The Movements and Habits of Climbing Plants
by
Charles Darwin
Part 3 out of 3
power of spontaneously revolving, for it occurs in tendrils, such as
those of Lathyrus grandiflorus and Ampelopsis hederacea, which do not
revolve. It is not necessarily related to the curling of the tips
round a support, as we see with the Ampelopsis and Bignonia
capreolata, in which the development of adherent discs suffices to
cause spiral contraction. Yet in some cases this contraction seems
connected with the curling or clasping movement, due to contact with
a support; for not only does it soon follow this act, but the
contraction generally begins close to the curled extremity, and
travels downwards to the base. If, however, a tendril be very slack,
the whole length almost simultaneously becomes at first flexuous and
then spiral. Again, the tendrils of some few plants never contract
spirally unless they have first seized hold of some object; if they
catch nothing they hang down, remaining straight, until they wither
and drop off: this is the case with the tendrils of Bignonia, which
consist of modified leaves, and with those of three genera of the
Vitaceae, which are modified flower-peduncles. But in the great
majority of cases, tendrils which have never come in contact with any
object, after a time contract spirally. All these facts taken
together, show that the act of clasping a support and the spiral
contraction of the whole length of the tendril, are phenomena not
necessarily connected.
The spiral contraction which ensues after a tendril has caught a
support is of high service to the plant; hence its almost universal
occurrence with species belonging to widely different orders. When a
shoot is inclined and its tendril has caught an object above, the
spiral contraction drags up the shoot. When the shoot is upright,
the growth of the stem, after the tendrils have seized some object
above, would leave it slack, were it not for the spiral contraction
which draws up the stem as it increases in length. Thus there is no
waste of growth, and the stretched stem ascends by the shortest
course. When a terminal branchlet of the tendril of Cobaea catches a
stick, we have seen how well the spiral contraction successively
brings the other branchlets, one after the other, into contact with
the stick, until the whole tendril grasps it in an inextricable knot.
When a tendril has caught a yielding object, this is sometimes
enveloped and still further secured by the spiral folds, as I have
seen with Passiflora quadrangularis; but this action is of little
importance.
A far more important service rendered by the spiral contraction of
the tendrils is that they are thus made highly elastic. As before
remarked under Ampelopsis, the strain is thus distributed equally
between the several attached branches; and this renders the whole far
stronger than it otherwise would be, as the branches cannot break
separately. It is this elasticity which protects both branched and
simple tendrils from being torn away from their supports during
stormy weather. I have more than once gone on purpose during a gale
to watch a Bryony growing in an exposed hedge, with its tendrils
attached to the surrounding bushes; and as the thick and thin
branches were tossed to and fro by the wind, the tendrils, had they
not been excessively elastic, would instantly have been torn off and
the plant thrown prostrate. But as it was, the Bryony safely rode
out the gale, like a ship with two anchors down, and with a long
range of cable ahead to serve as a spring as she surges to the storm.
When an unattached tendril contracts spirally, the spire always runs
in the same direction from tip to base. A tendril, on the other
hand, which has caught a support by its extremity, although the same
side is concave from end to end, invariably becomes twisted in one
part in one direction, and in another part in the opposite direction;
the oppositely turned spires being separated by a short straight
portion. This curious and symmetrical structure has been noticed by
several botanists, but has not been sufficiently explained. {35} It
occurs without exception with all tendrils which after catching an
object contract spirally, but is of course most conspicuous in the
longer tendrils. It never occurs with uncaught tendrils; and when
this appears to have occurred, it will be found that the tendril had
originally seized some object and had afterwards been torn free.
Commonly, all the spires at one end of an attached tendril run in one
direction, and all those at the other end in the opposite direction,
with a single short straight portion in the middle; but I have seen a
tendril with the spires alternately turning five times in opposite
directions, with straight pieces between them; and M. Leon has seen
seven or eight such alternations. Whether the spires turn once or
more than once in opposite directions, there are as many turns in the
one direction as in the other. For instance, I gathered ten attached
tendrils of the Bryony, the longest with 33, and the shortest with
only 8 spiral turns; and the number of turns in the one direction was
in every case the same (within one) as in the opposite direction.
The explanation of this curious little fact is not difficult. I will
not attempt any geometrical reasoning, but will give only a practical
illustration. In doing this, I shall first have to allude to a point
which was almost passed over when treating of Twining-plants. If we
hold in our left hand a bundle of parallel strings, we can with our
right hand turn these round and round, thus imitating the revolving
movement of a twining plant, and the strings do not become twisted.
But if we hold at the same time a stick in our left hand, in such a
position that the strings become spirally turned round it, they will
inevitably become twisted. Hence a straight coloured line, painted
along the internodes of a twining plant before it has wound round a
support, becomes twisted or spiral after it has wound round. I
painted a red line on the straight internodes of a Humulus, Mikania,
Ceropegia, Convolvulus, and Phaseolus, and saw it become twisted as
the plant wound round a stick. It is possible that the stems of some
plants by spontaneously turning on their own axes, at the proper rate
and in the proper direction, might avoid becoming twisted; but I have
seen no such case.
In the above illustration, the parallel strings were wound round a
stick; but this is by no means necessary, for if wound into a hollow
coil (as can be done with a narrow slip of elastic paper) there is
the same inevitable twisting of the axis. When, therefore, a free
tendril coils itself into a spire, it must either become twisted
along its whole length (and this never occurs), or the free extremity
must turn round as many times as there are spires formed. It was
hardly necessary to observe this fact; but I did so by affixing
little paper vanes to the extreme points of the tendrils of
Echinocystis and Passiflora quadrangularis; and as the tendril
contracted itself into successive spires, the vane slowly revolved.
We can now understand the meaning of the spires being invariably
turned in opposite directions, in tendrils which from having caught
some object are fixed at both ends. Let us suppose a caught tendril
to make thirty spiral turns all in the same direction; the inevitable
result would be that it would become twisted thirty times on its own
axis. This twisting would not only require considerable force, but,
as I know by trial, would burst the tendril before the thirty turns
were completed. Such cases never really occur; for, as already
stated, when a tendril has caught a support and is spirally
contracted, there are always as many turns in one direction as in the
other; so that the twisting of the axis in the one direction is
exactly compensated by the twisting in the opposite direction. We
can further see how the tendency is given to make the later formed
coils opposite to those, whether turned to the right or to the left,
which are first made. Take a piece of string, and let it hang down
with the lower end fixed to the floor; then wind the upper end
(holding the string quite loosely) spirally round a perpendicular
pencil, and this will twist the lower part of the string; and after
it has been sufficiently twisted, it will be seen to curve itself
into an open spire, with the curves running in an opposite direction
to those round the pencil, and consequently with a straight piece of
string between the opposed spires. In short, we have given to the
string the regular spiral arrangement of a tendril caught at both
ends. The spiral contraction generally begins at the extremity which
has clasped a support; and these first-formed spires give a twist to
the axis of the tendril, which necessarily inclines the basal part
into an opposite spiral curvature. I cannot resist giving one other
illustration, though superfluous: when a haberdasher winds up ribbon
for a customer, he does not wind it into a single coil; for, if he
did, the ribbon would twist itself as many times as there were coils;
but he winds it into a figure of eight on his thumb and little
finger, so that he alternately takes turns in opposite directions,
and thus the ribbon is not twisted. So it is with tendrils, with
this sole difference, that they take several consecutive turns in one
direction and then the same number in an opposite direction; but in
both cases the self-twisting is avoided.
Summary on the Nature and Action of Tendrils.
With the majority of tendril-bearing plants the young internodes
revolve in more or less broad ellipses, like those made by twining
plants; but the figures described, when carefully traced, generally
form irregular ellipsoidal spires. The rate of revolution varies
from one to five hours in different species, and consequently is in
some cases more rapid than with any twining plant, and is never so
slow as with those many twiners which take more than five hours for
each revolution. The direction is variable even in the same
individual plant. In Passiflora, the internodes of only one species
have the power of revolving. The Vine is the weakest revolver
observed by me, apparently exhibiting only a trace of a former power.
In the Eccremocarpus the movement is interrupted by many long pauses.
Very few tendril-bearing plants can spirally twine up an upright
stick. Although the power of twining has generally been lost, either
from the stiffness or shortness of the internodes, from the size of
the leaves, or from some other unknown cause, the revolving movement
of the stem serves to bring the tendrils into contact with
surrounding objects.
The tendrils themselves also spontaneously revolve. The movement
begins whilst the tendril is young, and is at first slow. The mature
tendrils of Bignonia littoralis move much slower than the internodes.
Generally, the internodes and tendrils revolve together at the same
rate; in Cissus, Cobaea, and most Passiflorae, the tendrils alone
revolve; in other cases, as with Lathyrus aphaca, only the internodes
move, carrying with them the motionless tendrils; and, lastly (and
this is the fourth possible case), neither internodes nor tendrils
spontaneously revolve, as with Lathyrus grandiflorus and Ampelopsis.
In most Bignonias, Eccremocarpus Mutisia, and the Fumariaceae, the
internodes, petioles and tendrils all move harmoniously together. In
every case the conditions of life must be favourable in order that
the different parts should act in a perfect manner.
Tendrils revolve by the curvature of their whole length, excepting
the sensitive extremity and the base, which parts do not move, or
move but little. The movement is of the same nature as that of the
revolving internodes, and, from the observations of Sachs and H. de
Vries, no doubt is due to the same cause, namely, the rapid growth of
a longitudinal band, which travels round the tendril and successively
bows each part to the opposite side. Hence, if a line be painted
along that surface which happens at the time to be convex, the line
becomes first lateral, then concave, then lateral, and ultimately
again convex. This experiment can be tried only on the thicker
tendrils, which are not affected by a thin crust of dried paint. The
extremities are often slightly curved or hooked, and the curvature of
this part is never reversed; in this respect they differ from the
extremities of twining shoots, which not only reverse their
curvature, or at least become periodically straight, but curve
themselves in a greater degree than the lower part. In most other
respects a tendril acts as if it were one of several revolving
internodes, which all move together by successively bending to each
point of the compass. There is, however, in many cases this
unimportant difference, that the curving tendril is separated from
the curving internode by a rigid petiole. With most tendril-bearers
the summit of the stem or shoot projects above the point from which
the tendril arises; and it is generally bent to one side, so as to be
out of the way of the revolutions swept by the tendril. In those
plants in which the terminal shoot is not sufficiently out of the
way, as we have seen with the Echinocystis, as soon as the tendril
comes in its revolving course to this point, it stiffens and
straightens itself, and thus rising vertically up passes over the
obstacle in an admirable manner.
All tendrils are sensitive, but in various degrees, to contact with
an object, and curve towards the touched side. With several plants a
single touch, so slight as only just to move the highly flexible
tendril, is enough to induce curvature. Passiflora gracilis
possesses the most sensitive tendrils which I have observed: a bit
of platina wire 0.02 of a grain (1.23 mg.) in weight, gently placed
on the concave point, caused a tendril to become hooked, as did a
loop of soft, thin cotton thread weighing one thirty-second of a
grain (2.02 mg.) With the tendrils of several other plants, loops
weighing one sixteenth of a grain (4.05 mg.) sufficed. The point of
a tendril of Passiflora gracilis began to move distinctly in 25
seconds after a touch, and in many cases after 30 seconds. Asa Gray
also saw movement in the tendrils of the Cucurbitaceous genus,
Sicyos, in 30 seconds. The tendrils of some other plants, when
lightly rubbed, moved in a few minutes; with Dicentra in half-an-
hour; with Smilax in an hour and a quarter or half; and with
Ampelopsis still more slowly. The curling movement consequent on a
single touch continues to increase for a considerable time, then
ceases; after a few hours the tendril uncurls itself, and is again
ready to act. When the tendrils of several kinds of plants were
caused to bend by extremely light weights suspended on them, they
seemed to grow accustomed to so slight a stimulus, and straightened
themselves, as if the loops had been removed. It makes no difference
what sort of object a tendril touches, with the remarkable exception
of other tendrils and drops of water, as was observed with the
extremely sensitive-tendrils of Passiflora gracilis and of the
Echinocystis. I have, however, seen tendrils of the Bryony which had
temporarily caught other tendrils, and often in the case of the vine.
Tendrils of which the extremities are permanently and slightly
curved, are sensitive only on the concave surface; other tendrils,
such as those of the Cobaea (though furnished with horny hooks
directed to one side) and those of Cissus discolor, are sensitive on
all sides. Hence the tendrils of this latter plant, when stimulated
by a touch of equal force on opposite sides, did not bend. The
inferior and lateral surfaces of the tendrils of Mutisia are
sensitive, but not the upper surface. With branched tendrils, the
several branches act alike; but in the Hanburya the lateral spur-like
branch does not acquire (for excellent reasons which have been
explained) its sensitiveness nearly so soon as the main branch. With
most tendrils the lower or basal part is either not at all sensitive,
or sensitive only to prolonged contact. We thus see that the
sensitiveness of tendrils is a special and localized capacity. It is
quite independent of the power of spontaneously revolving; for the
curling of the terminal portion from touch does not in the least
interrupt the former movement. In Bignonia unguis and its close
allies, the petioles of the leaves, as well as the tendrils, are
sensitive to a touch.
Twining plants when they come into contact with a stick, curl round
it invariably in the direction of their revolving movement; but
tendrils curl indifferently to either side, in accordance with the
position of the stick and the side which is first touched. The
clasping movement of the extremity is apparently not steady, but
undulatory or vermicular in its nature, as may be inferred from the
curious manner in which the tendrils of the Echinocystis slowly
crawled round a smooth stick.
As with a few exceptions tendrils spontaneously revolve, it may be
asked,--why have they been endowed with sensitiveness?--why, when
they come into contact with a stick, do they not, like twining
plants, spirally wind round it? One reason may be that they are in
most cases so flexible and thin, that when brought into contact with
any object, they would almost certainly yield and be dragged onwards
by the revolving movement. Moreover, the sensitive extremities have
no revolving power as far as I have observed, and could not by this
means curl round a support. With twining plants, on the other hand,
the extremity spontaneously bends more than any other part; and this
is of high importance for the ascent of the plant, as may be seen on
a windy day. It is, however, possible that the slow movement of the
basal and stiffer parts of certain tendrils, which wind round sticks
placed in their path, may be analogous to that of twining plants.
But I hardly attended sufficiently to this point, and it would have
been difficult to distinguish between a movement due to extremely
dull irritability, from the arrestment of the lower part, whilst the
upper part continued to move onwards.
Tendrils which are only three-fourths grown, and perhaps even at an
earlier age, but not whilst extremely young, have the power of
revolving and of grasping any object which they touch. These two
capacities are generally acquired at about the same period, and both
fail when the tendril is full grown. But in Cobaea and Passiflora
punctata the tendrils begin to revolve in a useless manner, before
they have become sensitive. In the Echinocystis they retain their
sensitiveness for some time after they have ceased to revolve and
after they have sunk downwards; in this position, even if they were
able to seize an object, such power would be of no service in
supporting the stem. It is a rare circumstance thus to detect any
superfluity or imperfection in the action of tendrils--organs which
are so excellently adapted for the functions which they have to
perform; but we see that they are not always perfect, and it would be
rash to assume that any existing tendril has reached the utmost limit
of perfection.
Some tendrils have their revolving motion accelerated or retarded, in
moving to or from the light; others, as with the Pea, seem
indifferent to its action; others move steadily from the light to the
dark, and this aids them in an important manner in finding a support.
For instance, the tendrils of Bignonia capreolata bend from the light
to the dark as truly as a wind-vane from the wind. In the
Eccremocarpus the extremities alone twist and turn about so as to
bring their finer branches and hooks into close contact with any dark
surface, or into crevices and holes.
A short time after a tendril has caught a support, it contracts with
some rare exceptions into a spire; but the manner of contraction and
the several important advantages thus gained have been discussed so
lately, that nothing need here be repeated on the subject. Tendrils
soon after catching a support grow much stronger and thicker, and
sometimes more durable to a wonderful degree; and this shows how much
their internal tissues must be changed. Occasionally it is the part
which is wound round a support which chiefly becomes thicker and
stronger; I have seen, for instance, this part of a tendril of
Bignonia aequinoctialis twice as thick and rigid as the free basal
part. Tendrils which have caught nothing soon shrink and wither; but
in some species of Bignonia they disarticulate and fall off like
leaves in autumn.
Any one who had not closely observed tendrils of many kinds would
probably infer that their action was uniform. This is the case with
the simpler kinds, which simply curl round an object of moderate
thickness, whatever its nature may be. {36} But the genus Bignonia
shows us what diversity of action there may be between the tendrils
of closely allied species. In all the nine species observed by me,
the young internodes revolve vigorously; the tendrils also revolve,
but in some of the species in a very feeble manner; and lastly the
petioles of nearly all revolve, though with unequal power. The
petioles of three of the species, and the tendrils of all are
sensitive to contact. In the first-described species, the tendrils
resemble in shape a bird's foot, and they are of no service to the
stem in spirally ascending a thin upright stick, but they can seize
firm hold of a twig or branch. When the stem twines round a somewhat
thick stick, a slight degree of sensitiveness possessed by the
petioles is brought into play, and the whole leaf together with the
tendril winds round it. In B. unguis the petioles are more
sensitive, and have greater power of movement than those of the last
species; they are able, together with the tendrils, to wind
inextricably round a thin upright stick; but the stem does not twine
so well. B. Tweedyana has similar powers, but in addition, emits
aerial roots which adhere to the wood. In B. venusta the tendrils
are converted into elongated three-pronged grapnels, which move
spontaneously in a conspicuous manner; the petioles, however, have
lost their sensitiveness. The stem of this species can twine round
an upright stick, and is aided in its ascent by the tendrils seizing
the stick alternately some way above and then contracting spirally.
In B. littoralis the tendrils, petioles, and internodes, all revolve
spontaneously. The stem, however, cannot twine, but ascends an
upright stick by seizing it above with both tendrils together, which
then contract into a spire. The tips of these tendrils become
developed into adhesive discs. B. speciosa possesses similar powers
of movement as the last species, but it cannot twine round a stick,
though it can ascend by clasping the stick horizontally with one or
both of its unbranched tendrils. These tendrils continually insert
their pointed ends into minute crevices or holes, but as they are
always withdrawn by the subsequent spiral contraction, the habit
seems to us in our ignorance useless. Lastly, the stem of B.
capreolata twines imperfectly; the much-branched tendrils revolve in
a capricious manner, and bend from the light to the dark; their
hooked extremities, even whilst immature, crawl into crevices, and,
when mature, seize any thin projecting point; in either case they
develop adhesive discs, and these have the power of enveloping the
finest fibres.
In the allied Eccremocarpus the internodes, petioles, and much-
branched tendrils all spontaneously revolve together. The tendrils
do not as a whole turn from the light; but their bluntly-hooked
extremities arrange themselves neatly on any surface with which they
come into contact, apparently so as to avoid the light. They act
best when each branch seizes a few thin stems, like the culms of a
grass, which they afterwards draw together into a solid bundle by the
spiral contraction of all the branches. In Cobaea the finely-
branched tendrils alone revolve; the branches terminate in sharp,
hard, double, little hooks, with both points directed to the same
side; and these turn by well-adapted movements to any object with
which they come into contact. The tips of the branches also crawl
into dark crevices or holes. The tendrils and internodes of
Ampelopsis have little or no power of revolving; the tendrils are but
little sensitive to contact; their hooked extremities cannot seize
thin objects; they will not even clasp a stick, unless in extreme
need of a support; but they turn from the light to the dark, and,
spreading out their branches in contact with any nearly flat surface,
develop discs. These adhere by the secretion of some cement to a
wall, or even to a polished surface; and this is more than the discs
of the Bignonia capreolata can effect.
The rapid development of these adherent discs is one of the most
remarkable peculiarities possessed by any tendrils. We have seen
that such discs are formed by two species of Bignonia, by Ampelopsis,
and, according to Naudin, {37} by the Cucurbitaceous genus Peponopsis
adhaerens. In Anguria the lower surface of the tendril, after it has
wound round a stick, forms a coarsely cellular layer, which closely
fits the wood, but is not adherent; whilst in Hanburya a similar
layer is adherent. The growth of these cellular out-growths depends,
(except in the case of the Haplolophium and of one species of
Ampelopsis,) on the stimulus from contact. It is a singular fact
that three families, so widely distinct as the Bignoniaceae,
Vitaceae, and Cucurbitaceae, should possess species with tendrils
having this remarkable power.
Sachs attributes all the movements of tendrils to rapid growth on the
side opposite to that which becomes concave. These movements consist
of revolving nutation, the bending to and from the light, and in
opposition to gravity, those caused by a touch, and spiral
contraction. It is rash to differ from so great an authority, but I
cannot believe that one at least of these movements--curvature from a
touch--is thus caused. {38} In the first place it may be remarked
that the movement of nutation differs from that due to a touch, in so
far that in some cases the two powers are acquired by the same
tendril at different periods of growth; and the sensitive part of the
tendril does not seem capable of nutation. One of my chief reasons
for doubting whether the curvature from a touch is the result of
growth, is the extraordinary rapidity of the movement. I have seen
the extremity of a tendril of Passiflora gracilis, after being
touched, distinctly bent in 25 seconds, and often in 30 seconds; and
so it is with the thicker tendril of Sicyos. It appears hardly
credible that their outer surfaces could have actually grown in
length, which implies a permanent modification of structure, in so
short a time. The growth, moreover, on this view must be
considerable, for if the touch has been at all rough the extremity is
coiled in two or three minutes into a spire of several turns.
When the extreme tip of the tendril of Echinocystis caught hold of a
smooth stick, it coiled itself in a few hours (as described at p.
132) twice or thrice round the stick, apparently by an undulatory
movement. At first I attributed this movement to the growth of the
outside; black marks were therefore made, and the interspaces
measured, but I could not thus detect any increase in length. Hence
it seems probable in this case and in others, that the curvature of
the tendril from a touch depends on the contraction of the cells
along the concave side. Sachs himself admits {39} that "if the
growth which takes place in the entire tendril at the time of contact
with a support is small, a considerable acceleration occurs on the
convex surface, but in general there is no elongation on the concave
surface, or there may even be a contraction; in the case of a tendril
of Cucurbita this contraction amounted to nearly one-third of the
original length." In a subsequent passage Sachs seems to feel some
difficulty in accounting for this kind of contraction. It must not
however be supposed from the foregoing remarks that I entertain any
doubt, after reading De Vries' observations, about the outer and
stretched surfaces of attached tendrils afterwards increasing in
length by growth. Such increase seems to me quite compatible with
the first movement being independent of growth. Why a delicate touch
should cause one side of a tendril to contract we know as little as
why, on the view held by Sachs, it should lead to extraordinarily
rapid growth of the opposite side. The chief or sole reason for the
belief that the curvature of a tendril when touched is due to rapid
growth, seems to be that tendrils lose their sensitiveness and power
of movement after they have grown to their full length; but this fact
is intelligible, if we bear in mind that all the functions of a
tendril are adapted to drag up the terminal growing shoot towards the
light. Of what use would it be, if an old and full-grown tendril,
arising from the lower part of a shoot, were to retain its power of
clasping a support? This would be of no use; and we have seen with
tendrils so many instances of close adaptation and of the economy of
means, that we may feel assured that they would acquire irritability
and the power of clasping a support at the proper age--namely, youth-
-and would not uselessly retain such power beyond the proper age.
CHAPTER V.--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 Muller has remarked, {40} 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. {41}
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 helix), Ficus repens, and F. barbatus,
have no power of movement, not even from the light to the dark. As
previously stated, the Hoya carnosa (Asclepiadaceae) 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 (Bignoniaceae),
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, {42} 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 aerial 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. {43}
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
Cucurbitaceae, Passifloraceae, and Vitaceae. 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 Tropaeolum, 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 laminae 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 laminae are still occasionally
retained. In four genera of the Fumariaceae 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 Tropaeolum tricolorum we have another
kind of passage; for the leaves which are first formed on the young
stems are entirely destitute of laminae, and must be called tendrils,
whilst the later formed leaves have well-developed laminae. 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
laminae 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
Antirrhineae, 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 Fumariaceae 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 Muller. 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 {44} 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. {45} 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? {46}
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 Bignoniaceae, 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, {47} 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. {48} 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 aerial roots--all possess this power.
The first action of a tendril is to place itself in a proper
position. For instance, the tendril of Cobaea 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, Cobaea, 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 Compositae, Rubiaceae,
Scrophulariaceae, Liliaceae, &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:
{1} 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.
{2} 'Proc. Amer. Acad. of Arts and Sciences,' vol. iv. Aug. 12,
1858, p. 98.
{3} 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.
{4} "Des Mouvements revolutife Respontanes," &c., 'Comptes Rendus,'
tom. xvii. (1843) p. 989; "Recherches sur la Volubilite des Tiges,"
&c., tom. xix. (1844) p. 295.
{5} 'Bull. Bot Soc. de France,' tom. v. 1858, p. 356.
{6} This whole subject has been ably discussed and explained by H.
de Vries, 'Arbeiten des Bot. Instituts in Wurzburg,' 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."
{7} 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.
{8} 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.
{9} 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.
{10} 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."
{11} 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.
{12} Dr. H. de Vries states (ibid. p. 322) that the stem of Cuscuta
is irritable like a tendril.
{13} See Dr. H. de Vries (ibid. p. 324) on this subject.
{14} Comptes Rendus, 1844, tom. xix. p. 295, and Annales des Sc. Nat
3rd series, Bot., tom. ii. p. 163.
{15} 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.
{16} 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.
{17} I raised nine plants of the hybrid Loasa Herbertii, and six of
these also reversed their spire in ascending a support.
{18} In another genus, namely Davilla, belonging to the same family
with Hibbertia, Fritz Muller 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."
{19} Fritz Muller 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 Menispermaceae. 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.
{20} Fritz Muller has published some interesting facts and views on
the structure of the wood of climbing plants in 'Bot. Zeitung,' 1866,
pp. 57, 66.
{21} 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.
{22} 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.
{23} 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.
{24} 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 Muller
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.
{25} 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.
{26} Fritz Muller states (ibid. p. 348) that in South Brazil the
trifid tendrils of Haplolophium, (one of the Bignoniaceae) without
having come into contact with any object, terminate in smooth shining
discs. These, however, after adhering to any object, sometimes
become considerably enlarged.
{27} Comptes Rendus, tom. xvii. 1843, p. 989.
{28} 'Lecons de Botanique,' &c., 1841, p. 170.
{29} I am indebted to Prof. Oliver for information on this head. In
the Bulletin de la Societe Botanique de France, 1857, there are
numerous discussions on the nature of the tendrils in this family.
{30} 'Gardeners' Chronicle,' 1864, p. 721. From the affinity of the
Cucurbitaceae to the Passifloraceae, 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."
{31} Trans. Phil. Soc. 1812, p. 314.
{32} 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 Bignoniaceae.
{33} Fritz Muller 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.
{34} 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., 38.5 m., and
46 m. One half-revolution was performed in 15 m.
{35} See M. Isid. Leon 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.
{36} 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.
{37} Annales des Sc. Nat. Bot. 4th series, tom. xii. p. 89.
{38} It occurred to me that the movement of notation and that from a
touch might be differently affected by anaesthetics, 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 1.5 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.
{39} Text-Book of Botany, 1875, p. 779.
{40} 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.
{41} 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.
{42} 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.
{43} Fritz Muller 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 aerial 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 aerial roots of some other species of
Philodendron hang vertically downwards, sometimes for a length of
more than fifty feet.
{44} Quoted by Cohn, in his remarkable memoir, "Contractile Gewebe
im Pflanzenreiche," 'Abhandl. der Schlesischen Gesell. 1861, Heft i.
s. 35.
{45} 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 Muller 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.
{46} 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.
{47} Annales des Sc. Nat. 4th series, Bot. tom. vi. 1856, p. 31.
{48} Moquin-Tandon (Elements de Teratologie. 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.
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