The Movements and Habits of Climbing Plants
by
Charles Darwin
Part 2 out of 3
FUMARIACEAE.--Fumaria officinalis.--It could not have been
anticipated that so lowly a plant as this Fumaria should have been a
climber. It climbs by the aid of the main and lateral petioles of
its compound leaves; and even the much-flattened terminal portion of
the petiole can seize a support. I have seen a substance as soft as
a withered blade of grass caught. Petioles which have clasped any
object ultimately become rather thicker and more cylindrical. On
lightly rubbing several petioles with a twig, they became perceptibly
curved in 1 hr. 15 m., and subsequently straightened themselves. A
stick gently placed in the angle between two sub-petioles excited
them to move, and was almost clasped in 9 hrs. A loop of thread,
weighing one-eighth of a grain, caused, after 12 hrs. and before 20
hrs, had elapsed, a considerable curvature; but it was never fairly
clasped by the petiole. The young internodes are in continual
movement, which is considerable in extent, but very irregular; a
zigzag line, or a spire crossing itself; or a figure of 8 being
formed. The course during 12 hrs., when traced on a bell-glass,
apparently represented about four ellipses. The leaves themselves
likewise move spontaneously, the main petioles curving themselves in
accordance with the movements of the internodes; so that when the
latter moved to one side, the petioles moved to the same side, then,
becoming straight, reversed their curvature. The petioles, however,
do not move over a wide space, as could be seen when a shoot was
securely tied to a stick. The leaf in this case followed an
irregular course, like that made by the internodes.
Adlumia cirrhosa.--I raised some plants late in the summer; they
formed very fine leaves, but threw up no central stem. The first-
formed leaves were not sensitive; some of the later ones were so, but
only towards their extremities, which were thus enabled to clasp
sticks. This could be of no service to the plant, as these leaves
rose from the ground; but it showed what the future character of the
plant would have been, had it grown tall enough to climb. The tip of
one of these basal leaves, whilst young, described in 1 hr. 36 m. a
narrow ellipse, open at one end, and exactly three inches in length;
a second ellipse was broader, more irregular, and shorter, viz., only
2.5 inches in length, and was completed in 2 hrs. 2 m. From the
analogy of Fumaria and Corydalis, I have no doubt that the internodes
of Adlumia have the power of revolving.
Corydalis claviculata.--This plant is interesting from being in a
condition so exactly intermediate between a leaf-climber and a
tendril-bearer, that it might have been described under either head;
but, for reasons hereafter assigned, it has been classed amongst
tendril-bearers.
Besides the plants already described, Bignonia unguis and its close
allies, though aided by tendrils, have clasping petioles. According
to Mohl (p. 40), Cocculus Japonicus (one of the Menispermaceae) and a
fern, the Ophioglossum Japonicum (p. 39), climb by their leaf-stalks.
We now come to a small section of plants which climb by means of the
produced midribs or tips of their leaves.
LILIACEAE.--Gloriosa Plantii.--The stem of a half-grown plant
continually moved, generally describing an irregular spire, but
sometimes oval figures with the longer axes directed in different
lines. It either followed the sun, or moved in an opposite course,
and sometimes stood still before reversing its direction. One oval
was completed in 3 hrs. 40 m.; of two horseshoe-shaped figures, one
was completed in 4 hrs. 35 m. and the other in 3 hrs. The shoots, in
their movements, reached points between four and five inches asunder.
The young leaves, when first developed, stand up nearly vertically;
but by the growth of the axis, and by the spontaneous bending down of
the terminal half of the leaf, they soon become much inclined, and
ultimately horizontal. The end of the leaf forms a narrow, ribbon-
like, thickened projection, which at first is nearly straight, but by
the time the leaf gets into an inclined position, the end bends
downwards into a well-formed hook. This hook is now strong and rigid
enough to catch any object, and, when caught, to anchor the plant and
stop the revolving movement. Its inner surface is sensitive, but not
in nearly so high a degree as that of the many before-described
petioles; for a loop of string, weighing 1.64 grain, produced no
effect. When the hook has caught a thin twig or even a rigid fibre,
the point may be perceived in from 1 hr. to 3 hrs. to have curled a
little inwards; and, under favourable circumstances, it curls round
and permanently seizes an object in from 8 hrs. to 10 hrs. The hook
when first formed, before the leaf has bent downwards, is but little
sensitive. If it catches hold of nothing, it remains open and
sensitive for a long time; ultimately the extremity spontaneously and
slowly curls inwards, and makes a button-like, flat, spiral coil at
the end of the leaf. One leaf was watched, and the hook remained
open for thirty-three days; but during the last week the tip had
curled so much inwards that only a very thin twig could have been
inserted within it. As soon as the tip has curled so much inwards
that the hook is converted into a ring, its sensibility is lost; but
as long as it remains open some sensibility is retained.
Whilst the plant was only about six inches in height, the leaves,
four or five in number, were broader than those subsequently
produced; their soft and but little-attenuated tips were not
sensitive, and did not form hooks; nor did the stem then revolve. At
this early period of growth, the plant can support itself; its
climbing powers are not required, and consequently are not developed.
So again, the leaves on the summit of a full-grown flowering plant,
which would not require to climb any higher, were not sensitive and
could not clasp a stick. We thus see how perfect is the economy of
nature.
COMMELYNACEAE.--Flagellaria Indica.--From dried specimens it is
manifest that this plant climbs exactly like the Gloriosa. A young
plant 12 inches in height, and bearing fifteen leaves, had not a
single leaf as yet produced into a hook or tendril-like filament; nor
did the stem revolve. Hence this plant acquires its climbing powers
later in life than does the Gloriosa lily. According to Mohl (p.
41), Uvularia (Melanthaceae) also climbs like Gloriosa.
These three last-named genera are Monocotyledons; but there is one
Dicotyledon, namely Nepenthes, which is ranked by Mohl (p. 41)
amongst tendril-bearers; and I hear from Dr. Hooker that most of the
species climb well at Kew. This is effected by the stalk or midrib
between the leaf and the pitcher coiling round any support. The
twisted part becomes thicker; but I observed in Mr. Veitch's hothouse
that the stalk often takes a turn when not in contact with any
object, and that this twisted part is likewise thickened. Two
vigorous young plants of N. laevis and N. distillatoria, in my
hothouse, whilst less than a foot in height, showed no sensitiveness
in their leaves, and had no power of climbing. But when N. laevis
had grown to a height of 16 inches, there were signs of these powers.
The young leaves when first formed stand upright, but soon become
inclined; at this period they terminate in a stalk or filament, with
the pitcher at the extremity hardly at all developed. The leaves now
exhibited slight spontaneous movements; and when the terminal
filaments came into contact with a stick, they slowly bent round and
firmly seized it. But owing to the subsequent growth of the leaf,
this filament became after a time quite slack, though still remaining
firmly coiled round the stick. Hence it would appear that the chief
use of the coiling, at least whilst the plant is young, is to support
the pitcher with its load of secreted fluid.
Summary on Leaf-climbers.--Plants belonging to eight families are
known to have clasping petioles, and plants belonging to four
families climb by the tips of their leaves. In all the species
observed by me, with one exception, the young internodes revolve more
or less regularly, in some cases as regularly as those of a twining
plant. They revolve at various rates, in most cases rather rapidly.
Some few can ascend by spirally twining round a support. Differently
from most twiners, there is a strong tendency in the same shoot to
revolve first in one and then in an opposite direction. The object
gained by the revolving movement is to bring the petioles or the tips
of the leaves into contact with surrounding objects; and without this
aid the plant would be much less successful in climbing. With rare
exceptions, the petioles are sensitive only whilst young. They are
sensitive on all sides, but in different degrees in different plants;
and in some species of Clematis the several parts of the same petiole
differ much in sensitiveness. The hooked tips of the leaves of the
Gloriosa are sensitive only on their inner or inferior surfaces. The
petioles are sensitive to a touch and to excessively slight continued
pressure, even from a loop of soft thread weighing only the one-
sixteenth of a grain (4.05 mg.); and there is reason to believe that
the rather thick and stiff petioles of Clematis flammula are
sensitive to even much less weight if spread over a wide surface.
The petioles always bend towards the side which is pressed or
touched, at different rates in different species, sometimes within a
few minutes, but generally after a much longer period. After
temporary contact with any object, the petiole continues to bend for
a considerable time; afterwards it slowly becomes straight again, and
can then re-act. A petiole excited by an extremely slight weight
sometimes bends a little, and then becomes accustomed to the
stimulus, and either bends no more or becomes straight again, the
weight still remaining suspended. Petioles which have clasped an
object for some little time cannot recover their original position.
After remaining clasped for two or three days, they generally
increase much in thickness either throughout their whole diameter or
on one side alone; they subsequently become stronger and more woody,
sometimes to a wonderful degree; and in some cases they acquire an
internal structure like that of the stem or axis.
The young internodes of the Lophospermum as well as the petioles are
sensitive to a touch, and by their combined movement seize an object.
The flower-peduncles of the Maurandia semperflorens revolve
spontaneously and are sensitive to a touch, yet are not used for
climbing. The leaves of at least two, and probably of most, of the
species of Clematis, of Fumaria and Adlumia, spontaneously curve from
side to side, like the internodes, and are thus better adapted to
seize distant objects. The petioles of the perfect leaves of
Tropaeolum tricolorum, as well as the tendril-like filaments of the
plants whilst young, ultimately move towards the stem or the
supporting stick, which they then clasp. These petioles and
filaments also show some tendency to contract spirally. The tips of
the uncaught leaves of the Gloriosa, as they grow old, contract into
a flat spire or helix. These several facts are interesting in
relation to true tendrils.
With leaf climbers, as with twining plants, the first internodes
which rise from the ground do not, at least in the cases observed by
me, spontaneously revolve; nor are the petioles or tips of the first-
formed leaves sensitive. In certain species of Clematis, the large
size of the leaves, together with their habit of revolving, and the
extreme sensitiveness of their petioles, appear to render the
revolving movement of the internodes superfluous; and this latter
power has consequently become much enfeebled. In certain species of
Tropaeolum, both the spontaneous movements of the internodes and the
sensitiveness of the petioles have become much enfeebled, and in one
species have been completely lost.
CHAPTER III.--TENDRIL-BEARERS.
Nature of tendrils--BIGNONIACEAE, 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.--POLEMONIACEAE--Cobaea
scandens much branched and hooked tendrils, their manner of action--
LEGUMINOSAE--COMPOSITAE--SMILACEAE--Smilax aspera, its inefficient
tendrils--FUMARIACEAE--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, {24} 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: --Bignoniaceae, Polemoniaceae, Leguminosae, Compositae,
Smilaceae, Fumariaceae, Cucurbitaceae, Vitaceae, Sapindaceae,
Passifloraceae. {25}
BIGNONIACEAE.--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.
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. 6m. 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 aerial 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 aequinoctialis, 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 2.5 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 0.25th 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; {26} 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 (Bignoniaceae).--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 degrees with the
stem, in an hour afterwards making an angle of 106 degrees. 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.
POLEMONIACEAE.--Cobaea 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 Cobaea
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 Cobaea 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.
LEGUMINOSAE.--Pisum sativum.--The common pea was the subject of a
valuable memoir by Dutrochet, {27} 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.
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 1.125 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 degrees, 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 degrees 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.
COMPOSITAE.--Mutisia clematis.--The immense family of the Compositae
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. Muller 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.
SMILACEAE.--Smilax aspera, var. maculata.--Aug. St.-Hilaire {28}
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 1.5 to 1.75 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.
FUMARIACEAE.--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 laminae 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 1.5 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.
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 Bignoniaceae and Cobaea. 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.
CHAPTER IV.--TENDRIL-BEARERS--(continued).
CUCURBITACEAE.--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--VITACAE--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--SAPINDACEAE--PASSIFLORACEAE--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.
CUCURBITACEAE.--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.
{29} 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. {30}
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 degrees 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 degrees (in one case of only 37
degrees) 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 degrees beneath the horizon, the
uprising took two hours; in half an hour afterwards it rose to 23
degrees 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 1.75 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.
VITACEAE.--In this family and in the two following, namely, the
Sapindaceae and Passifloraceae, 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 Cucurbitaceae. The homological nature, however, of a
tendril seems to make no difference in its action.
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.
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 8.5 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 Vitaceae, 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 3.5 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, {31} from the light to the dark. I have
seen several tendrils move in less than 24 hours, through an angle of
180 degrees 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. {32} 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.
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!
SAPINDACEAE.--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 4.5 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.
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, {33} 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.
PASSIFLORACEAE.--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. {34} 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 Vitaceae, Sapindaceae and Passifloraceae, 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
Polygonaceae. In two or three species of Modecca, one of the
Papayaceae, 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 Tropaeolum 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, Smilax
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
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