TENDRIL-BEARERS.
Nature of tendrils—BIGNONIACEÆ, various species of, and their different
modes of climbing—Tendrils which avoid the light and creep into
crevices—Development of adhesive discs—Excellent adaptations for seizing
different kinds of supports.—POLEMONIACEÆ—_Cobæa scandens_ much branched
and hooked tendrils, their manner of
action—LEGUMINOSÆ—COMPOSITÆ—SMILACEÆ—_Smilax aspera_, its inefficient
tendrils—FUMARIACEÆ—_Corydalis claviculata_, its state intermediate
between that of a leaf-climber and a tendril-bearer.
BY tendrils I mean filamentary organs, sensitive to contact and used
exclusively for climbing. By this definition, spines, hooks and
rootlets, all of which are used for climbing, are excluded. True
tendrils are formed by the modification of leaves with their petioles, of
flower-peduncles, branches, {84} and perhaps stipules. Mohl, who
includes under the name of tendrils various organs having a similar
external appearance, classes them according to their homological nature,
as being modified leaves, flower-peduncles, &c. This would be an
excellent scheme; but I observe that botanists are by no means unanimous
on the homological nature of certain tendrils. Consequently I will
describe tendril-bearing plants by natural families, following Lindley’s
classification; and this will in most cases keep those of the same nature
together. The species to be described belong to ten families, and will
be given in the following order:—_Bignoniaceæ_, _Polemoniaceæ_,
_Leguminosæ_, _Compositæ_, _Smilaceæ_, _Fumariaceæ_, _Cucurbitaceæ_,
_Vitaceæ_, _Sapindaceæ_, _Passifloraceæ_. {85}
BIGNONIACEÆ.—This family contains many tendril-bearers, some twiners, and
some root-climbers. The tendrils always consist of modified leaves.
Nine species of _Bignonia_, selected by hazard, are here described, in
order to show what diversity of structure and action there may be within
the same genus, and to show what remarkable powers some tendrils possess.
The species, taken together, afford connecting links between twiners,
leaf-climbers, tendril-bearers, and root-climbers.
[Picture: Fig. 5. Bignonia. Unnamed species from Kew]
_Bignonia_ (an unnamed species from Kew, closely allied to _B. unguis_,
but with smaller and rather broader leaves).—A young shoot from a
cut-down plant made three revolutions against the sun, at an average rate
of 2 hrs. 6 m. The stem is thin and flexible; it twined round a slender
vertical stick, ascending from left to right, as perfectly and as
regularly as any true twining-plant. When thus ascending, it makes no
use of its tendrils or petioles; but when it twined round a rather thick
stick, and its petioles were brought into contact with it, these curved
round the stick, showing that they have some degree of irritability. The
petioles also exhibit a slight degree of spontaneous movement; for in one
case they certainly described minute, irregular, vertical ellipses. The
tendrils apparently curve themselves spontaneously to the same side with
the petioles; but from various causes, it was difficult to observe the
movement of either the tendrils or petioles, in this and the two
following species. The tendrils are so closely similar in all respects
to those of _B. unguis_, that one description will suffice.
_Bignonia unguis_.—The young shoots revolve, but less regularly and less
quickly than those of the last species. The stem twines imperfectly
round a vertical stick, sometimes reversing its direction, in the same
manner as described in so many leaf-climbers; and this plant though
possessing tendrils, climbs to a certain extent like a leaf-climber.
Each leaf consists of a petiole bearing a pair of leaflets, and
terminates in a tendril, which is formed by the modification of three
leaflets, and closely resembles that above figured (fig. 5). But it is a
little larger, and in a young plant was about half an inch in length. It
is curiously like the leg and foot of a small bird, with the hind toe cut
off. The straight leg or tarsus is longer than the three toes, which are
of equal length, and diverging, lie in the same plane. The toes
terminate in sharp, hard claws, much curved downwards, like those on a
bird’s foot. The petiole of the leaf is sensitive to contact; even a
small loop of thread suspended for two days caused it to bend upwards;
but the sub-petioles of the two lateral leaflets are not sensitive. The
whole tendril, namely, the tarsus and the three toes, are likewise
sensitive to contact, especially on their under surfaces. When a shoot
grows in the midst of thin branches, the tendrils are soon brought by the
revolving movement of the internodes into contact with them; and then one
toe of the tendril or more, commonly all three, bend, and after several
hours seize fast hold of the twigs, like a bird when perched. If the
tarsus of the tendril comes into contact with a twig, it goes on slowly
bending, until the whole foot is carried quite round, and the toes pass
on each side of the tarsus and seize it. In like manner, if the petiole
comes into contact with a twig, it bends round, carrying the tendril,
which then seizes its own petiole or that of the opposite leaf. The
petioles move spontaneously, and thus, when a shoot attempts to twine
round an upright stick, those on both sides after a time come into
contact with it, and are excited to bend. Ultimately the two petioles
clasp the stick in opposite directions, and the foot-like tendrils,
seizing on each other or on their own petioles, fasten the stem to the
support with surprising security. The tendrils are thus brought into
action, if the stem twines round a thin vertical stick; and in this
respect the present species differs from the last. Both species use
their tendrils in the same manner when passing through a thicket. This
plant is one of the most efficient climbers which I have observed; and it
probably could ascend a polished stem incessantly tossed by heavy storms.
To show how important vigorous health is for the action of all the parts,
I may mention that when I first examined a plant which was growing
moderately well, though not vigorously, I concluded that the tendrils
acted only like the hooks on a bramble, and that it was the most feeble
and inefficient of all climbers!
_Bignonia Tweedyana_.—This species is closely allied to the last, and
behaves in the same manner; but perhaps twines rather better round a
vertical stick. On the same plant, one branch twined in one direction
and another in an opposite direction. The internodes in one case made
two circles, each in 2 hrs. 33 m. I was enabled to observe the
spontaneous movements of the petioles better in this than in the two
preceding species: one petiole described three small vertical ellipses in
the course of 11 hrs., whilst another moved in an irregular spire. Some
little time after a stem has twined round an upright stick, and is
securely fastened to it by the clasping petioles and tendrils, it emits
aërial roots from the bases of its leaves; and these roots curve partly
round and adhere to the stick. This species of _Bignonia_, therefore,
combines four different methods of climbing generally characteristic of
distinct plants, namely, twining, leaf-climbing, tendril-climbing, and
root-climbing.
In the three foregoing species, when the foot-like tendril has caught an
object, it continues to grow and thicken, and ultimately becomes
wonderfully strong, in the same manner as the petioles of leaf-climbers.
If the tendril catches nothing, it first slowly bends downwards, and then
its power of clasping is lost. Very soon afterwards it disarticulates
itself from the petiole, and drops off like a leaf in autumn. I have
seen this process of disarticulation in no other tendrils, for these,
when they fail to catch an object, merely wither away.
_Bignonia venusta_.—The tendrils differ considerably from those of the
previous species. The lower part, or tarsus, is four times as long as
the three toes; these are of equal length and diverge equally, but do not
lie in the same plane; their tips are bluntly hooked, and the whole
tendril makes an excellent grapnel. The tarsus is sensitive on all
sides; but the three toes are sensitive only on their outer surfaces.
The sensitiveness is not much developed; for a slight rubbing with a twig
did not cause the tarsus or the toes to become curved until an hour had
elapsed, and then only in a slight degree. Subsequently they
straightened themselves. Both the tarsus and toes can seize well hold of
sticks. If the stem is secured, the tendrils are seen spontaneously to
sweep large ellipses; the two opposite tendrils moving independently of
one another. I have no doubt, from the analogy of the two following
allied species, that the petioles also move spontaneously; but they are
not irritable like those of _B. unguis_ and _B. Tweedyana_. The young
internodes sweep large circles, one being completed in 2 hrs. 15 m., and
a second in 2 hrs. 55 m. By these combined movements of the internodes,
petioles, and grapnel-like tendrils, the latter are soon brought into
contact with surrounding objects. When a shoot stands near an upright
stick, it twines regularly and spirally round it. As it ascends, it
seizes the stick with one of its tendrils, and, if the stick be thin, the
right—and left-hand tendrils are alternately used. This alternation
follows from the stem necessarily taking one twist round its own axis for
each completed circle.
The tendrils contract spirally a short time after catching any object;
those which catch nothing merely bend slowly downwards. But the whole
subject of the spiral contraction of tendrils will be discussed after all
the tendril-bearing species have been described.
_Bignonia littoralis_.—The young internodes revolve in large ellipses.
An internode bearing immature tendrils made two revolutions, each in 3
hrs. 50 m.; but when grown older with the tendrils mature, it made two
ellipses, each at the rate of 2 hrs. 44 m. This species, unlike the
preceding, is incapable of twining round a stick: this does not appear to
be due to any want of flexibility in the internodes or to the action of
the tendrils, and certainly not to any want of the revolving power; nor
can I account for the fact. Nevertheless the plant readily ascends a
thin upright stick by seizing a point above with its two opposite
tendrils, which then contract spirally. If the tendrils seize nothing,
they do not become spiral.
The species last described, ascended a vertical stick by twining spirally
and by seizing it alternately with its opposite tendrils, like a sailor
pulling himself up a rope, hand over hand; the present species pulls
itself up, like a sailor seizing with both hands together a rope above
his head.
The tendrils are similar in structure to those of the last species. They
continue growing for some time, even after they have clasped an object.
When fully grown, though borne by a young plant, they are 9 inches in
length. The three divergent toes are shorter relatively to the tarsus
than in the former species; they are blunt at their tips and but slightly
hooked; they are not quite equal in length, the middle one being rather
longer than the others. Their outer surfaces are highly sensitive; for
when lightly rubbed with a twig, they became perceptibly curved in 4 m.
and greatly curved in 7 m. In 7 hrs. they became straight again and were
ready to re-act. The tarsus, for the space of one inch close to the
toes, is sensitive, but in a rather less degree than the toes; for the
latter after a slight rubbing, became curved in about half the time.
Even the middle part of the tarsus is sensitive to prolonged contact, as
soon as the tendril has arrived at maturity. After it has grown old, the
sensitiveness is confined to the toes, and these are only able to curl
very slowly round a stick. A tendril is perfectly ready to act, as soon
as the three toes have diverged, and at this period their outer surfaces
first become irritable. The irritability spreads but little from one
part when excited to another: thus, when a stick was caught by the part
immediately beneath the three toes, these seldom clasped it, but remained
sticking straight out.
The tendrils revolve spontaneously. The movement begins before the
tendril is converted into a three-pronged grapnel by the divergence of
the toes, and before any part has become sensitive; so that the revolving
movement is useless at this early period. The movement is, also, now
slow, two ellipses being completed conjointly in 24 hrs. 18 m. A mature
tendril made an ellipse in 6 hrs.; so that it moved much more slowly than
the internodes. The ellipses which were swept, both in a vertical and
horizontal plane, were of large size. The petioles are not in the least
sensitive, but revolve like the tendrils. We thus see that the young
internodes, the petioles, and the tendrils all continue revolving
together, but at different rates. The movements of the tendrils which
rise opposite one another are quite independent. Hence, when the whole
shoot is allowed freely to revolve, nothing can be more intricate than
the course followed by the extremity of each tendril. A wide space is
thus irregularly searched for some object to be grasped.
One other curious point remains to be mentioned. In the course of a few
days after the toes have closely clasped a stick, their blunt extremities
become developed, though not invariably, into irregular disc-like balls
which have the power of adhering firmly to the wood. As similar cellular
outgrowths will be fully described under _B. capreolata_, I will here say
nothing more about them.
_Bignonia æquinoctialis_, var. _Chamberlaynii_.—The internodes, the
elongated non-sensitive petioles, and the tendrils all revolve. The stem
does not twine, but ascends a vertical stick in the same manner as the
last species. The tendrils also resemble those of the last species, but
are shorter; the three toes are more unequal in length, the two outer
ones being about one-third shorter and rather thinner than the middle
toe; but they vary in this respect. They terminate in small hard points;
and what is important, cellular adhesive discs are not developed. The
reduced size of two of the toes as well as their lessened sensitiveness,
seem to indicate a tendency to abortion; and on one of my plants the
first-formed tendrils were sometimes simple, that is, were not divided
into three toes. We are thus naturally led to the three following
species with undivided tendrils:—
_Bignonia speciosa_.—The young shoots revolve irregularly, making narrow
ellipses, spires or circles, at rates varying from 3 hrs. 30 m. to 4 hrs.
40 m.; but they show no tendency to twine. Whilst the plant is young and
does not require a support, tendrils are not developed. Those borne by a
moderately young plant were five inches in length. They revolve
spontaneously, as do the short and non-sensitive petioles. When rubbed,
they slowly bend to the rubbed side and subsequently straighten
themselves; but they are not highly sensitive. There is something
strange in their behaviour: I repeatedly placed close to them, thick and
thin, rough and smooth sticks and posts, as well as string suspended
vertically, but none of these objects were well seized. After clasping
an upright stick, they repeatedly loosed it again, and often would not
seize it at all, or their extremities did not coil closely round. I have
observed hundreds of tendrils belonging to various Cucurbitaceous,
Passifloraceous, and Leguminous plants, and never saw one behave in this
manner. When, however, my plant had grown to a height of eight or nine
feet, the tendrils acted much better. They now seized a thin, upright
stick horizontally, that is, at a point on their own level, and not some
way up the stick as in the case of all the previous species.
Nevertheless, the non-twining stem was enabled by this means to ascend
the stick.
The extremity of the tendril is almost straight and sharp. The whole
terminal portion exhibits a singular habit, which in an animal would be
called an instinct; for it continually searches for any little crevice or
hole into which to insert itself. I had two young plants; and, after
having observed this habit, I placed near them posts, which had been
bored by beetles, or had become fissured by drying. The tendrils, by
their own movement and by that of the internodes, slowly travelled over
the surface of the wood, and when the apex came to a hole or fissure it
inserted itself; in order to effect this the extremity for a length of
half or quarter of an inch, would often bend itself at right angles to
the basal part. I have watched this process between twenty and thirty
times. The same tendril would frequently withdraw from one hole and
insert its point into a second hole. I have also seen a tendril keep its
point, in one case for 20 hrs. and in another for 36 hrs., in a minute
hole, and then withdraw it. Whilst the point is thus temporarily
inserted, the opposite tendril goes on revolving.
The whole length of a tendril often fits itself closely to any surface of
wood with which it has come into contact; and I have observed one bent at
right angles, from having entered a wide and deep fissure, with its apex
abruptly re-bent and inserted into a minute lateral hole. After a
tendril has clasped a stick, it contracts spirally; if it remains
unattached it hangs straight downwards. If it has merely adapted itself
to the inequalities of a thick post, though it has clasped nothing, or if
it has inserted its apex into some little fissure, this stimulus suffices
to induce spiral contraction; but the contraction always draws the
tendril away from the post. So that in every case these movements, which
seem so nicely adapted for some purpose, were useless. On one occasion,
however, the tip became permanently jammed into a narrow fissure. I
fully expected, from the analogy of _B. capreolata_ and _B. littoralis_,
that the tips would have been developed into adhesive discs; but I could
never detect even a trace of this process. There is therefore at present
something unintelligible about the habits of this plant.
_Bignonia picta_.—This species closely resembles the last in the
structure and movements of its tendrils. I also casually examined a fine
growing plant of the allied _B. Lindleyi_, and this apparently behaved in
all respects in the same manner.
_Bignonia capreolata_.—We now come to a species having tendrils of a
different type; but first for the internodes. A young shoot made three
large revolutions, following the sun, at an average rate of 2 hrs. 23 m.
The stem is thin and flexible, and I have seen one make four regular
spiral turns round a thin upright stick, ascending of course from right
to left, and therefore in a reversed direction compared with the before
described species. Afterwards, from the interference of the tendrils, it
ascended either straight up the stick or in an irregular spire. The
tendrils are in some respects highly remarkable. In a young plant they
were about 2½ inches in length and much branched, the five chief branches
apparently representing two pairs of leaflets and a terminal one. Each
branch is, however, bifid or more commonly trifid towards the extremity,
with the points blunt yet distinctly hooked. A tendril bends to any side
which is lightly rubbed, and subsequently becomes straight again; but a
loop of thread weighing ¼th of a grain produced no effect. On two
occasions the terminal branches became slightly curved in 10 m. after
they had touched a stick; and in 30 m. the tips were curled quite round
it. The basal part is less sensitive. The tendrils revolved in an
apparently capricious manner, sometimes very slightly or not at all; at
other times they described large regular ellipses. I could detect no
spontaneous movement in the petioles of the leaves.
Whilst the tendrils are revolving more or less regularly, another
remarkable movement takes place, namely, a slow inclination from the
light towards the darkest side of the house. I repeatedly changed the
position of my plants, and some little time after the revolving movement
had ceased, the successively formed tendrils always ended by pointing to
the darkest side. When I placed a thick post near a tendril, between it
and the light, the tendril pointed in that direction. In two instances a
pair of leaves stood so that one of the two tendrils was directed towards
the light and the other to the darkest side of the house; the latter did
not move, but the opposite one bent itself first upwards and then right
over its fellow, so that the two became parallel, one above the other,
both pointing to the dark: I then turned the plant half round; and the
tendril which had turned over recovered its original position, and the
opposite one which had not before moved, now turned over to the dark
side. Lastly, on another plant, three pairs of tendrils were produced at
the same time by three shoots, and all happened to be differently
directed: I placed the pot in a box open only on one side, and obliquely
facing the light; in two days all six tendrils pointed with unerring
truth to the darkest corner of the box, though to do this each had to
bend in a different manner. Six wind-vanes could not have more truly
shown the direction of the wind, than did these branched tendrils the
course of the stream of light which entered the box. I left these
tendrils undisturbed for above 24 hrs., and then turned the pot half
round; but they had now lost their power of movement, and could not any
longer avoid the light.
When a tendril has not succeeded in clasping a support, either through
its own revolving movement or that of the shoot, or by turning towards
any object which intercepts the light, it bends vertically downwards and
then towards its own stem, which it seizes together with the supporting
stick, if there be one. A little aid is thus given in keeping the stem
secure. If the tendril seizes nothing, it does not contract spirally,
but soon withers away and drops off. If it seizes an object, all the
branches contract spirally.
I have stated that after a tendril has come into contact with a stick, it
bends round it in about half an hour; but I repeatedly observed, as in
the case of _B. speciosa_ and its allies, that it often again loosed the
stick; sometimes seizing and loosing the same stick three or four times.
Knowing that the tendrils avoided the light, I gave them a glass tube
blackened within, and a well-blackened zinc plate: the branches curled
round the tube and abruptly bent themselves round the edges of the zinc
plate; but they soon recoiled from these objects with what I can only
call disgust, and straightened themselves. I then placed a post with
extremely rugged bark close to a pair of tendrils; twice they touched it
for an hour or two, and twice they withdrew; at last one of the hooked
extremities curled round and firmly seized an excessively minute
projecting point of bark, and then the other branches spread themselves
out, following with accuracy every inequality of the surface. I
afterwards placed near the plant a post without bark but much fissured,
and the points of the tendrils crawled into all the crevices in a
beautiful manner. To my surprise, I observed that the tips of the
immature tendrils, with the branches not yet fully separated, likewise
crawled just like roots into the minutest crevices. In two or three days
after the tips had thus crawled into the crevices, or after their hooked
ends had seized minute points, the final process, now to be described,
commenced.
This process I discovered by having accidentally left a piece of wool
near a tendril; and this led me to bind a quantity of flax, moss, and
wool loosely round sticks, and to place them near tendrils. The wool
must not be dyed, for these tendrils are excessively sensitive to some
poisons. The hooked points soon caught hold of the fibres, even loosely
floating fibres, and now there was no recoiling; on the contrary, the
excitement caused the hooks to penetrate the fibrous mass and to curl
inwards, so that each hook caught firmly one or two fibres, or a small
bundle of them. The tips and the inner surfaces of the hooks now began
to swell, and in two or three days were visibly enlarged. After a few
more days the hooks were converted into whitish, irregular balls, rather
above the 0.05th of an inch (1.27 mm.) in diameter, formed of coarse
cellular tissue, which sometimes wholly enveloped and concealed the hooks
themselves. The surfaces of these balls secrete some viscid resinous
matter, to which the fibres of the flax, &c., adhere. When a fibre has
become fastened to the surface, the cellular tissue does not grow
directly beneath it, but continues to grow closely on each side; so that
when several adjoining fibres, though excessively thin, were caught, so
many crests of cellular matter, each not as thick as a human hair, grew
up between them, and these, arching over on both sides, adhered firmly
together. As the whole surface of the ball continues to grow, fresh
fibres adhere and are afterwards enveloped; so that I have seen a little
ball with between fifty and sixty fibres of flax crossing it at various
angles and all embedded more or less deeply. Every gradation in the
process could be followed—some fibres merely sticking to the surface,
others lying in more or less deep furrows, or deeply embedded, or passing
through the very centre of the cellular ball. The embedded fibres are so
closely clasped that they cannot be withdrawn. The outgrowing tissue has
so strong a tendency to unite, that two balls produced by distinct
tendrils sometimes unite and grow into a single one.
On one occasion, when a tendril had curled round a stick, half an inch in
diameter, an adhesive disc was formed; but this does not generally occur
in the case of smooth sticks or posts. If, however, the tip catches a
minute projecting point, the other branches form discs, especially if
they find crevices to crawl into. The tendrils failed to attach
themselves to a brick wall.
I infer from the adherence of the fibres to the discs or balls, that
these secrete some resinous adhesive matter; and more especially from
such fibres becoming loose if immersed in sulphuric ether. This fluid
likewise removes small, brown, glistening points which can generally be
seen on the surfaces of the older discs. If the hooked extremities of
the tendrils do not touch anything, discs, as far as I have seen, are
never formed; {102} but temporary contact during a moderate time suffices
to cause their development. I have seen eight discs formed on the same
tendril. After their development the tendrils contract spirally, and
become woody and very strong. A tendril in this state supported nearly
seven ounces, and would apparently have supported a considerably greater
weight, had not the fibres of flax to which the discs were attached
yielded.
From the facts now given, we may infer that though the tendrils of this
Bignonia can occasionally adhere to smooth cylindrical sticks and often
to rugged bark, yet that they are specially adapted to climb trees
clothed with lichens, mosses, or other such productions; and I hear from
Professor Asa Gray that the _Polypodium incanum_ abounds on the
forest-trees in the districts of North America where this species of
Bignonia grows. Finally, I may remark how singular a fact it is that a
leaf should be metamorphosed into a branched organ which turns from the
light, and which can by its extremities either crawl like roots into
crevices, or seize hold of minute projecting points, these extremities
afterwards forming cellular outgrowths which secrete an adhesive cement,
and then envelop by their continued growth the finest fibres.
_Eccremocarpus scaber_ (_Bignoniaceæ_).—Plants, though growing pretty
well in my green-house, showed no spontaneous movements in their shoots
or tendrils; but when removed to the hot-house, the young internodes
revolved at rates varying from 3 hrs. 15 m. to 1 hr. 13 m. One large
circle was swept at this latter unusually quick rate; but generally the
circles or ellipses were small, and sometimes the course pursued was
quite irregular. An internode, after making several revolutions,
sometimes stood still for 12 hrs. or 18 hrs., and then recommenced
revolving. Such strongly marked interruptions in the movements of the
internodes I have observed in hardly any other plant.
The leaves bear four leaflets, themselves subdivided, and terminate in
much-branched tendrils. The main petiole of the leaf, whilst young,
moves spontaneously, and follows nearly the same irregular course and at
about the same rate as the internodes. The movement to and from the stem
is the most conspicuous, and I have seen the chord of a curved petiole
which formed an angle of 59° with the stem, in an hour afterwards making
an angle of 106°. The two opposite petioles do not move together, and
one is sometimes so much raised as to stand close to the stem, whilst the
other is not far from horizontal. The basal part of the petiole moves
less than the distal part. The tendrils, besides being carried by the
moving petioles and internodes, themselves move spontaneously; and the
opposite tendrils occasionally move in opposite directions. By these
combined movements of the young internodes, petioles, and tendrils, a
considerable space is swept in search of a support.
In young plants the tendrils are about three inches in length: they bear
two lateral and two terminal branches; and each branch bifurcates twice,
with the tips terminating in blunt double hooks, having both points
directed to the same side. All the branches are sensitive on all sides;
and after being lightly rubbed, or after coming into contact with a
stick, bend in about 10 m. One which had become curved in 10 m. after a
light rub, continued bending for between 3 hrs. and 4 hrs., and became
straight again in 8 hrs. or 9 hrs. Tendrils, which have caught nothing,
ultimately contract into an irregular spire, as they likewise do, only
much more quickly, after clasping a support. In both cases the main
petiole bearing the leaflets, which is at first straight and inclined a
little upwards, moves downwards, with the middle part bent abruptly into
a right angle; but this is seen in _E. miniatus_ more plainly than in _E.
scaber_. The tendrils in this genus act in some respects like those of
_Bignonia capreolata_; but the whole does not move from the light, nor do
the hooked tips become enlarged into cellular discs. After the tendrils
have come into contact with a moderately thick cylindrical stick or with
rugged bark, the several branches may be seen slowly to lift themselves
up, change their positions, and again come into contact with the
supporting surface. The object of these movements is to bring the
double-hooks at the extremities of the branches, which naturally face in
all directions, into contact with the wood. I have watched a tendril,
half of which had bent itself at right angles round the sharp corner of a
square post, neatly bring every single hook into contact with both
rectangular surfaces. The appearance suggested the belief, that though
the whole tendril is not sensitive to light, yet that the tips are so,
and that they turn and twist themselves towards any dark surface.
Ultimately the branches arrange themselves very neatly to all the
irregularities of the most rugged bark, so that they resemble in their
irregular course a river with its branches, as engraved on a map. But
when a tendril has wound round a rather thick stick, the subsequent
spiral contraction generally draws it away and spoils the neat
arrangement. So it is, but not in quite so marked a manner, when a
tendril has spread itself over a large, nearly flat surface of rugged
bark. We may therefore conclude that these tendrils are not perfectly
adapted to seize moderately thick sticks or rugged bark. If a thin stick
or twig is placed near a tendril, the terminal branches wind quite round
it, and then seize their own lower branches or the main stem. The stick
is thus firmly, but not neatly, grasped. What the tendrils are really
adapted for, appears to be such objects as the thin culms of certain
grasses, or the long flexible bristles of a brush, or thin rigid leaves
such as those of the Asparagus, all of which they seize in an admirable
manner. This is due to the extremities of the branches close to the
little hooks being extremely sensitive to a touch from the thinnest
object, which they consequently curl round and clasp. When a small
brush, for instance, was placed near a tendril, the tips of each
sub-branch seized one, two, or three of the bristles; and then the spiral
contraction of the several branches brought all these little parcels
close together, so that thirty or forty bristles were drawn into a single
bundle, which afforded an excellent support.
POLEMONIACEÆ.—_Cobæa scandens_.—This is an excellently constructed
climber. The tendrils on a fine plant were eleven inches long, with the
petiole bearing two pairs of leaflets, only two and a half inches in
length. They revolve more rapidly and vigorously than those of any other
tendril-bearer observed by me, with the exception of one kind of
Passiflora. Three large, nearly circular sweeps, directed against the
sun were completed, each in 1 hr. 15 m.; and two other circles in 1 hr.
20 m. and 1 hr. 23 m. Sometimes a tendril travels in a much inclined
position, and sometimes nearly upright. The lower part moves but little
and the petiole not at all; nor do the internodes revolve; so that here
we have the tendril alone moving. On the other hand, with most of the
species of _Bignonia_ and the _Eccremocarpus_, the internodes, tendrils,
and petioles all revolved. The long, straight, tapering main stem of the
tendril of the _Cobæa_ bears alternate branches; and each branch is
several times divided, with the finer branches as thin as very thin
bristles and extremely flexible, so that they are blown about by a breath
of air; yet they are strong and highly elastic. The extremity of each
branch is a little flattened, and terminates in a minute double (though
sometimes single) hook, formed of a hard, translucent, woody substance,
and as sharp as the finest needle. On a tendril which was eleven inches
long I counted ninety-four of these beautifully constructed little hooks.
They readily catch soft wood, or gloves, or the skin of the naked hand.
With the exception of these hardened hooks, and of the basal part of the
central stem, every part of every branchlet is highly sensitive on all
sides to a slight touch, and bends in a few minutes towards the touched
side. By lightly rubbing several sub-branches on opposite sides, the
whole tendril rapidly assumed an extraordinarily crooked shape. These
movements from contact do not interfere with the ordinary revolving
movement. The branches, after becoming greatly curved from being
touched, straighten themselves at a quicker rate than in almost any other
tendril seen by me, namely, in between half an hour and an hour. After
the tendril has caught any object, spiral contraction likewise begins
after an unusually short interval of time, namely, in about twelve hours.
Before the tendril is mature, the terminal branchlets cohere, and the
hooks are curled closely inwards. At this period no part is sensitive to
a touch; but as soon as the branches diverge and the hooks stand out,
full sensitiveness is acquired. It is a singular circumstance that
immature tendrils revolve at their full velocity before they become
sensitive, but in a useless manner, as in this state they can catch
nothing. This want of perfect co-adaptation, though only for a short
time, between the structure and the functions of a climbing-plant is a
rare event. A tendril, as soon as it is ready to act, stands, together
with the supporting petiole, vertically upwards. The leaflets borne by
the petiole are at this time quite small, and the extremity of the
growing stem is bent to one side so as to be out of the way of the
revolving tendril, which sweeps large circles directly over head. The
tendrils thus revolve in a position well adapted for catching objects
standing above; and by this means the ascent of the plant is favoured.
If no object is caught, the leaf with its tendril bends downwards and
ultimately assumes a horizontal position. An open space is thus left for
the next succeeding and younger tendril to stand vertically upwards and
to revolve freely. As soon as an old tendril bends downwards, it loses
all power of movement, and contracts spirally into an entangled mass.
Although the tendrils revolve with unusual rapidity, the movement lasts
for only a short time. In a plant placed in the hot-house and growing
vigorously, a tendril revolved for not longer than 36 hours, counting
from the period when it first became sensitive; but during this period it
probably made at least 27 revolutions.
When a revolving tendril strikes against a stick, the branches quickly
bend round and clasp it. The little hooks here play an important part,
as they prevent the branches from being dragged away by the rapid
revolving movement, before they have had time to clasp the stick
securely. This is especially the case when only the extremity of a
branch has caught hold of a support. As soon as a tendril has bent a
smooth stick or a thick rugged post, or has come into contact with planed
wood (for it can adhere temporarily even to so smooth a surface as this),
the same peculiar movements may be observed as those described under
_Bignonia capreolata_ and _Eccremocarpus_. The branches repeatedly lift
themselves up and down; those which have their hooks already directed
downwards remaining in this position and securing the tendril, whilst the
others twist about until they succeed in arranging themselves in
conformity with every irregularity of the surface, and in bringing their
hooks into contact with the wood. The use of the hooks was well shown by
giving the tendrils tubes and slips of glass to catch; for these, though
temporarily seized, were invariably lost, either during the
re-arrangement of the branches or ultimately when spiral contraction
ensued.
The perfect manner in which the branches arranged themselves, creeping
like rootlets over every inequality of the surface and into any deep
crevice, is a pretty sight; for it is perhaps more effectually performed
by this than by any other species. The action is certainly more
conspicuous, as the upper surfaces of the main stem, as well as of every
branch to the extreme hooks, are angular and green, whilst the lower
surfaces are rounded and purple. I was led to infer, as in former cases,
that a less amount of light guided these movements of the branches of the
tendrils. I made many trials with black and white cards and glass tubes
to prove it, but failed from various causes; yet these trials
countenanced the belief. As a tendril consists of a leaf split into
numerous segments, there is nothing surprising in all the segments
turning their upper surfaces towards the light, as soon as the tendril is
caught and the revolving movement is arrested. But this will not account
for the whole movement, for the segments actually bend or curve to the
dark side besides turning round on their axes so that their upper
surfaces may face the light.
When the _Cobæa_ grows in the open air, the wind must aid the extremely
flexible tendrils in seizing a support, for I found that a mere breath
sufficed to cause the extreme branches to catch hold by their hooks of
twigs, which they could not have reached by the revolving movement. It
might have been thought that a tendril, thus hooked by the extremity of a
single branch, could not have fairly grasped its support. But several
times I watched cases like the following: tendril caught a thin stick by
the hooks of one of its two extreme branches; though thus held by the
tip, it still tried to revolve, bowing itself to all sides, and by this
movement the other extreme branch soon caught the stick. The first
branch then loosed itself, and, arranging its hooks, again caught hold.
After a time, from the continued movement of the tendril, the hooks of a
third branch caught hold. No other branches, as the tendril then stood,
could possibly have touched the stick. But before long the upper part of
the main stem began to contract into an open spire. It thus dragged the
shoot which bore the tendril towards the stick; and as the tendril
continually tried to revolve, a fourth branch was brought into contact.
And lastly, from the spiral contraction travelling down both the main
stem and the branches, all of them, one after another, were ultimately
brought into contact with the stick. They then wound themselves round it
and round one another, until the whole tendril was tied together in an
inextricable knot. The tendrils, though at first quite flexible, after
having clasped a support for a time, become more rigid and stronger than
they were at first. Thus the plant is secured to its support in a
perfect manner.
LEGUMINOSÆ.—_Pisum sativum_.—The common pea was the subject of a valuable
memoir by Dutrochet, {111} who discovered that the internodes and
tendrils revolve in ellipses. The ellipses are generally very narrow,
but sometimes approach to circles. I several times observed that the
longer axis slowly changed its direction, which is of importance, as the
tendril thus sweeps a wider space. Owing to this change of direction,
and likewise to the movement of the stem towards the light, the
successive irregular ellipses generally form an irregular spire. I have
thought it worth while to annex a tracing of the course pursued by the
upper internode (the movement of the tendril being neglected) of a young
plant from 8.40 A.M. to 9.15 P.M. The course was traced on a
hemispherical glass placed over the plant, and the dots with figures give
the hours of observation; each dot being joined by a straight line. No
doubt all the lines would have been curvilinear if the course had been
observed at much shorter intervals. The extremity of the petiole, from
which the young tendril arose, was two inches from the glass, so that if
a pencil two inches in length could have been affixed to the petiole, it
would have traced the annexed figure on the under side of the glass; but
it must be remembered that the figure is reduced by one-half. Neglecting
the first great sweep towards the light from the figure 1 to 2, the end
of the petiole swept a space 4 inches across in one direction, and 3
inches in another. As a full-grown tendril is considerably above two
inches in length, and as the tendril itself bends and revolves in harmony
with the internode, a considerably wider space is swept than is here
represented on a reduced scale. Dutrochet observed the completion of an
ellipse in 1 hr. 20 m.; and I saw one completed in 1 hr. 30 m. The
direction followed is variable, either with or against the sun.
[Picture: Fig. 6. Side of room with window] {113}
Dutrochet asserts that the petioles of the leaves spontaneously revolve,
as well as the young internodes and tendrils; but he does not say that he
secured the internodes; when this was done, I could never detect any
movement in the petiole, except to and from the light.
The tendrils, on the other hand, when the internodes and petioles are
secured, describe irregular spires or regular ellipses, exactly like
those made by the internodes. A young tendril, only 1⅛ of an inch in
length, revolved. Dutrochet has shown that when a plant is placed in a
room, so that the light enters laterally, the internodes travel much
quicker to the light than from it: on the other hand, he asserts that the
tendril itself moves from the light towards the dark side of the room.
With due deference to this great observer, I think he was mistaken, owing
to his not having secured the internodes. I took a young plant with
highly sensitive tendrils, and tied the petiole so that the tendril alone
could move; it completed a perfect ellipse in 1 hr. 30 m.; I then turned
the plant partly round, but this made no change in the direction of the
succeeding ellipse. The next day I watched a plant similarly secured
until the tendril (which was highly sensitive) made an ellipse in a line
exactly to and from the light; the movement was so great that the tendril
at the two ends of its elliptical course bent itself a little beneath the
horizon, thus travelling more than 180 degrees; but the curvature was
fully as great towards the light as towards the dark side of the room. I
believe Dutrochet was misled by not having secured the internodes, and by
having observed a plant of which the internodes and tendrils no longer
curved in harmony together, owing to inequality of age.
Dutrochet made no observations on the sensitiveness of the tendrils.
These, whilst young and about an inch in length with the leaflets on the
petiole only partially expanded, are highly sensitive; a single light
touch with a twig on the inferior or concave surface near the tip caused
them to bend quickly, as did occasionally a loop of thread weighing
one-seventh of a grain (9.25 mg.). The upper or convex surface is barely
or not at all sensitive. Tendrils, after bending from a touch,
straighten themselves in about two hours, and are then ready to act
again. As soon as they begin to grow old, the extremities of their two
or three pairs of branches become hooked, and they then appear to form an
excellent grappling instrument; but this is not the case. For at this
period they have generally quite lost their sensitiveness; and when
hooked on to twigs, some were not at all affected, and others required
from 18 hrs. to 24 hrs. before clasping such twigs; nevertheless, they
were able to utilise the last vestige of irritability owing to their
extremities being hooked. Ultimately the lateral branches contract
spirally, but not the middle or main stem.
_Lathyrus aphaca_.—This plant is destitute of leaves, except during a
very early age, these being replaced by tendrils, and the leaves
themselves by large stipules. It might therefore have been expected that
the tendrils would have been highly organized, but this is not so. They
are moderately long, thin, and unbranched, with their tips slightly
curved. Whilst young they are sensitive on all sides, but chiefly on the
concave side of the extremity. They have no spontaneous revolving power,
but are at first inclined upwards at an angle of about 45°, then move
into a horizontal position, and ultimately bend downwards. The young
internodes, on the other hand, revolve in ellipses, and carry with them
the tendrils. Two ellipses were completed, each in nearly 5 hrs.; their
longer axes were directed at about an angle of 45° to the axis of the
previously made ellipse.
_Lathyrus grandiflorus_.—The plants observed were young and not growing
vigorously, yet sufficiently so, I think, for my observations to be
trusted. If so, we have the rare case of neither internodes nor tendrils
revolving. The tendrils of vigorous plants are above 4 inches in length,
and are often twice divided into three branches; the tips are curved and
are sensitive on their concave sides; the lower part of the central stem
is hardly at all sensitive. Hence this plant appears to climb simply by
its tendrils being brought, through the growth of the stem, or more
efficiently by the wind, into contact with surrounding objects, which
they then clasp. I may add that the tendrils, or the internodes, or
both, of _Vicia sativa_ revolve.
COMPOSITÆ.—_Mutisia clematis_.—The immense family of the Compositæ is
well known to include very few climbing plants. We have seen in the
Table in the first chapter that _Mikania scandens_ is a regular twiner,
and F. Müller informs me that in S. Brazil there is another species which
is a leaf-climber. _Mutisia_ is the only genus in the family, as far as
I can learn, which bears tendrils: it is therefore interesting to find
that these, though rather less metamorphosed from their primordial foliar
condition than are most other tendrils, yet display all the ordinary
characteristic movements, both those that are spontaneous and those which
are excited by contact.
The long leaf bears seven or eight alternate leaflets, and terminates in
a tendril which, in a plant of considerable size, was 5 inches in length.
It consists generally of three branches; and these, although much
elongated, evidently represent the petioles and midribs of three
leaflets; for they closely resemble the same parts in an ordinary leaf,
in being rectangular on the upper surface, furrowed, and edged with
green. Moreover, the green edging of the tendrils of young plants
sometimes expands into a narrow lamina or blade. Each branch is curved a
little downwards, and is slightly hooked at the extremity.
A young upper internode revolved, judging from three revolutions, at an
average rate of 1 hr. 38 m.; it swept ellipses with the longer axes
directed at right angles to one another; but the plant, apparently,
cannot twine. The petioles and the tendrils are both in constant
movement. But their movement is slower and much less regularly
elliptical than that of the internodes. They appear to be much affected
by the light, for the whole leaf usually sinks down during the night and
rises during the day, moving, also, during the day in a crooked course to
the west. The tip of the tendril is highly sensitive on the lower
surface; and one which was just touched with a twig became perceptibly
curved in 3 m., and another in 5 m.; the upper surface is not at all
sensitive; the sides are moderately sensitive, so that two branches which
were rubbed on their inner sides converged and crossed each other. The
petiole of the leaf and the lower parts of the tendril, halfway between
the upper leaflet and the lowest branch, are not sensitive. A tendril
after curling from a touch became straight again in about 6 hrs., and was
ready to re-act; but one that had been so roughly rubbed as to have
coiled into a helix did not become perfectly straight until after 13 hrs.
The tendrils retain their sensibility to an unusually late age; for one
borne by a leaf with five or six fully developed leaves above, was still
active. If a tendril catches nothing, after a considerable interval of
time the tips of the branches curl a little inwards; but if it clasps
some object, the whole contracts spirally.
[Picture: Fig. 7. Smilax aspera]
SMILACEÆ.—_Smilax aspera_, var. _maculata_.—Aug. St.-Hilaire {118}
considers that the tendrils, which rise in pairs from the petiole, are
modified lateral leaflets; but Mohl (p. 41) ranks them as modified
stipules. These tendrils are from 1½ to 1¾ inches in length, are thin,
and have slightly curved, pointed extremities. They diverge a little
from each other, and stand at first nearly upright. When lightly rubbed
on either side, they slowly bend to that side, and subsequently become
straight again. The back or convex side when placed in contact with a
stick became just perceptibly curved in 1 hr. 20 m., but did not
completely surround it until 48 hrs. had elapsed; the concave side of
another became considerably curved in 2 hrs. and clasped a stick in 5
hrs. As the pairs of tendrils grow old, one tendril diverges more and
more from the other, and both slowly bend backwards and downwards, so
that after a time they project on the opposite side of the stem to that
from which they arise. They then still retain their sensitiveness, and
can clasp a support placed _behind_ the stem. Owing to this power, the
plant is able to ascend a thin upright stick. Ultimately the two
tendrils belonging to the same petiole, if they do not come into contact
with any object, loosely cross each other behind the stem, as at B, in
fig. 7. This movement of the tendrils towards and round the stem is, to
a certain extent, guided by their avoidance of the light; for when a
plant stood so that one of the two tendrils was compelled in thus slowly
moving to travel towards the light, and the other from the light, the
latter always moved, as I repeatedly observed, more quickly than its
fellow. The tendrils do not contract spirally in any case. Their chance
of finding a support depends on the growth of the plant, on the wind, and
on their own slow backward and downward movement, which, as we have just
seen, is guided, to a certain extent, by the avoidance of the light; for
neither the internodes nor the tendrils have any proper revolving
movement. From this latter circumstance, from the slow movements of the
tendrils after contact (though their sensitiveness is retained for an
unusual length of time), from their simple structure and shortness, this
plant is a less perfect climber than any other tendril-bearing species
observed by me. The plant whilst young and only a few inches in height,
does not produce any tendrils; and considering that it grows to only
about 8 feet in height, that the stem is zigzag and is furnished, as well
as the petioles, with spines, it is surprising that it should be provided
with tendrils, comparatively inefficient though these are. The plant
might have been left, one would have thought, to climb by the aid of its
spines alone, like our brambles. As, however, it belongs to a genus,
some of the species of which are furnished with much longer tendrils, we
may suspect that it possesses these organs solely from being descended
from progenitors more highly organized in this respect.
FUMARIACEÆ.—_Corydalis claviculata_.—According to Mohl (p. 43), the
extremities of the branched stem, as well as the leaves, are converted
into tendrils. In the specimens examined by me all the tendrils were
certainly foliar, and it is hardly credible that the same plant should
produce tendrils of a widely different homological nature. Nevertheless,
from this statement by Mohl, I have ranked this species amongst the
tendril-bearers; if classed exclusively by its foliar tendrils, it would
be doubtful whether it ought not to have been placed amongst the
leaf-climbers, with its allies, _Fumaria_ and _Adlumia_. A large
majority of its so-called tendrils still bear leaflets, though
excessively reduced in size; but some few of them may properly be
designated as tendrils, for they are completely destitute of laminæ or
blades. Consequently, we here behold a plant in an actual state of
transition from a leaf-climber to a tendril-bearer. Whilst the plant is
rather young, only the outer leaves, but when full-grown all the leaves,
have their extremities converted into more or less perfect tendrils. I
have examined specimens from one locality alone, viz. Hampshire; and it
is not improbable that plants growing under different conditions might
have their leaves a little more or less changed into true tendrils.
Whilst the plant is quite young, the first-formed leaves are not modified
in any way, but those next formed have their terminal leaflets reduced in
size, and soon all the leaves assume the structure represented in the
following drawing. This leaf bore nine leaflets; the lower ones being
much subdivided. The terminal portion of the petiole, about 1½ inch in
length (above the leaflet _f_), is thinner and more elongated than the
lower part, and may be considered as the tendril. The leaflets borne by
this part are greatly reduced in size, being, on an average, about the
tenth of an inch in length and very narrow; one small leaflet measured
one-twelfth of an inch in length and one-seventy-fifth in breadth (2.116
mm. and 0.339 mm.), so that it was almost microscopically minute. All
the reduced leaflets have branching nerves, and terminate in little
spines, like those of the fully developed leaflets. Every gradation
could be traced, until we come to branchlets (as _a_ and _d_ in the
figure) which show no vestige of a lamina or blade. Occasionally all the
terminal branchlets of the petiole are in this condition, and we then
have a true tendril.
[Picture: Fig. 8. Corydalis claviculata. Leaf-tendril of natural size]
The several terminal branches of the petiole bearing the much reduced
leaflets (_a_, _b_, _c_, _d_) are highly sensitive, for a loop of thread
weighing only the one-sixteenth of a grain (4.05 mg.) caused them to
become greatly curved in under 4 hrs. When the loop was removed, the
petioles straightened themselves in about the same time. The petiole
(_e_) was rather less sensitive; and in another specimen, in which the
corresponding petiole bore rather larger leaflets, a loop of thread
weighing one-eighth of a grain did not cause curvature until 18 hrs. had
elapsed. Loops of thread weighing one-fourth of a grain, left suspended
on the lower petioles (_f_ to _l_) during several days, produced no
effect. Yet the three petioles _f_, _g_, and _h_ were not quite
insensible, for when left in contact with a stick for a day or two they
slowly curled round it. Thus the sensibility of the petiole gradually
diminishes from the tendril-like extremity to the base. The internodes
of the stem are not at all sensitive, which makes Mohl’s statement that
they are sometimes converted into tendrils the more surprising, not to
say improbable.
The whole leaf, whilst young and sensitive, stands almost vertically
upwards, as we have seen to be the case with many tendrils. It is in
continual movement, and one that I observed swept at an average rate of
about 2 hrs. for each revolution, large, though irregular, ellipses,
which were sometimes narrow, sometimes broad, with their longer axes
directed to different points of the compass. The young internodes,
likewise revolved irregularly in ellipses or spires; so that by these
combined movements a considerable space was swept for a support. If the
terminal and attenuated portion of a petiole fails to seize any object,
it ultimately bends downwards and inwards, and soon loses all
irritability and power of movement. This bending down differs much in
nature from that which occurs with the extremities of the young leaves in
many species of _Clematis_; for these, when thus bent downwards or
hooked, first acquire their full degree of sensitiveness.
_Dicentra thalictrifolia_.—In this allied plant the metamorphosis of the
terminal leaflets is complete, and they are converted into perfect
tendrils. Whilst the plant is young, the tendrils appear like modified
branches, and a distinguished botanist thought that they were of this
nature; but in a full-grown plant there can be no doubt, as I am assured
by Dr. Hooker, that they are modified leaves. When of full size, they
are above 5 inches in length; they bifurcate twice, thrice, or even four
times; their extremities are hooked and blunt. All the branches of the
tendrils are sensitive on all sides, but the basal portion of the main
stem is only slightly so. The terminal branches when lightly rubbed with
a twig became curved in the course of from 30 m. to 42 m., and
straightened themselves in between 10 hrs. and 20 hrs. A loop of thread
weighing one-eighth of a grain plainly caused the thinner branches to
bend, as did occasionally a loop weighing one-sixteenth of a grain; but
this latter weight, though left suspended, was not sufficient to cause a
permanent flexure. The whole leaf with its tendril, as well as the young
upper internodes, revolves vigorously and quickly, though irregularly,
and thus sweeps a wide space. The figure traced on a bell-glass was
either an irregular spire or a zigzag line. The nearest approach to an
ellipse was an elongated figure of 8, with one end a little open, and
this was completed in 1 hr. 53 m. During a period of 6 hrs. 17 m.
another shoot made a complex figure, apparently representing three and a
half ellipses. When the lower part of the petiole bearing the leaflets
was securely fastened, the tendril itself described similar but much
smaller figures.
This species climbs well. The tendrils after clasping a stick become
thicker and more rigid; but the blunt hooks do not turn and adapt
themselves to the supporting surface, as is done in so perfect a manner
by some Bignoniaceæ and Cobæa. The tendrils of young plants, two or
three feet in height, are only half the length of those borne by the same
plant when grown taller, and they do not contract spirally after clasping
a support, but only become slightly flexuous. Full-sized tendrils, on
the other hand, contract spirally, with the exception of the thick basal
portion. Tendrils which have caught nothing simply bend downwards and
inwards, like the extremities of the leaves of the _Corydalis
claviculata_. But in all cases the petiole after a time is angularly and
abruptly bent downwards like that of Eccremocarpus.