ON CLIMBING PLANTS
Robin-run-the-Hedge--Bramble bushes--Climbing roses--Spiny,
wiry stems of smilax--The weak young stem of a liane--The way
in which stems revolve--The hop and its little harpoons--A
climbing palm--Rapidity of turners--The effect of American
life on them--Living bridges--Rope bridges in India--The
common stitchwort--Tendrils--Their behaviour when stroked or
tickled--Their sensibility--Their grasping power--The quickness
with which they curve and their sense of weight--Charles
Darwin--Reasonableness of plants--Corkscrew spirals--The pads
of the Virginian Creeper--The ivy--Does it do harm?--Embracing
roots--Tree ivy.
There are many plants which depend upon and cling to other more
sturdy kinds, and which would be quite unable to live upon the earth
at all if they had not developed the most beautiful methods of doing
so.
In autumn, as soon as the leaves of the Hawthorn have fallen off,
one is sure to find upon the hedges the common Robin-run-the-Hedge
(Goosegrass, Cleavers, or Sticky Willie, for it is known by all these
nicknames as well as by its proper name, _Galium aparine_).
Its stem is exceedingly weak, but it will be found sometimes to be
six or seven feet long. It does not support itself, but is resting
amongst and entangled in the outer twigs of the hedge in such a
manner that it cannot be blown away by the wind or indeed picked
out without its being broken. The young stems grow upright and are
vigorous at first, but soon they cannot bear their own weight, and
fall back upon a branch of the hedge. There are small curved little
roughnesses along the stem and on the under side of the leaves of the
Galium; these hitch on to the twig. Up to this point then the stem
is supported, and the young part above grows until it also gets a
lodgment, and so it goes on until it sometimes reaches right over the
top of the hedge.
Its young flowering branches grow out towards the light away from
the main stem, and the yellow withered stem in autumn rests upon the
hedge just as a piece of string laid upon it might do.
The Bramble and Rose manage to get a support in very much the same
way, but in Great Britain the Bramble generally grows in open ground
and its branches take root.
The peculiar, curved-back prickles of the Bramble and its arching
sideways growth would of course hang it on to any horizontal branches
in the neighbourhood. Kerner measured the length of the stem of
a Bramble which had interwoven itself into the boughs of a tree,
and found that it was over twenty feet long, although it was only
one-third of an inch thick. In Chile one often finds hedges of
Brambles ten to fifteen feet in height, which have been formed by the
aid of other plants, and also by the way in which the branches become
entangled with one another.
Some Climbing Roses act in a very similar way, especially if grown on
trellis, but the flower shoots always turn to the light like those of
the Galium.
But it is the creepers and lianes of the tropical forests that are
the most remarkable of all climbing plants. They twine round the
stems and hang in great loops and grotesque folds from the branches.
Sometimes in the dense shade it may be difficult to see the main
stem, for it is quite thin, though as strong as a piece of steel
wire. It often happens, when hurrying through a rather open part of
the forest after game, that one's leg suddenly catches in a thin,
spiny, wiry stem of Smilax or some such creeper. The first that one
knows of the creeper is when a quarter of an inch of the spine is
buried in one's flesh.
Away up amongst the branches and foliage far above one's head, leaves
and flowers are developed on numerous branches which have vigorously
pushed out as soon as they got near the sunlight, this tough, spiny,
thread-like stem being their only connexion with the ground.
The development of these climbing plants is probably connected with
the dense shade of forests. In such places a young stem growing up
will become long and drawn out; its tip will droop over and hang
downwards. But there is a curious peculiarity in the growth of all
stems. The stem generally grows more rapidly at any one time on one
side, say on the north, and therefore bends over to the opposite
side. After a time it will be growing most rapidly on the eastern
side and then its head points westwards, and so on. The result is
that the tip of the stem swings in an irregular circle round the stem
itself. Its head turns to every point of the compass in succession.
Supposing a stone is tied to the end of a piece of string, and one
swings the stone horizontally in a circle, then, if an upright stick
is put in the ground and the string comes against it, the string
will coil itself round the stick because the stone goes on swinging
horizontally.
Our young climbing plant in the shade of the forest acts in exactly
the same way. If there is any trunk of a suitable size, it will
in the course of its revolving or sweeping round first touch and
then coil itself round and round the trunk.[137] Of these twining
stems, one of the most interesting and beautiful is the common Hop.
The young shoots or suckers which come from the ground may be seen
waving their stems helplessly round in the air. If they cannot find
something to cling to, then they form weak limp curves, but if one
such shoot touches a pole it very soon obtains a hold, wraps itself
round the support, and easily climbs up to a height of many feet. But
the Hop is worth examining closely. If one passes the fingers along
the stem, it feels rough and prickly. With the aid of a hand-lens, a
whole series of most exquisite little hooks will be discovered. They
are like small pimples with two or three very fine and minute, sharp
grappling-hooks on the top. These prevent the stem from slipping off.
It is also helped in climbing by its leaves, which curve outwards,
and are also provided with grappling prickles on the under side. At
the top of the stem the young leaves are close together, and folded
near the point, so as not to interfere with the tip finding its way
in and out of a trellis-work or amongst branches.
[137] Henslow, _Origin of Plant Structures_; Warming, _Rev. Gen.
de Bot._, tom. 5, p. 213.
These grappling-hooks on the Hop are as perfect in their way, though
by no means so beautiful and elegant as those which are found in the
climbing palm, Desmoncus, so well described by Kerner in his _Natural
History of Plants_. It is one of the rotang palms which reach lengths
of 600 feet, though their stem may be no more than 1-1/3 to 2 inches
thick. The leaflets towards the end of the leaf are transformed
into strong spiny barbs which are exquisitely adapted to hang on to
other plants. In many places, thickets in which these rotang palms
have developed are so matted and tangled together that it is
quite impossible even to cut into them, and they are practically
impenetrable.
[Illustration: IN A KENTISH HOP GARDEN.]
Some of our common British twiners climb very quickly. A complete
turn round the supporting pole was made in England, at Charles
Darwin's home, in the following times. The Hop took 2 hrs. 8 mins.,
Wistaria 2 hrs. 5 mins., Convolvulus 1 hr. 42 mins., and Phaseolus 1
hr. 54 mins. A Honeysuckle took 7 hrs. 30 mins. to make one complete
turn round the support.
Recently Miss Elizabeth A. Simons timed the rate of growth of the
same plants at the University of Pennsylvania. They seem to have been
stimulated by the exhilarating atmosphere of the United States, for
they were all growing faster. The Hop did its turn in 1 hr. 5 mins.,
Phaseolus took from 1 hr. to 1 hr. 20 mins., Convolvulus 57 mins.
only, Lonicera from 1 hr. 43 mins. to 2 hrs. 48 mins., and Wistaria 2
hrs.[138] But there are curious variations in the rate at which these
plants revolve.
[138] _Trans. and Proc. Bot. Soc. Pennsylvania_, Session 1897-8,
vol. 1, No. 1.
Thus when coming towards the light they go as fast as they can, but
revolve more slowly, and as it were reluctantly, away from it. It has
been found in one case that the shoot took thirty-five minutes to do
the semicircle towards the light, and an hour and fifteen to twenty
minutes going away from it, but this is not always the case, for
sometimes the reverse takes place[139] (Baranetzki).
[139] Pfeffer, _Pflanzen-Physiologie_, vol. 2, p. 412.
These twining plants are not very common in Great Britain, and indeed
in Europe. Some of them move or twine to the right (in the same
direction as the hands of a watch or of the sun), such as Convolvulus
(Bindweed), Phaseolus, Ipomoea, and Aristolochia. Others, like the
Hop, Polygonum, Convolvulus, Honeysuckle, and Elephant's Foot, move
in the opposite way from right to left, or "widder-shins." But there
is nothing very important in this distinction, for the Bittersweet
may be found twining in either direction, and in some plants part of
a stem may be twining one way and the other in the opposite direction.
It is in the tropics, and especially in the rank, dark,
moisture-laden atmosphere of the coast jungle forests, that these
twiners attain their greatest development.
They show the most extraordinary variety. Sometimes a twiner hangs
in elegant festoons from branch to branch, forming a convenient
suspension bridge for monkeys. Sometimes four or five are wound round
one another or twisted together, so that they look like some gigantic
cable. In other cases they are knotted, looped, tangled, and twisted
in the most inextricable manner.
Some creepers are flat, like green ribbons or broad bands. In others
the dense mass of old, thick creepers and twiners round some sturdy
trunk becomes so thick and so fused together that when the trunk dies
the lattice-like arrangement of these creepers may keep them upright
although the original supporting trunk is quite rotten and decayed
away.
More usually, a tree will become unhealthy because its branches are
overladen with the dense foliage and flowers of heavy lianes, and
because both trunk and branches are so strangled in the embrace of
great creepers that they cannot expand and develop in the proper way.
Then a storm will overthrow the dead giant of the forest, and these
creepers, entangled with all the surrounding trees, will produce ruin
and destruction all around.
A regular duty of the foresters in India is to cut the stems of
climbing plants. These twining, trailing, rope-like creepers are,
in fact, natural ropes, and are used as such in India, Burma, and
other places. Sometimes they form natural bridges of living plants
extending across a stream. The great suspension bridges in the
valleys of the Himalayas are sometimes made without a single nail
or plank. They are just three ropes (one for the feet and two to
hold on by) made of jungle creepers. Crossing one of these swinging,
swaying creeper-bridges is not an easy matter for those whose heads
are unaccustomed to depths of hundreds of feet below them, especially
if combined with a motion of the creeper-bridge sufficient in itself
to produce violent seasickness. Yet the natives run across them with
loads on their heads!
But it is not necessary to go to the tropics to find interesting and
ingenious climbing plants.
There is a very common little British plant, _Stellaria holostea_
(the Star of Bethlehem, Great Starwort, or Stitchwort), which is
common in shady places, light woods, and by hedges. In the spring
it grows very quickly, and the pairs of leaves are shut together
over the growing point, so that the end of the stem is narrow and
can insert itself between the leaves and twigs of the neighbouring
plants. As soon as such a growing end gets out of the foliage into
the light, each pair of leaves opens out and curves backwards, making
a pair of broad, curved hooks excellently suited to hang the stem on
to the leaves or twigs. Then another period of growth follows, and
again a new pair of hook-like leaves opens out. The stem may be five
or six feet long.
In a rather rare Speedwell (_Veronica scutellata_) a very similar
method is used, but the leaves have special little backward-pointing
teeth on their edges which assist in the attachment process.
But these leaves are not to be compared as regards perfection of
mechanism with the tendrils by means of which plants climb. These
tendrils are thin, flexible, twining threads, which may be formed
by the modification of whole leaves, in other cases of leaflets, or
sometimes of branches. Sweet Peas, Vetches, Passion-flowers, Vines,
and many other plants possess them.
They are like twining plants in the way in which they revolve or
twine so as to wrap themselves round anything which they touch. They
move much faster than twining plants. A Cobæa tendril only takes
twenty-five minutes to make a complete turn, Passion-flowers take
from half to three-quarters of an hour, and the Vine tendril takes a
little over an hour to make one complete turn.
But in one way they differ altogether, for they are sensitive to
contact. If tickled, they contract and embrace closely the object
which is touching them. They show a most extraordinary sensibility
and sensitiveness.
As a matter of fact, these tendrils have a finer sense of touch and
a much more delicate feeling of weight than any human being. They
detect the weight of twenty-seven inches of a spider's thread.
It is, however, best to explain what happens. A half-grown curved
tendril of the Passion-flower is perhaps the most interesting to
experiment with, but any sort of tendril does quite well. If one very
gently rubs the inner or concave side of its little hook, then in a
very few minutes, or even seconds, the tendril distinctly curves. If
this has happened naturally, as when for instance it has been rubbing
upon a pea-stick, this curve makes it curl round the stick, and the
more it touches the more it curls, until the whole tendril is wrapped
round the support.
It is, of course, quite impossible to explain it all exactly: the
sensitive part on the inside of the curve differs from the outside
or convex part of the tendril; the former has a layer of elongated,
thin-walled cells, full of the living matter, protoplasm, which are
absent on the outer side. Immediately the tendril touches the stick,
the outer convex surface begins to grow rapidly. It grows from forty
to 200 times as fast as the inner side which touches the stick! Very
soon after it has clasped the stick the tendril becomes woody and
forms a strong, woody, spiral coil.
These tendrils can be made to curve by a weight exceedingly small.
The most sensitive part of our own skins is quite unable to
distinguish so small a weight as is perceived by these tendrils. Even
the sensation of taste can only be produced by a weight eight times
as great as that shown by some of them. Tendrils curve very quickly
after they have been touched. In twenty seconds some tendrils curve
(_Cyclanthera_), others (_Passiflora_) take thirty seconds, and some
of them require four to five minutes or even longer before they make
up their minds to coil.
Even more remarkable, however, is the fact that they do not coil
when raindrops fall on them, giving a much harder blow than small
weights. If one tendril touches or rubs against another, it is said
not to curve. They are persevering little things also, for Darwin got
a passion-flower tendril to curve when struck or rubbed no less than
twenty-one times during fifty-four hours.
If one reflects on all these curious facts, it is difficult to
help feeling that these plants behave very much in the way that a
reasonable animal would do. There are many other cases in which some
vegetable does exactly what we should expect of reasonable beings
under the circumstances. The tip of the root (see p. 89), the
Sensitive Plant, the Monkey and Barberry flowers, are all well-known
cases.
So that it is difficult to find anything in science to contradict the
comfortable belief that wide-open flowers and stretched-out leaves
of plants as they drink in the warm rays of the sunlight are really
enjoying themselves, whilst they are doing their day's work.
All these interesting facts are so beautifully described and so
carefully summed up by Charles Darwin, that we shall only earnestly
recommend our readers to get first that fascinating book _The Power
of Movement in Plants_, and then read all the rest of his works.[140]
[140] For the above facts: Pfeffer, _Pflanzen-Physiologie_, vol.
2, pp. 423-8; Green, _Vegetable Physiology_, p. 389; Kerner,
_l.c._, p. 697; Bonnier, _l.c._, p. 305.
There are an extraordinary number of these plants and the tendrils
are formed exactly where they will be most useful. Every part of a
leaf may become a tendril. The whole leaf is changed into one in
some kinds of Lathyrus. In a very beautiful creeper which is not so
often grown in greenhouses as it might be (_Gloriosa superba_), the
tip of the leaf only acts as a tendril. Leaflets are often made into
tendrils. The Clematis is the most economical of them all, for the
leaf-stalk coils round and forms little woody rings which hold up the
plant.
Before leaving the subject of tendrils, it may be interesting to
notice the queer corkscrew spirals in which they roll themselves up.
These spirals are formed after the end of the tendril has tied itself
to the support and become woody. The free part between the end and
its own stem goes on revolving; now if you tie a piece of string at
both ends and make it revolve, you will see at once that it must coil
itself into a double spiral, one part in one direction and the other
in the opposite way, with a flat piece between them.
One might be disposed to think no more about these double coils; but
here comes in one of the curious, inexplicable coincidences which
happen so often in plant life. Such a coil is much stronger than a
straight bit of wire or string would be, because if pulled out it
yields and is springy. That of course makes it less probable that the
tendril will be broken. Attached by a series of wiry springs, the
plant yields and sways to the wind, and it is not likely that it will
be torn away. Besides this, the coiling of the tendril pulls the stem
closer to its support, which is also a great advantage.
Certain Virginian Creepers and Vines behave in quite a different
manner. The tendrils grow away from the light and so seek the shadow
of the leaves. They are also divided into little branches. At the
tip of each little branch is a small knob; if this should touch the
wall or the trunk of a tree, etc., it immediately secretes a drop
of cement and glues itself firmly to the wall. There is a curious
difference in different sorts of _Ampelopsis_ in this respect. There
is no adhesive pad in one of them (_Ampelopsis hederacea_) until
it touches, whilst _A. Veitchii_ has them more or less ready for
gluing before they touch (though they become much larger and better
developed as soon as they rub against the wall).[141]
[141] Henslow, _Origin of Plant Structures_, p. 223.
One of the most interesting of our common climbers, "that rare old
plant the Ivy green," has not yet been mentioned. It is exceedingly
decorative on walls, especially on ruins and on old tree-trunks in
winter time, where its dark, brilliant green is most effective.
A violent controversy rages as to whether it does good or harm.
Unhappily it does not do any good to trees. It does not suck their
sap, for its roots do not get through the bark, but it does choke,
with its clinging branches, young tree-stems, and prevents their
growing properly.
Also, in winter storms an ivy-covered tree is much more likely to be
blown down. But on walls the ivy certainly does good, for it sucks
up the moisture, and ivy-covered walls are much more dry inside than
those which are exposed to rain.
Its method of climbing is very curious. All along the stem quantities
of little roots are produced. They dislike light, like most roots,
and creep into crevices and cracks, where they wedge themselves
in by growing thicker. Thus the stem is anchored all along its
length. It is curious to find that these roots are formed before
a twig is actually touching the wall, so as to be ready for any
emergencies.[142]
[142] Henslow, _l.c._
One interesting little point in the growth of the ivy on a tree is
perhaps worth mentioning. The main stem runs nearly straight up the
trunk, and when young is pulled down into the crevices or cracks
in the bark. But its branches leave the main stem at an angle of
forty-five degrees or so to it; these latter may often grow in this
direction for a foot or eighteen inches, but then they gradually
begin to turn more and more distinctly up the tree. Still these
branches firmly clasp the trunk like arms spread out on either side
of it, and make it almost impossible to dislodge the main stem.
Old plants of ivy entirely surround the trunk. The flowering branches
grow straight out into the air, and have no tendency to cling to the
bark. Their leaves are also different.
The ivy may be considered as a root-climber, although the branches
assist by growing round the stem.
A curious instance has been given me of the longevity of ivy and its
power of clinging to life. A correspondent mentions the case of a
Scotch fir whose life was threatened by an ivy. The trunk of the ivy
was sawn through. That did not kill it, at any rate immediately!
Probably the rain soaked up by the leaves, and by the roots in the
crevices of the bark, kept it sufficiently fresh to cling to life. As
it refused to die, a ladder was brought, and it was dragged off the
tree. No doubt it would have died if the weather had been at all dry.
There are some very beautiful tropical plants which also climb by
means of their roots. These roots, the so-called girdle roots, grow
right round the stem and embrace it, so that the climber is perfectly
supported.
It is impossible not to be impressed with the extraordinary variety
of all these contrivances by which plants are able to escape the
trouble of supporting themselves. But such ways of life involve
certain disadvantages. Supposing there is nothing on which to climb,
the stems trail feebly on the ground, and are probably soon choked
by the surrounding grasses. Curiously enough, there are varieties of
the Ivy, Wistaria, and the French Bean which are upright, and do not
climb at all. The Tree Ivy has all its leaves like the leaves of the
flowering shoot in the common form. In America, _Wistaria sinensis_
is often grown as a standard tree, and does not send out the long
shoots, sometimes thirty feet in length, which are common when it
grows on walls. The dwarf French Bean has a thick stem and requires
no support, yet it often puts out a long slender shoot which tries to
twine round something.
In a tropical forest also, the creepers, though they damage the
trees, yet manage to find space for their leaves and flowers: more
vegetable matter is formed per square yard of ground than would be
the case if there were no climbing plants.