and, when mature, seize any thin

projecting point; in either case they develop adhesive discs, and these

have the power of enveloping the finest fibres.

In the allied Eccremocarpus the internodes, petioles, and much-branched

tendrils all spontaneously revolve together. The tendrils do not as a

whole turn from the light; but their bluntly-hooked extremities arrange

themselves neatly on any surface with which they come into contact,

apparently so as to avoid the light. They act best when each branch

seizes a few thin stems, like the culms of a grass, which they afterwards

draw together into a solid bundle by the spiral contraction of all the

branches. In Cobæa the finely-branched tendrils alone revolve; the

branches terminate in sharp, hard, double, little hooks, with both points

directed to the same side; and these turn by well-adapted movements to

any object with which they come into contact. The tips of the branches

also crawl into dark crevices or holes. The tendrils and internodes of

Ampelopsis have little or no power of revolving; the tendrils are but

little sensitive to contact; their hooked extremities cannot seize thin

objects; they will not even clasp a stick, unless in extreme need of a

support; but they turn from the light to the dark, and, spreading out

their branches in contact with any nearly flat surface, develop discs.

These adhere by the secretion of some cement to a wall, or even to a

polished surface; and this is more than the discs of the _Bignonia

capreolata_ can effect.

The rapid development of these adherent discs is one of the most

remarkable peculiarities possessed by any tendrils. We have seen that

such discs are formed by two species of Bignonia, by Ampelopsis, and,

according to Naudin, {179} by the Cucurbitaceous genus _Peponopsis

adhærens_. In Anguria the lower surface of the tendril, after it has

wound round a stick, forms a coarsely cellular layer, which closely fits

the wood, but is not adherent; whilst in Hanburya a similar layer is

adherent. The growth of these cellular out-growths depends, (except in

the case of the _Haplolophium_ and of one species of Ampelopsis,) on the

stimulus from contact. It is a singular fact that three families, so

widely distinct as the Bignoniaceæ, Vitaceæ, and Cucurbitaceæ, should

possess species with tendrils having this remarkable power.

* * * * *

Sachs attributes all the movements of tendrils to rapid growth on the

side opposite to that which becomes concave. These movements consist of

revolving nutation, the bending to and from the light, and in opposition

to gravity, those caused by a touch, and spiral contraction. It is rash

to differ from so great an authority, but I cannot believe that one at

least of these movements—curvature from a touch—is thus caused. {180} In

the first place it may be remarked that the movement of nutation differs

from that due to a touch, in so far that in some cases the two powers are

acquired by the same tendril at different periods of growth; and the

sensitive part of the tendril does not seem capable of nutation. One of

my chief reasons for doubting whether the curvature from a touch is the

result of growth, is the extraordinary rapidity of the movement. I have

seen the extremity of a tendril of _Passiflora gracilis_, after being

touched, distinctly bent in 25 seconds, and often in 30 seconds; and so

it is with the thicker tendril of Sicyos. It appears hardly credible

that their outer surfaces could have actually grown in length, which

implies a permanent modification of structure, in so short a time. The

growth, moreover, on this view must be considerable, for if the touch has

been at all rough the extremity is coiled in two or three minutes into a

spire of several turns.

When the extreme tip of the tendril of Echinocystis caught hold of a

smooth stick, it coiled itself in a few hours (as described at p. 132)

twice or thrice round the stick, apparently by an undulatory movement.

At first I attributed this movement to the growth of the outside; black

marks were therefore made, and the interspaces measured, but I could not

thus detect any increase in length. Hence it seems probable in this case

and in others, that the curvature of the tendril from a touch depends on

the contraction of the cells along the concave side. Sachs himself

admits {181} that “if the growth which takes place in the entire tendril

at the time of contact with a support is small, a considerable

acceleration occurs on the convex surface, but in general there is no

elongation on the concave surface, or there may even be a _contraction_;

in the case of a tendril of Cucurbita this contraction amounted to nearly

one-third of the original length.” In a subsequent passage Sachs seems

to feel some difficulty in accounting for this kind of contraction. It

must not however be supposed from the foregoing remarks that I entertain

any doubt, after reading De Vries’ observations, about the outer and

stretched surfaces of attached tendrils afterwards increasing in length

by growth. Such increase seems to me quite compatible with the first

movement being independent of growth. Why a delicate touch should cause

one side of a tendril to contract we know as little as why, on the view

held by Sachs, it should lead to extraordinarily rapid growth of the

opposite side. The chief or sole reason for the belief that the

curvature of a tendril when touched is due to rapid growth, seems to be

that tendrils lose their sensitiveness and power of movement after they

have grown to their full length; but this fact is intelligible, if we

bear in mind that all the functions of a tendril are adapted to drag up

the terminal growing shoot towards the light. Of what use would it be,

if an old and full-grown tendril, arising from the lower part of a shoot,

were to retain its power of clasping a support? This would be of no use;

and we have seen with tendrils so many instances of close adaptation and

of the economy of means, that we may feel assured that they would acquire

irritability and the power of clasping a support at the proper

age—namely, youth—and would not uselessly retain such power beyond the

proper age.