HOOK AND ROOT-CLIMBERS.—CONCLUDING REMARKS.
Plants climbing by the aid of hooks, or merely scrambling over other
plants—Root-climbers, adhesive matter secreted by the rootlets—General
conclusions with respect to climbing plants, and the stages of their
development.
_Hook-Climbers_.—In my introductory remarks, I stated that, besides the
two first great classes of climbing plants, namely, those which twine
round a support, and those endowed with irritability enabling them to
seize hold of objects by means of their petioles or tendrils, there are
two other classes, hook-climbers and root-climbers. Many plants,
moreover, as Fritz Müller has remarked, {183} climb or scramble up
thickets in a still more simple fashion, without any special aid,
excepting that their leading shoots are generally long and flexible. It
may, however, be suspected from what follows, that these shoots in some
cases tend to avoid the light. The few hook-climbers which I have
observed, namely, _Galium aparine_, _Rubus australis_, and some climbing
Roses, exhibit no spontaneous revolving movement. If they had possessed
this power, and had been capable of twining, they would have been placed
in the class of Twiners; for some twiners are furnished with spines or
hooks, which aid them in their ascent. For instance, the Hop, which is a
twiner, has reflexed hooks as large as those of the _Galium_; some other
twiners have stiff reflexed hairs; and _Dipladenia_ has a circle of blunt
spines at the bases of its leaves. I have seen only one tendril-bearing
plant, namely, _Smilax aspera_, which is furnished with reflexed spines;
but this is the case with several branch-climbers in South Brazil and
Ceylon; and their branches graduate into true tendrils. Some few plants
apparently depend solely on their hooks for climbing, and yet do so
efficiently, as certain palms in the New and Old Worlds. Even some
climbing Roses will ascend the walls of a tall house, if covered with a
trellis. How this is effected I know not; for the young shoots of one
such Rose, when placed in a pot in a window, bent irregularly towards the
light during the day and from the light during the night, like the shoots
of any common plant; so that it is not easy to understand how they could
have got under a trellis close to the wall. {184}
_Root-climbers_.—A good many plants come under this class, and are
excellent climbers. One of the most remarkable is the _Marcgravia
umbellata_, the stem of which in the tropical forests of South America,
as I hear from Mr. Spruce, grows in a curiously flattened manner against
the trunks of trees; here and there it puts forth claspers (roots), which
adhere to the trunk, and, if the latter be slender, completely embrace
it. When this plant has climbed to the light, it produces free branches
with rounded stems, clad with sharp-pointed leaves, wonderfully different
in appearance from those borne by the stem as long as it remains
adherent. This surprising difference in the leaves, I have also observed
in a plant of _Marcgravia dubia_ in my hothouse. Root-climbers, as far
as I have seen, namely, the Ivy (_Hedera heliæ_), _Ficus repens_, and _F.
barbatus_, have no power of movement, not even from the light to the
dark. As previously stated, the _Hoya carnosa_ (Asclepiadaceæ) is a
spiral twiner, and likewise adheres by rootlets even to a flat wall. The
tendril-bearing _Bignonia Tweedyana_ emits roots, which curve half round
and adhere to thin sticks. The _Tecoma radicans_ (Bignoniaceæ), which is
closely allied to many spontaneously revolving species, climbs by
rootlets; nevertheless, its young shoots apparently move about more than
can be accounted for by the varying action of the light.
I have not closely observed many root-climbers, but can give one curious
fact. _Ficus repens_ climbs up a wall just like Ivy; and when the young
rootlets are made to press lightly on slips of glass, they emit after
about a week’s interval, as I observed several times, minute drops of
clear fluid, not in the least milky like that exuded from a wound. This
fluid is slightly viscid, but cannot be drawn out into threads. It has
the remarkable property of not soon drying; a drop, about the size of
half a pin’s head, was slightly spread out on glass, and I scattered on
it some minute grains of sand. The glass was left exposed in a drawer
during hot and dry weather, and if the fluid had been water, it would
certainly have dried in a few minutes; but it remained fluid, closely
surrounding each grain of sand, during 128 days: how much longer it would
have remained I cannot say. Some other rootlets were left in contact
with the glass for about ten days or a fortnight, and the drops of
secreted fluid were now rather larger, and so viscid that they could be
drawn out into threads. Some other rootlets were left in contact during
twenty-three days, and these were firmly cemented to the glass. Hence we
may conclude that the rootlets first secrete a slightly viscid fluid,
subsequently absorb the watery parts, (for we have seen that the fluid
will not dry by itself,) and ultimately leave a cement. When the
rootlets were torn from the glass, atoms of yellowish matter were left on
it, which were partly dissolved by a drop of bisulphide of carbon; and
this extremely volatile fluid was rendered very much less volatile by
what it had dissolved.
As the bisulphide of carbon has a strong power of softening indurated
caoutchouc, I soaked in it during a short time several rootlets of a
plant which had grown up a plaistered wall; and I then found many
extremely thin threads of transparent, not viscid, excessively elastic
matter, precisely like caoutchouc, attached to two sets of rootlets on
the same branch. These threads proceeded from the bark of the rootlet at
one end, and at the other end were firmly attached to particles of silex
or mortar from the wall. There could be no mistake in this observation,
as I played with the threads for a long time under the microscope,
drawing them out with my dissecting-needles and letting them spring back
again. Yet I looked repeatedly at other rootlets similarly treated, and
could never again discover these elastic threads. I therefore infer that
the branch in question must have been slightly moved from the wall at
some critical period, whilst the secretion was in the act of drying,
through the absorption of its watery parts. The genus _Ficus_ abounds
with caoutchouc, and we may conclude from the facts just given that this
substance, at first in solution and ultimately modified into an unelastic
cement, {187} is used by the _Ficus repens_ to cement its rootlets to any
surface which it ascends. Whether other plants, which climb by their
rootlets, emit any cement I do not know; but the rootlets of the Ivy,
placed against glass, barely adhered to it, yet secreted a little
yellowish matter. I may add, that the rootlets of the _Marcgravia dubia_
can adhere firmly to smooth painted wood.
_Vanilla aromatica_ emits aërial roots a foot in length, which point
straight down to the ground. According to Mohl (p. 49), these crawl into
crevices, and when they meet with a thin support, wind round it, as do
tendrils. A plant which I kept was young, and did not form long roots;
but on placing thin sticks in contact with them, they certainly bent a
little to that side, in the course of about a day, and adhered by their
rootlets to the wood; but they did not bend quite round the sticks, and
afterwards they re-pursued their downward course. It is probable that
these slight movements of the roots are due to the quicker growth of the
side exposed to the light, in comparison with the other side, and not
because the roots are sensitive to contact in the same manner as true
tendrils. According to Mohl, the rootlets of certain species of
_Lycopodium_ act as tendrils. {188}
_Concluding Remarks on Climbing Plants_.
Plants become climbers, in order, as it may be presumed, to reach the
light, and to expose a large surface of their leaves to its action and to
that of the free air. This is effected by climbers with wonderfully
little expenditure of organized matter, in comparison with trees, which
have to support a load of heavy branches by a massive trunk. Hence, no
doubt, it arises that there are so many climbing plants in all quarters
of the world, belonging to so many different orders. These plants have
been arranged under four classes, disregarding those which merely
scramble over bushes without any special aid. Hook-climbers are the
least efficient of all, at least in our temperate countries, and can
climb only in the midst of an entangled mass of vegetation.
Root-climbers are excellently adapted to ascend naked faces of rock or
trunks of trees; when, however, they climb trunks they are compelled to
keep much in the shade; they cannot pass from branch to branch and thus
cover the whole summit of a tree, for their rootlets require
long-continued and close contact with a steady surface in order to
adhere. The two great classes of twiners and of plants with sensitive
organs, namely, leaf-climbers and tendril-bearers taken together, far
exceed in number and in the perfection of their mechanism the climbers of
the two first classes. Those which have the power of spontaneously
revolving and of grasping objects with which they come in contact, easily
pass from branch to branch, and securely ramble over a wide, sun-lit
surface.
The divisions containing twining plants, leaf-climbers, and
tendril-bearers graduate to a certain extent into one another, and nearly
all have the same remarkable power of spontaneously revolving. Does this
gradation, it may be asked, indicate that plants belonging to one
subdivision have actually passed during the lapse of ages, or can pass,
from one state to the other? Has, for instance, any tendril-bearing
plant assumed its present structure without having previously existed as
a leaf-climber or a twiner? If we consider leaf-climbers alone, the idea
that they were primordially twiners is forcibly suggested. The
internodes of all, without exception, revolve in exactly the same manner
as twiners; some few can still twine well, and many others in an
imperfect manner. Several leaf-climbing genera are closely allied to
other genera which are simple twiners. It should also be observed, that
the possession of leaves with sensitive petioles, and with the consequent
power of clasping an object, would be of comparatively little use to a
plant, unless associated with revolving internodes, by which the leaves
are brought into contact with a support; although no doubt a scrambling
plant would be apt, as Professor Jaeger has remarked, to rest on other
plants by its leaves. On the other hand, revolving internodes, without
any other aid, suffice to give the power of climbing; so that it seems
probable that leaf-climbers were in most cases at first twiners, and
subsequently became capable of grasping a support; and this, as we shall
presently see, is a great additional advantage.
From analogous reasons, it is probable that all tendril-bearers were
primordially twiners, that is, are the descendants of plants having this
power and habit. For the internodes of the majority revolve; and, in a
few species, the flexible stem still retains the capacity of spirally
twining round an upright stick. Tendril-bearers have undergone much more
modification than leaf-climbers; hence it is not surprising that their
supposed primordial habits of revolving and twining have been more
frequently lost or modified than in the case of leaf-climbers. The three
great tendril-bearing families in which this loss has occurred in the
most marked manner, are the Cucurbitaceæ, Passifloraceæ, and Vitaceæ. In
the first, the internodes revolve; but I have heard of no twining form,
with the exception (according to Palm, p. 29. 52) of _Momordica
balsamina_, and this is only an imperfect twiner. In the two other
families I can hear of no twiners; and the internodes rarely have the
power of revolving, this power being confined to the tendrils. The
internodes, however, of _Passiflora gracilis_ have the power in a perfect
manner, and those of the common Vine in an imperfect degree: so that at
least a trace of the supposed primordial habit has been retained by some
members of all the larger tendril-bearing groups.
On the view here given, it may be asked, Why have the species which were
aboriginally twiners been converted in so many groups into leaf-climbers
or tendril-bearers? Of what advantage has this been to them? Why did
they not remain simple twiners? We can see several reasons. It might be
an advantage to a plant to acquire a thicker stem, with short internodes
bearing many or large leaves; and such stems are ill fitted for twining.
Any one who will look during windy weather at twining plants will see
that they are easily blown from their support; not so with
tendril-bearers or leaf-climbers, for they quickly and firmly grasp their
support by a much more efficient kind of movement. In those plants which
still twine, but at the same time possess tendrils or sensitive petioles,
as some species of Bignonia, Clematis, and Tropæolum, it can readily be
observed how incomparably better they grasp an upright stick than do
simple twiners. Tendrils, from possessing this power of grasping an
object, can be made long and thin; so that little organic matter is
expended in their development, and yet they sweep a wide circle in search
of a support. Tendril-bearers can, from their first growth, ascend along
the outer branches of any neighbouring bush, and they are thus always
fully exposed to the light; twiners, on the contrary, are best fitted to
ascend bare stems, and generally have to start in the shade. Within tall
and dense tropical forests, twining plants would probably succeed better
than most kinds of tendril-bearers; but the majority of twiners, at least
in our temperate regions, from the nature of their revolving movement,
cannot ascend thick trunks, whereas this can be affected by
tendril-bearers if the trunks are branched or bear twigs, and by some
species if the bark is rugged.
The advantage gained by climbing is to reach the light and free air with
as little expenditure of organic matter as possible; now, with twining
plants, the stem is much longer than is absolutely necessary; for
instance, I measured the stem of a kidney-bean, which had ascended
exactly two feet in height, and it was three feet in length: the stem of
a pea, on the other hand, which had ascended to the same height by the
aid of its tendrils, was but little longer than the height reached. That
this saving of the stem is really an advantage to climbing plants, I
infer from the species that still twine but are aided by clasping
petioles or tendrils, generally making more open spires than those made
by simple twiners. Moreover, the plants thus aided, after taking one or
two turns in one direction, generally ascend for a space straight, and
then reverse the direction of their spire. By this means they ascend to
a considerably greater height, with the same length of stem, than would
otherwise have been possible; and they do this with safety, as they
secure themselves at intervals by their clasping petioles or tendrils.
We have seen that tendrils consist of various organs in a modified state,
namely, leaves, flower-peduncles, branches, and perhaps stipules. With
respect to leaves, the evidence of their modification is ample. In young
plants of Bignonia the lower leaves often remain quite unchanged, whilst
the upper ones have their terminal leaflets converted into perfect
tendrils; in _Eccremocarpus_ I have seen a single lateral branch of a
tendril replaced by a perfect leaflet; in _Vicia sativa_, on the other
hand, leaflets are sometimes replaced by tendril-branches; and many other
such cases could be given. But he who believes in the slow modification
of species will not be content simply to ascertain the homological nature
of different kinds of tendrils; he will wish to learn, as far as is
possible, by what actual steps leaves, flower-peduncles, &c., have had
their functions wholly changed, and have come to serve merely as
prehensile organs.
In the whole group of leaf-climbers abundant evidence has been given that
an organ, still subserving the functions of a leaf, may become sensitive
to a touch, and thus grasp an adjoining object. With several
leaf-climbers the true leaves spontaneously revolve; and their petioles,
after clasping a support grow thicker and stronger. We thus see that
leaves may acquire all the leading and characteristic qualities of
tendrils, namely, sensitiveness, spontaneous movement, and subsequently
increased strength. If their blades or laminæ were to abort, they would
form true tendrils. And of this process of abortion we can follow every
step, until no trace of the original nature of the tendril is left. In
_Mutisia clematis_, the tendril, in shape and colour, closely resembles
the petiole of one of the ordinary leaves, together with the midribs of
the leaflets, but vestiges of the laminæ are still occasionally retained.
In four genera of the Fumariaceæ we can follow the whole process of
transformation. The terminal leaflets of the leaf-climbing _Fumaria
officinalis_ are not smaller than the other leaflets; those of the
leaf-climbing _Adlumia cirrhosa_ are greatly reduced; those of _Corydalis
claviculata_ (a plant which may indifferently be called a leaf-climber or
a tendril-bearer) are either reduced to microscopical dimensions or have
their blades wholly aborted, so that this plant is actually in a state of
transition; and, finally, in the _Dicentra_ the tendrils are perfectly
characterized. If, therefore, we could behold at the same time all the
progenitors of _Dicentra_, we should almost certainly see a series like
that now exhibited by the above-named three genera. In _Tropæolum
tricolorum_ we have another kind of passage; for the leaves which are
first formed on the young stems are entirely destitute of laminæ, and
must be called tendrils, whilst the later formed leaves have
well-developed laminæ. In all cases the acquirement of sensitiveness by
the mid-ribs of the leaves appears to stand in some close relation with
the abortion of their laminæ or blades.
On the view here given, leaf-climbers were primordially twiners, and
tendril-bearers (when formed of modified leaves) were primordially
leaf-climbers. The latter, therefore, are intermediate in nature between
twiners and tendril-bearers, and ought to be related to both. This is
the case: thus the several leaf-climbing species of the Antirrhineæ, of
Solanum, Cocculus, and Gloriosa, have within the same family and even
within the same genus, relatives which are twiners. In the genus
Mikania, there are leaf-climbing and twining species. The leaf-climbing
species of Clematis are very closely allied to the tendril-bearing
Naravelia. The Fumariaceæ include closely allied genera which are
leaf-climbers and tendril-bearers. Lastly, a species of Bignonia is at
the same time both a leaf-climber and a tendril-bearer; and other closely
allied species are twiners.
Tendrils of another kind consist of modified flower-peduncles. In this
case we likewise have many interesting transitional states. The common
Vine (not to mention the _Cardiospermum_) gives us every possible
gradation between a perfectly developed tendril and a flower-peduncle
covered with flowers, yet furnished with a branch, forming the
flower-tendril. When the latter itself bears a few flowers, as we know
sometimes is the case, and still retains the power of clasping a support,
we see an early condition of all those tendrils which have been formed by
the modification of flower-peduncles.
According to Mohl and others, some tendrils consist of modified branches:
I have not observed any such cases, and know nothing of their
transitional states, but these have been fully described by Fritz Müller.
The genus Lophospermum also shows us how such a transition is possible;
for its branches spontaneously revolve and are sensitive to contact.
Hence, if the leaves on some of the branches of the Lophospermum were to
abort, these branches would be converted into true tendrils. Nor is
there anything improbable in certain branches alone being thus modified,
whilst others remained unaltered; for we have seen with certain varieties
of _Phaseolus_, that some of the branches are thin, flexible, and twine,
whilst other branches on the same plant are stiff and have no such power.
If we inquire how a petiole, a branch or flower-peduncle first became
sensitive to a touch, and acquired the power of bending towards the
touched side, we get no certain answer. Nevertheless an observation by
Hofmeister {197} well deserves attention, namely, that the shoots and
leaves of all plants, whilst young, move after being shaken. Kerner also
finds, as we have seen, that the flower-peduncles of a large number of
plants, if shaken or gently rubbed bend to this side. And it is young
petioles and tendrils, whatever their homological nature may be, which
move on being touched. It thus appears that climbing plants have
utilized and perfected a widely distributed and incipient capacity, which
capacity, as far as we can see, is of no service to ordinary plants. If
we further inquire how the stems, petioles, tendrils, and
flower-peduncles of climbing plants first acquired their power of
spontaneously revolving, or, to speak more accurately, of successively
bending to all points of the compass, we are again silenced, or at most
can only remark that the power of moving, both spontaneously and from
various stimulants, is far more common with plants, than is generally
supposed to be the case by those who have not attended to the subject. I
have given one remarkable instance, namely that of the _Maurandia
semperflorens_, the young flower-peduncles of which spontaneously revolve
in very small circles, and bend when gently rubbed to the touched side;
yet this plant certainly does not profit by these two feebly developed
powers. A rigorous examination of other young plants would probably show
slight spontaneous movements in their stems, petioles or peduncles, as
well as sensitiveness to a touch. {198} We see at least that the
_Maurandia_ might, by a little augmentation of the powers which it
already possesses, come first to grasp a support by its flower-peduncles,
and then, by the abortion of some of its flowers (as with _Vitis_ or
_Cardiospermum_), acquire perfect tendrils.
There is one other interesting point which deserves notice. We have seen
that some tendrils owe their origin to modified leaves, and others to
modified flower-peduncles; so that some are foliar and others axial in
their nature. It might therefore have been expected that they would have
presented some difference in function. This is not the case. On the
contrary, they present the most complete identity in their several
characteristic powers. Tendrils of both kinds spontaneously revolve at
about the same rate. Both, when touched, bend quickly to the touched
side, and afterwards recover themselves and are able to act again. In
both the sensitiveness is either confined to one side or extends all
round the tendril. Both are either attracted or repelled by the light.
The latter property is seen in the foliar tendrils of _Bignonia
capreolata_ and in the axial tendrils of _Ampelopsis_. The tips of the
tendrils in these two plants become, after contact, enlarged into discs,
which are at first adhesive by the secretion of some cement. Tendrils of
both kinds, soon after grasping a support, contract spirally; they then
increase greatly in thickness and strength. When we add to these several
points of identity the fact that the petiole of _Solanum jasminoides_,
after it has clasped a support, assumes one of the most characteristic
features of the axis, namely, a closed ring of woody vessels, we can
hardly avoid asking, whether the difference between foliar and axial
organs can be of so fundamental a nature as is generally supposed? {199}
We have attempted to trace some of the stages in the genesis of climbing
plants. But, during the endless fluctuations of the conditions of life
to which all organic beings have been exposed, it might be expected that
some climbing plants would have lost the habit of climbing. In the cases
given of certain South African plants belonging to great twining
families, which in their native country never twine, but reassume this
habit when cultivated in England, we have a case in point. In the
leaf-climbing _Clematis flammula_, and in the tendril-bearing Vine, we
see no loss in the power of climbing, but only a remnant of the revolving
power which is indispensable to all twiners, and is so common as well as
so advantageous to most climbers. In _Tecoma radicans_, one of the
Bignoniaceæ, we see a last and doubtful trace of the power of revolving.
With respect to the abortion of tendrils, certain cultivated varieties of
_Cucurbita pepo_ have, according to Naudin, {200} either quite lost these
organs or bear semi-monstrous representatives of them. In my limited
experience, I have met with only one apparent instance of their natural
suppression, namely, in the common bean. All the other species of
_Vicia_, I believe, bear tendrils; but the bean is stiff enough to
support its own stem, and in this species, at the end of the petiole,
where, according to analogy, a tendril ought to have existed, a small
pointed filament projects, about a third of an inch in length, and which
is probably the rudiment of a tendril. This may be the more safely
inferred, as in young and unhealthy specimens of other tendril-bearing
plants similar rudiments may occasionally be observed. In the bean these
filaments are variable in shape, as is so frequently the case with
rudimentary organs; they are either cylindrical, or foliaceous, or are
deeply furrowed on the upper surface. They have not retained any vestige
of the power of revolving. It is a curious fact, that many of these
filaments, when foliaceous, have on their lower surfaces, dark-coloured
glands like those on the stipules, which excrete a sweet fluid; so that
these rudiments have been feebly utilized.
One other analogous case, though hypothetical, is worth giving. Nearly
all the species of _Lathyrus_ possesses tendrils; but _L. nissolia_ is
destitute of them. This plant has leaves, which must have struck
everyone with surprise who has noticed them, for they are quite unlike
those of all common papilionaceous plants, and resemble those of a grass.
In another species, _L. aphaca_, the tendril, which is not highly
developed (for it is unbranched, and has no spontaneous revolving-power),
replaces the leaves, the latter being replaced in function by large
stipules. Now if we suppose the tendrils of _L. aphaca_ to become
flattened and foliaceous, like the little rudimentary tendrils of the
bean, and the large stipules to become at the same time reduced in size,
from not being any longer wanted, we should have the exact counterpart of
_L. nissolia_, and its curious leaves are at once rendered intelligible
to us.
It may be added, as serving to sum up the foregoing views on the origin
of tendril-bearing plants, that _L. nissolia_ is probably descended from
a plant which was primordially a twiner; this then became a leaf-climber,
the leaves being afterwards converted by degrees into tendrils, with the
stipules greatly increased in size through the law of compensation. {202}
After a time the tendrils lost their branches and became simple; they
then lost their revolving-power (in which state they would have resembled
the tendrils of the existing _L. aphaca_), and afterwards losing their
prehensile power and becoming foliaceous would no longer be thus
designated. In this last stage (that of the existing _L. nissolia_) the
former tendrils would reassume their original function of leaves, and the
stipules which were recently much developed being no longer wanted, would
decrease in size. If species become modified in the course of ages, as
almost all naturalists now admit, we may conclude that _L. nissolia_ has
passed through a series of changes, in some degree like those here
indicated.
The most interesting point in the natural history of climbing plants is
the various kinds of movement which they display in manifest relation to
their wants. The most different organs—stems, branches,
flower-peduncles, petioles, mid-ribs of the leaf and leaflets, and
apparently aërial roots—all possess this power.
The first action of a tendril is to place itself in a proper position.
For instance, the tendril of Cobæa first rises vertically up, with its
branches divergent and with the terminal hooks turned outwards; the young
shoot at the extremity of the stem is at the same time bent to one side,
so as to be out of the way. The young leaves of Clematis, on the other
hand, prepare for action by temporarily curving themselves downwards, so
as to serve as grapnels.
Secondly, if a twining plant or a tendril gets by any accident into an
inclined position, it soon bends upwards, though secluded from the light.
The guiding stimulus no doubt is the attraction of gravity, as Andrew
Knight showed to be the case with germinating plants. If a shoot of any
ordinary plant be placed in an inclined position in a glass of water in
the dark, the extremity will, in a few hours, bend upwards; and if the
position of the shoot be then reversed, the downward-bent shoot reverses
its curvature; but if the stolen of a strawberry, which has no tendency
to grow upwards, be thus treated, it will curve downwards in the
direction of, instead of in opposition to, the force of gravity. As with
the strawberry, so it is generally with the twining shoots of the
_Hibbertia dentata_, which climbs laterally from bush to bush; for these
shoots, if placed in a position inclined downwards, show little and
sometimes no tendency to curve upwards.
Thirdly, climbing plants, like other plants, bend towards the light by a
movement closely analogous to the incurvation which causes them to
revolve, so that their revolving movement is often accelerated or
retarded in travelling to or from the light. On the other hand, in a few
instances tendrils bend towards the dark.
Fourthly, we have the spontaneous revolving movement which is independent
of any outward stimulus, but is contingent on the youth of the part, and
on vigorous health; and this again of course depends on a proper
temperature and other favourable conditions of life.
Fifthly, tendrils, whatever their homological nature may be, and the
petioles or tips of the leaves of leaf-climbers, and apparently certain
roots, all have the power of movement when touched, and bend quickly
towards the touched side. Extremely slight pressure often suffices. If
the pressure be not permanent, the part in question straightens itself
and is again ready to bend on being touched.
Sixthly, and lastly, tendrils, soon after clasping a support, but not
after a mere temporary curvature, contract spirally. If they have not
come into contact with any object, they ultimately contract spirally,
after ceasing to revolve; but in this case the movement is useless, and
occurs only after a considerable lapse of time.
With respect to the means by which these various movements are effected,
there can be little doubt from the researches of Sachs and H. de Vries,
that they are due to unequal growth; but from the reasons already
assigned, I cannot believe that this explanation applies to the rapid
movements from a delicate touch.
Finally, climbing plants are sufficiently numerous to form a conspicuous
feature in the vegetable kingdom, more especially in tropical forests.
America, which so abounds with arboreal animals, as Mr. Bates remarks,
likewise abounds according to Mohl and Palm with climbing plants; and of
the tendril-bearing plants examined by me, the highest developed kinds
are natives of this grand continent, namely, the several species of
_Bignonia_, _Eccremocarpus_, _Cobæa_, and _Ampelopsis_. But even in the
thickets of our temperate regions the number of climbing species and
individuals is considerable, as will be found by counting them. They
belong to many and widely different orders. To gain some rude idea of
their distribution in the vegetable series, I marked, from the lists
given by Mohl and Palm (adding a few myself, and a competent botanist, no
doubt, could have added many more), all those families in Lindley’s
‘Vegetable Kingdom’ which include twiners, leaf-climbers, or
tendril-bearers. Lindley divides Phanerogamic plants into fifty-nine
Alliances; of these, no less than thirty-five include climbing plants of
the above kinds, hook and root-climbers being excluded. To these a few
Cryptogamic plants must be added. When we reflect on the wide separation
of these plants in the series, and when we know that in some of the
largest, well-defined orders, such as the Compositæ, Rubiaceæ,
Scrophulariaceæ, Liliaceæ, &c., species in only two or three genera have
the power of climbing, the conclusion is forced on our minds that the
capacity of revolving, on which most climbers depend, is inherent, though
undeveloped, in almost every plant in the vegetable kingdom.
It has often been vaguely asserted that plants are distinguished from
animals by not having the power of movement. It should rather be said
that plants acquire and display this power only when it is of some
advantage to them; this being of comparatively rare occurrence, as they
are affixed to the ground, and food is brought to them by the air and
rain. We see how high in the scale of organization a plant may rise,
when we look at one of the more perfect tendril-bearers. It first places
its tendrils ready for action, as a polypus places its tentacula. If the
tendril be displaced, it is acted on by the force of gravity and rights
it self. It is acted on by the light, and bends towards or from it, or
disregards it, whichever may be most advantageous. During several days
the tendrils or internodes, or both, spontaneously revolve with a steady
motion. The tendril strikes some object, and quickly curls round and
firmly grasps it. In the course of some hours it contracts into a spire,
dragging up the stem, and forming an excellent spring. All movements now
cease. By growth the tissues soon become wonderfully strong and durable.
The tendril has done its work, and has done it in an admirable manner.
FOOTNOTES.
{iv} An English translation of the ‘Lehrbuch der Botanik’ by Professor
Sachs, has recently (1875), appeared under the title of ‘Text-Book of
Botany,’ and this is a great boon to all lovers of natural science in
England.
{1a} ‘Proc. Amer. Acad. of Arts and Sciences,’ vol. iv. Aug. 12, 1858,
p. 98.
{1b} Ludwig H. Palm, ‘Ueber das Winden der Pflanzen;’ Hugo von Mohl,
‘Ueber den Bau und des Winden der Ranken und Schlingpflanzen,’ 1827.
Palm’s Treatise was published only a few weeks before Mohl’s. See also
‘The Vegetable Cell’ (translated by Henfrey), by H. von Mohl, p. 147 to
end.
{1c} “Des Mouvements révolutife Respontanés,” &c., ‘Comptes Rendus,’
tom. xvii. (1843) p. 989; “Recherches sur la Volubilité des Tiges,” &c.,
tom. xix. (1844) p. 295.
{8} ‘Bull. Bot Soc. de France,’ tom. v. 1858, p. 356.
{9a} This whole subject has been ably discussed and explained by H. de
Vries, ‘Arbeiten des Bot. Instituts in Würzburg,’ Heft iii. pp. 331, 336.
See also Sachs (‘Text-Book of Botany,’ English translation, 1875, p.
770), who concludes “that torsion is the result of growth continuing in
the outer layers after it has ceased or begun to cease in the inner
layers.”
{9b} Professor Asa Gray has remarked to me, in a letter, that in _Thuja
occidentalis_ the twisting of the bark is very conspicuous. The twist is
generally to the right of the observer; but, in noticing about a hundred
trunks, four or five were observed to be twisted in an opposite
direction. The Spanish chestnut is often much twisted: there is an
interesting article on this subject in the ‘Scottish Farmer,’ 1865, p.
833.
{10} It is well known that the stems of many plants occasionally become
spirally twisted in a monstrous manner; and after my paper was read
before the Linnean Society, Dr. Maxwell Masters remarked to me in a
letter that “some of these cases, if not all, are dependent upon some
obstacle or resistance to their upward growth.” This conclusion agrees
with what I have said about the twisting of stems, which have twined
round rugged supports; but does not preclude the twisting being of
service to the plant by giving greater rigidity to the stem.
{12} The view that the revolving movement or nutation of the stems of
twining plants is due to growth is that advanced by Sachs and H. de
Vries; and the truth of this view is proved by their excellent
observations.
{14} The mechanism by which the end of the shoot remains hooked appears
to be a difficult and complex problem, discussed by Dr. H. de Vries
(ibid. p. 337): he concludes that “it depends on the relation between the
rapidity of torsion and the rapidity of nutation.”
{16} Dr. H. de Vries also has shown (ibid. p. 321 and 325) by a better
method than that employed by me, that the stems of twining plants are not
irritable, and that the cause of their winding up a support is exactly
what I have described.
{17} Dr. H. de Vries states (ibid. p. 322) that the stem of Cuscuta is
irritable like a tendril.
{18} See Dr. H. de Vries (ibid. p. 324) on this subject.
{19} Comptes Rendus, 1844, tom. xix. p. 295, and Annales des Sc. Nat 3rd
series, Bot., tom. ii. p. 163.
{24} I am much indebted to Dr. Hooker for having sent me many plants
from Kew; and to Mr. Veitch, of the Royal Exotic Nursery, for having
generously given me a collection of fine specimens of climbing plants.
Professor Asa Gray, Prof. Oliver, and Dr. Hooker have afforded me, as on
many previous occasions, much information and many references.
{33} Journal of the Linn. Soc. (Bot.) vol. ix. p. 344. I shall have
occasion often to quote this interesting paper, in which he corrects or
confirms various statements made by me.
{34} I raised nine plants of the hybrid _Loasa Herbertii_, and six of
these also reversed their spire in ascending a support.
{36} In another genus, namely Davilla, belonging to the same family with
Hibbertia, Fritz Müller says (ibid. p. 349) that “the stem twines
indifferently from left to right, or from right to left; and I once saw a
shoot which ascended a tree about five inches in diameter, reverse its
course in the same manner as so frequently occurs with Loasa.”
{37} Fritz Müller states (ibid. p. 349) that he saw on one occasion in
the forests of South Brazil a trunk about five feet in circumference
spirally ascended by a plant, apparently belonging to the Menispermaceæ.
He adds in his letter to me that most of the climbing plants which there
ascend thick trees, are root-climbers; some being tendril-bearers.
{44} Fritz Müller has published some interesting facts and views on the
structure of the wood of climbing plants in ‘Bot. Zeitung,’ 1866, pp. 57,
66.
{68} It appears from A. Kerner’s interesting observations, that the
flower-peduncles of a large number of plants are irritable, and bend when
they are rubbed or shaken: Die Schutzmittel des Pollens, 1873, p. 34.
{71} I have already referred to the case of the twining stem of Cuscuta,
which, according to H. de Vries (ibid. p. 322) is sensitive to a touch
like a tendril.
{75} Dr. Maxwell Masters informs me that in almost all petioles which
are cylindrical, such as those bearing peltate leaves, the woody vessels
form a closed ring; semilunar bands of vessels being confined to petioles
which are channelled along their upper surfaces. In accordance with this
statement, it may be observed that the enlarged and clasped petiole of
the _Solanum_, with its closed ring of woody vessels, has become more
cylindrical than it was in its original unclasped condition.
{84} Never having had the opportunity of examining tendrils produced by
the modification of branches, I spoke doubtfully about them in this essay
when originally published. But since then Fritz Müller has described
(Journal of Linn. Soc. vol. ix. p. 344) many striking cases in South
Brazil. In speaking of plants which climb by the aid of their branches,
more or less modified, he states that the following stages of development
can be traced: (1.) Plants supporting themselves simply by their branches
stretched out at right angles—for example, _Chiococca_. (2.) Plants
clasping a support with their unmodified branches, as with _Securidaca_.
(3.) Plants climbing by the extremities of their branches which appear
like tendrils, as is the case according to Endlicher with _Helinus_.
(4.) Plants with their branches much modified and temporarily converted
into tendrils, but which may be again transformed into branches, as with
certain Papilionaceous plants. (5.) Plants with their branches forming
true tendrils, and used exclusively for climbing—as with _Strychnos_ and
_Caulotretus_. Even the unmodified branches become much thickened when
they wind round a support. I may add that Mr. Thwaites sent me from
Ceylon a specimen of an Acacia which had climbed up the trunk of a rather
large tree, by the aid of tendril-like, curved or convoluted branchlets,
arrested in their growth and furnished with sharp recurved hooks.
{85} As far as I can make out, the history of our knowledge of tendrils
is as follows:—We have seen that Palm and von Mohl observed about the
same time the singular phenomenon of the spontaneous revolving movement
of twining-plants. Palm (p. 58), I presume, observed likewise the
revolving movement of tendrils; but I do not feel sure of this, for he
says very little on the subject. Dutrochet fully described this movement
of the tendril in the common pea. Mohl first discovered that tendrils
are sensitive to contact; but from some cause, probably from observing
too old tendrils, he was not aware how sensitive they were, and thought
that prolonged pressure was necessary to excite their movement.
Professor Asa Gray, in a paper already quoted, first noticed the extreme
sensitiveness and rapidity of the movements of the tendrils of certain
Cucurbitaceous plants.
{102} Fritz Müller states (ibid. p. 348) that in South Brazil the trifid
tendrils of Haplolophium, (one of the Bignoniaceæ) without having come
into contact with any object, terminate in smooth shining discs. These,
however, after adhering to any object, sometimes become considerably
enlarged.
{111} Comptes Rendus, tom. xvii. 1843, p. 989.
{113} Diagram showing the movement of the upper internode of the common
Pea, traced on a hemispherical glass, and transferred to paper; reduced
one-half in size (Aug. 1st)
No. H. M.
1 8 46 A.M.
2 10 0
3 11 0
4 11 37
5 12 7 P.M.
6 12 30
7 1 0
8 1 30
9 1 44
10 2 25
11 3 0
12 3 30
13 3 48
14 4 40
15 5 5
16 5 25
17 5 50
18 6 25
19 7 0
20 7 45
21 8 30
22 9 15
{118} ‘Leçons de Botanique,’ &c., 1841, p. 170.
{127a} I am indebted to Prof. Oliver for information on this head. In
the Bulletin de la Société Botanique de France, 1857, there are numerous
discussions on the nature of the tendrils in this family.
{127b} ‘Gardeners’ Chronicle,’ 1864, p. 721. From the affinity of the
Cucurbitaceæ to the Passifloraceæ, it might be argued that the tendrils
of the former are modified flower-peduncles, as is certainly the case
with those of Passion flowers. Mr. R. Holland (Hardwicke’s
‘Science-Gossip,’ 1865, p. 105) states that “a cucumber grew, a few years
ago in my own garden, where one of the short prickles upon the fruit had
grown out into a long, curled tendril.”
{145} Trans. Phil. Soc. 1812, p. 314.
{146} Dr. M’Nab remarks (Trans. Bot. Soc. Edinburgh, vol xi. p. 292)
that the tendrils of _Amp. Veitchii_ bear small globular discs before
they have came into contact with any object; and I have since observed
the same fact. These discs, however, increase greatly in size, if they
press against and adhere to any surface. The tendrils, therefore, of one
species of _Ampelopsis_ require the stimulus of contact for the first
development of their discs, whilst those of another species do not need
any such stimulus. We have seen an exactly parallel case with two
species of _Bignoniaceæ_.
{152} Fritz Müller remarks (ibid. p. 348) that a related genus,
Serjania, differs from Cardiospermum in bearing only a single tendril;
and that the common peduncle contracts spirally, when, as frequently
happens, the tendril has clasped the plant’s own stem.
{154} Prof. Asa Gray informs me that the tendrils of _P. sicyoides_
revolve even at a quicker rate than those of _P. gracilis_; four
revolutions were completed (the temperature varying from 88 degrees-92
degrees Fahr.) in the following times, 40 m., 45 m., 38½ m., and 46 m.
One half-revolution was performed in 15 m.
{165} See M. Isid. Léon in Bull. Soc. Bot. de France, tom. v. 1858, p.
650. Dr. H. de Vries points out (p. 306) that I have overlooked, in the
first edition of this essay, the following sentence by Mohl: “After a
tendril has caught a support, it begins in some days to wind into a
spire, which, since the tendril is made fast at both extremities, must of
necessity be in some places to the right, in others to the left.” But I
am not surprised that this brief sentence, without any further
explanation did not attract my attention.
{176} Sachs, however (‘Text-Book of Botany,’ Eng. Translation, 1875, p.
280), has shown that which I overlooked, namely, that the tendrils of
different species are adapted to clasp supports of different thicknesses.
He further shows that after a tendril has clasped a support it
subsequently tightens its hold.
{179} Annales des Sc. Nat. Bot. 4th series, tom. xii. p. 89.
{180} It occurred to me that the movement of notation and that from a
touch might be differently affected by anæsthetics, in the same manner as
Paul Bert has shown to be the case with the sleep-movements of Mimosa and
those from a touch. I tried the common pea and _Passiflora gracilis_,
but I succeeded only in observing that both movements were unaffected by
exposure for 1½ hrs. to a rather large dose of sulphuric ether. In this
respect they present a wonderful contrast with Drosera, owing no doubt to
the presence of absorbent glands in the latter plant.
{181} Text-Book of Botany, 1875, p. 779.
{183} Journal of Linn. Soc. vol. ix. p. 348. Professor G. Jaeger has
well remarked (‘In Sachen Darwin’s, insbesondere contra Wigand,’ 1874, p.
106) that it is highly characteristic of climbing plants to produce thin,
elongated, and flexible stems. He further remarks that plants growing
beneath other and taller species or trees, are naturally those which
would be developed into climbers; anti such plants, from stretching
towards the light, and from not being much agitated by the wind, tend to
produce long, thin and flexible shoots.
{184} Professor Asa Gray has explained, as it would appear, this
difficulty in his review (American Journal of Science, vol. xl. Sept.
1865, p. 282) of the present work. He has observed that the strong
summer shoots of the Michigan rose (_Rosa setigera_) are strongly
disposed to push into dark crevices and away from the light, so that they
would be almost sure to place themselves under a trellis. He adds that
the lateral shoots, made on the following spring emerged from the trellis
as they sought the light.
{187} Mr. Spiller has recently shown (Chemical Society, Feb. 16, 1865),
in a paper on the oxidation of india-rubber or caoutchouc, that this
substance, when exposed in a fine state of division to the air, gradually
becomes converted into brittle, resinous matter, very similar to
shell-lac.
{188} Fritz Müller informs me that he saw in the forests of South Brazil
numerous black strings, from some lines to nearly an inch in diameter,
winding spirally round the trunks of gigantic trees. At first sight he
thought that they were the stems of twining plants which were thus
ascending the trees: but he afterwards found that they were the aërial
roots of a Philodendron which grew on the branches above. These roots
therefore seem to be true twiners, though they use their powers to
descend, instead of to ascend like twining plants. The aërial roots of
some other species of Philodendron hang vertically downwards, sometimes
for a length of more than fifty feet.
{197} Quoted by Cohn, in his remarkable memoir, “Contractile Gewebe im
Pflanzenreiche,” ‘Abhandl. der Schlesischen Gesell. 1861, Heft i. s. 35.
{198} Such slight spontaneous movements, I now find, have been for some
time known to occur, for instance with the flower-stems of _Brassica
napus_ and with the leaves of many plants: Sachs’ ‘Text-Book of Botany’
1875, pp. 766, 785. Fritz Müller also has shown in relation to our
present subject (‘Jenaischen Zeitschrift,’ Bd. V. Heft 2, p. 133) that
the stems, whilst young, of an Alisma and of a Linum are continually
performing slight movements to all points of the compass, like those of
climbing plants.
{199} Mr. Herbert Spencer has recently argued (‘Principles of Biology,’
1865, p. 37 et seq.) with much force that there is no fundamental
distinction between the foliar and axial organs of plants.
{200} Annales des Sc. Nat. 4th series, Bot. tom. vi. 1856, p. 31.
{202} Moquin-Tandon (Eléments de Tératologie. 1841, p. 156) gives the
case of a monstrous bean, in which a case of compensation of this nature
was suddenly effected; for the leaves completely disappeared and the
stipules grew to an enormous size.