| By Author | [ A B C D E F G H I J K L M N O P Q R S T U V W X Y Z | Other Symbols ] |
| By Title | [ A B C D E F G H I J K L M N O P Q R S T U V W X Y Z | Other Symbols ] |
| By Language |
|
Download this book: [ ASCII | HTML | PDF ] Look for this book on Amazon Tweet |
Title: The Movements and Habits of Climbing Plants
Author: Darwin, Charles
Language: English
As this book started as an ASCII text book there are no pictures available.
*** Start of this LibraryBlog Digital Book "The Movements and Habits of Climbing Plants" ***
This book is indexed by ISYS Web Indexing system to allow the reader find any word or number within the document.
CLIMBING PLANTS***
Transcribed from the 1906 John Murray edition by David Price, email
ccx074@pglaf.org
[Picture: Book cover]
THE MOVEMENTS
AND HABITS OF
CLIMBING PLANTS.
BY CHARLES DARWIN, F.R.S.
* * * * *
POPULAR EDITION
* * * * *
LONDON:
JOHN MURRAY, ALBEMARLE STREET.
1906.
PREFACE
THIS Essay first appeared in the ninth volume of the ‘Journal of the
Linnean Society,’ published in 1865. It is here reproduced in a
corrected and, I hope, clearer form, with some additional facts. The
illustrations were drawn by my son, George Darwin. Fritz Müller, after
the publication of my paper, sent to the Linnean Society (Journal, vol.
ix., p. 344) some interesting observations on the climbing plants of
South Brazil, to which I shall frequently refer. Recently two important
memoirs, chiefly on the difference in growth between the upper and lower
sides of tendrils, and on the mechanism of the movements of
twining-plants, by Dr. Hugo de Vries, have appeared in the ‘Arbeiten des
Botanischen Instituts in Würzburg,’ Heft. iii., 1873. These memoirs
ought to be carefully studied by every one interested in the subject, as
I can here give only references to the more important points. This
excellent observer, as well as Professor Sachs, {iv} attributes all the
movements of tendrils to rapid growth along one side; but, from reasons
assigned towards the close of my fourth chapter, I cannot persuade myself
that this holds good with respect to those due to a touch. In order that
the reader may know what points have interested me most, I may call his
attention to certain tendril-bearing plants; for instance, Bignonia
capreolata, Cobæa, Echinocystis, and Hanburya, which display as beautiful
adaptations as can be found in any part of the kingdom of nature. It is,
also, an interesting fact that intermediate states between organs fitted
for widely different functions, may be observed on the same individual
plant of Corydalis claviculata and the common vine; and these cases
illustrate in a striking manner the principle of the gradual evolution of
species.
APPENDIX TO PREFACE (1882).
SINCE the publication of this Edition two papers by eminent botanists
have appeared; Schwendener, ‘Das Winden der Pflanzen’ (Monatsberichte der
Berliner Akademie, Dec. 1881), and J. Sachs, ‘Notiz über Schlingpflanzen’
(Arbeiten des botanischen Instituts in Würzburg, Bd. ii. p. 719, 1882).
The view “that the capacity of revolving, on which most climbers depend,
is inherent, though undeveloped, in almost every plant in the vegetable
kingdom” (‘Climbing Plants,’ p. 205), has been confirmed by the
observations on circumnutation since given in ‘The Power of Movement in
Plants.’
ERRATA.
On pp. 28, 32, 40, 53, statements are made with reference to the supposed
acceleration of the revolving movement towards the light. It appears
from the observations given in ‘The Power of Movement in Plants,’ p. 451,
that these conclusions were drawn from insufficient observations, and are
erroneous.
THE MOVEMENTS AND HABITS OF CLIMBING PLANTS.
CHAPTER I.
TWINING PLANTS.
Introductory remarks—Description of the twining of the Hop—Torsion of the
stems—Nature of the revolving movement, and manner of ascent—Stems not
irritable—Rate of revolution in various plants—Thickness of the support
round which plants can twine—Species which revolve in an anomalous
manner.
I WAS led to this subject by an interesting, but short paper by Professor
Asa Gray on the movements of the tendrils of some Cucurbitaceous plants.
{1a} My observations were more than half completed before I learnt that
the surprising phenomenon of the spontaneous revolutions of the stems and
tendrils of climbing plants had been long ago observed by Palm and by
Hugo von Mohl, {1b} and had subsequently been the subject of two memoirs
by Dutrochet. {1c} Nevertheless, I believe that my observations, founded
on the examination of above a hundred widely distinct living species,
contain sufficient novelty to justify me in publishing them.
Climbing plants may be divided into four classes. First, those which
twine spirally round a support, and are not aided by any other movement.
Secondly, those endowed with irritable organs, which when they touch any
object clasp it; such organs consisting of modified leaves, branches, or
flower-peduncles. But these two classes sometimes graduate to a certain
extent into one another. Plants of the third class ascend merely by the
aid of hooks; and those of the fourth by rootlets; but as in neither
class do the plants exhibit any special movements, they present little
interest, and generally when I speak of climbing plants I refer to the
two first great classes.
TWINING PLANTS.
This is the largest subdivision, and is apparently the primordial and
simplest condition of the class. My observations will be best given by
taking a few special cases. When the shoot of a Hop (_Humulus lupulus_)
rises from the ground, the two or three first-formed joints or internodes
are straight and remain stationary; but the next-formed, whilst very
young, may be seen to bend to one side and to travel slowly round towards
all points of the compass, moving, like the hands of a watch, with the
sun. The movement very soon acquires its full ordinary velocity. From
seven observations made during August on shoots proceeding from a plant
which had been cut down, and on another plant during April, the average
rate during hot weather and during the day is 2 hrs. 8 m. for each
revolution; and none of the revolutions varied much from this rate. The
revolving movement continues as long as the plant continues to grow; but
each separate internode, as it becomes old, ceases to move.
To ascertain more precisely what amount of movement each internode
underwent, I kept a potted plant, during the night and day, in a
well-warmed room to which I was confined by illness. A long shoot
projected beyond the upper end of the supporting stick, and was steadily
revolving. I then took a longer stick and tied up the shoot, so that
only a very young internode, 1¾ of an inch in length, was left free.
This was so nearly upright that its revolution could not be easily
observed; but it certainly moved, and the side of the internode which was
at one time convex became concave, which, as we shall hereafter see, is a
sure sign of the revolving movement. I will assume that it made at least
one revolution during the first twenty-four hours. Early the next
morning its position was marked, and it made a second revolution in 9
hrs.; during the latter part of this revolution it moved much quicker,
and the third circle was performed in the evening in a little over 3 hrs.
As on the succeeding morning I found that the shoot revolved in 2 hrs. 45
m., it must have made during the night four revolutions, each at the
average rate of a little over 3 hrs. I should add that the temperature
of the room varied only a little. The shoot had now grown 3½ inches in
length, and carried at its extremity a young internode 1 inch in length,
which showed slight changes in its curvature. The next or ninth
revolution was effected in 2 hrs. 30 m. From this time forward, the
revolutions were easily observed. The thirty-sixth revolution was
performed at the usual rate; so was the last or thirty-seventh, but it
was not completed; for the internode suddenly became upright, and after
moving to the centre, remained motionless. I tied a weight to its upper
end, so as to bow it slightly and thus detect any movement; but there was
none. Some time before the last revolution was half performed, the lower
part of the internode ceased to move.
A few more remarks will complete all that need be said about this
internode. It moved during five days; but the more rapid movements,
after the performance of the third revolution, lasted during three days
and twenty hours. The regular revolutions, from the ninth to
thirty-sixth inclusive, were effected at the average rate of 2 hrs. 31
m.; but the weather was cold, and this affected the temperature of the
room, especially during the night, and consequently retarded the rate of
movement a little. There was only one irregular movement, which
consisted in the stem rapidly making, after an unusually slow revolution,
only the segment of a circle. After the seventeenth revolution the
internode had grown from 1¾ to 6 inches in length, and carried an
internode 1⅞ inch long, which was just perceptibly moving; and this
carried a very minute ultimate internode. After the twenty-first
revolution, the penultimate internode was 2½ inches long, and probably
revolved in a period of about three hours. At the twenty-seventh
revolution the lower and still moving internode was 8⅜, the penultimate
3½, and the ultimate 2½ inches in length; and the inclination of the
whole shoot was such, that a circle 19 inches in diameter was swept by
it. When the movement ceased, the lower internode was 9 inches, and the
penultimate 6 inches in length; so that, from the twenty-seventh to
thirty-seventh revolutions inclusive, three internodes were at the same
time revolving.
The lower internode, when it ceased revolving, became upright and rigid;
but as the whole shoot was left to grow unsupported, it became after a
time bent into a nearly horizontal position, the uppermost and growing
internodes still revolving at the extremity, but of course no longer
round the old central point of the supporting stick. From the changed
position of the centre of gravity of the extremity, as it revolved, a
slight and slow swaying movement was given to the long horizontally
projecting shoot; and this movement I at first thought was a spontaneous
one. As the shoot grew, it hung down more and more, whilst the growing
and revolving extremity turned itself up more and more.
With the Hop we have seen that three internodes were at the same time
revolving; and this was the case with most of the plants observed by me.
With all, if in full health, two internodes revolved; so that by the time
the lower one ceased to revolve, the one above was in full action, with a
terminal internode just commencing to move. With _Hoya carnosa_, on the
other hand, a depending shoot, without any developed leaves, 32 inches in
length, and consisting of seven internodes (a minute terminal one, an
inch in length, being counted), continually, but slowly, swayed from side
to side in a semicircular course, with the extreme internodes making
complete revolutions. This swaying movement was certainly due to the
movement of the lower internodes, which, however, had not force
sufficient to swing the whole shoot round the central supporting stick.
The case of another Asclepiadaceous plant, viz., _Ceropegia Gardnerii_,
is worth briefly giving. I allowed the top to grow out almost
horizontally to the length of 31 inches; this now consisted of three long
internodes, terminated by two short ones. The whole revolved in a course
opposed to the sun (the reverse of that of the Hop), at rates between 5
hrs. 15 m. and 6 hrs. 45 m. for each revolution. The extreme tip thus
made a circle of above 5 feet (or 62 inches) in diameter and 16 feet in
circumference, travelling at the rate of 32 or 33 inches per hour. The
weather being hot, the plant was allowed to stand on my study-table; and
it was an interesting spectacle to watch the long shoot sweeping this
grand circle, night and day, in search of some object round which to
twine.
If we take hold of a growing sapling, we can of course bend it to all
sides in succession, so as to make the tip describe a circle, like that
performed by the summit of a spontaneously revolving plant. By this
movement the sapling is not in the least twisted round its own axis. I
mention this because if a black point be painted on the bark, on the side
which is uppermost when the sapling is bent towards the holder’s body, as
the circle is described, the black point gradually turns round and sinks
to the lower side, and comes up again when the circle is completed; and
this gives the false appearance of twisting, which, in the case of
spontaneously revolving plants, deceived me for a time. The appearance
is the more deceitful because the axes of nearly all twining-plants are
really twisted; and they are twisted in the same direction with the
spontaneous revolving movement. To give an instance, the internode of
the Hop of which the history has been recorded, was at first, as could be
seen by the ridges on its surface, not in the least twisted; but when,
after the 37th revolution, it had grown 9 inches long, and its revolving
movement had ceased, it had become twisted three times round its own
axis, in the line of the course of the sun; on the other hand, the common
Convolvulus, which revolves in an opposite course to the Hop, becomes
twisted in an opposite direction.
Hence it is not surprising that Hugo von Mohl (p. 105, 108, &c.) thought
that the twisting of the axis caused the revolving movement; but it is
not possible that the twisting of the axis of the Hop three times should
have caused thirty-seven revolutions. Moreover, the revolving movement
commenced in the young internode before any twisting of its axis could be
detected. The internodes of a young Siphomeris and Lecontea revolved
during several days, but became twisted only once round their own axes.
The best evidence, however, that the twisting does not cause the
revolving movement is afforded by many leaf-climbing and tendril-bearing
plants (as _Pisum sativum_, _Echinocystis lobata_, _Bignonia capreolata_,
_Eccremocarpus scaber_, and with the leaf-climbers, _Solanum jasminoides_
and various species of _Clematis_), of which the internodes are not
twisted, but which, as we shall hereafter see, regularly perform
revolving movements like those of true twining-plants. Moreover,
according to Palm (pp. 30, 95) and Mohl (p. 149), and Léon, {8}
internodes may occasionally, and even not very rarely, be found which are
twisted in an opposite direction to the other internodes on the same
plant, and to the course of their revolutions; and this, according to
Léon (p. 356), is the case with all the internodes of a certain variety
of _Phaseolus multiflorus_. Internodes which have become twisted round
their own axes, if they have not ceased to revolve, are still capable of
twining round a support, as I have several times observed.
Mohl has remarked (p. 111) that when a stem twines round a smooth
cylindrical stick, it does not become twisted. {9a} Accordingly I
allowed kidney-beans to run up stretched string, and up smooth rods of
iron and glass, one-third of an inch in diameter, and they became twisted
only in that degree which follows as a mechanical necessity from the
spiral winding. The stems, on the other hand, which had ascended
ordinary rough sticks were all more or less and generally much twisted.
The influence of the roughness of the support in causing axial twisting
was well seen in the stems which had twined up the glass rods; for these
rods were fixed into split sticks below, and were secured above to cross
sticks, and the stems in passing these places became much twisted. As
soon as the stems which had ascended the iron rods reached the summit and
became free, they also became twisted; and this apparently occurred more
quickly during windy than during calm weather. Several other facts could
be given, showing that the axial twisting stands in some relation to
inequalities in the support, and likewise to the shoot revolving freely
without any support. Many plants, which are not twiners, become in some
degree twisted round their own axes; {9b} but this occurs so much more
generally and strongly with twining-plants than with other plants, that
there must be some connexion between the capacity for twining and axial
twisting. The stem probably gains rigidity by being twisted (on the same
principle that a much twisted rope is stiffer than a slackly twisted
one), and is thus indirectly benefited so as to be enabled to pass over
inequalities in its spiral ascent, and to carry its own weight when
allowed to revolve freely. {10}
I have alluded to the twisting which necessarily follows on mechanical
principles from the spiral ascent of a stem, namely, one twist for each
spire completed. This was well shown by painting straight lines on
living stems, and then allowing them to twine; but, as I shall have to
recur to this subject under Tendrils, it may be here passed over.
The revolving movement of a twining plant has been compared with that of
the tip of a sapling, moved round and round by the hand held some way
down the stem; but there is one important difference. The upper part of
the sapling when thus moved remains straight; but with twining plants
every part of the revolving shoot has its own separate and independent
movement. This is easily proved; for when the lower half or two-thirds
of a long revolving shoot is tied to a stick, the upper free part
continues steadily revolving. Even if the whole shoot, except an inch or
two of the extremity, be tied up, this part, as I have seen in the case
of the Hop, Ceropegia, Convolvulus, &c., goes on revolving, but much more
slowly; for the internodes, until they have grown to some little length,
always move slowly. If we look to the one, two, or several internodes of
a revolving shoot, they will be all seen to be more or less bowed, either
during the whole or during a large part of each revolution. Now if a
coloured streak be painted (this was done with a large number of twining
plants) along, we will say, the convex surface, the streak will after a
time (depending on the rate of revolution) be found to be running
laterally along one side of the bow, then along the concave side, then
laterally on the opposite side, and, lastly, again on the originally
convex surface. This clearly proves that during the revolving movement
the internodes become bowed in every direction. The movement is, in
fact, a continuous self-bowing of the whole shoot, successively directed
to all points of the compass; and has been well designated by Sachs as a
revolving nutation.
As this movement is rather difficult to understand, it will be well to
give an illustration. Take a sapling and bend it to the south, and paint
a black line on the convex surface; let the sapling spring up and bend it
to the east, and the black line will be seen to run along the lateral
face fronting the north; bend it to the north, the black line will be on
the concave surface; bend it to the west, the line will again be on the
lateral face; and when again bent to the south, the line will be on the
original convex surface. Now, instead of bending the sapling, let us
suppose that the cells along its northern surface from the base to the
tip were to grow much more rapidly than on the three other sides, the
whole shoot would then necessarily be bowed to the south; and let the
longitudinal growing surface creep round the shoot, deserting by slow
degrees the northern side and encroaching on the western side, and so
round by the south, by the east, again to the north. In this case the
shoot would remain always bowed with the painted line appearing on the
several above specified surfaces, and with the point of the shoot
successively directed to each point of the compass. In fact, we should
have the exact kind of movement performed by the revolving shoots of
twining plants. {12}
It must not be supposed that the revolving movement is as regular as that
given in the above illustration; in very many cases the tip describes an
ellipse, even a very narrow ellipse. To recur once again to our
illustration, if we suppose only the northern and southern surfaces of
the sapling alternately to grow rapidly, the summit would describe a
simple arc; if the growth first travelled a very little to the western
face, and during the return a very little to the eastern face, a narrow
ellipse would be described; and the sapling would be straight as it
passed to and fro through the intermediate space; and a complete
straightening of the shoot may often be observed in revolving plants.
The movement is frequently such that three of the sides of the shoot seem
to be growing in due order more rapidly than the remaining side; so that
a semi-circle instead of a circle is described, the shoot becoming
straight and upright during half of its course.
When a revolving shoot consists of several internodes, the lower ones
bend together at the same rate, but one or two of the terminal ones bend
at a slower rate; hence, though at times all the internodes are in the
same direction, at other times the shoot is rendered slightly serpentine.
The rate of revolution of the whole shoot, if judged by the movement of
the extreme tip, is thus at times accelerated or retarded. One other
point must be noticed. Authors have observed that the end of the shoot
in many twining plants is completely hooked; this is very general, for
instance, with the Asclepiadaceæ. The hooked tip, in all the cases
observed by me, viz. in _Ceropegia_, _Sphærostemma_, _Clerodendron_,
_Wistaria_, _Stephania_, _Akebia_, and _Siphomeris_, has exactly the same
kind of movement as the other internodes; for a line painted on the
convex surface first becomes lateral and then concave; but, owing to the
youth of these terminal internodes, the reversal of the hook is a slower
process than that of the revolving movement. {14} This strongly marked
tendency in the young, terminal and flexible internodes, to bend in a
greater degree or more abruptly than the other internodes, is of service
to the plant; for not only does the hook thus formed sometimes serve to
catch a support, but (and this seems to be much more important) it causes
the extremity of the shoot to embrace the support much more closely than
it could otherwise have done, and thus aids in preventing the stem from
being blown away during windy weather, as I have many times observed. In
_Lonicera brachypoda_ the hook only straightens itself periodically, and
never becomes reversed. I will not assert that the tips of all twining
plants when hooked, either reverse themselves or become periodically
straight, in the manner just described; for the hooked form may in some
cases be permanent, and be due to the manner of growth of the species, as
with the tips of the shoots of the common vine, and more plainly with
those of _Cissus discolor_—plants which are not spiral twiners.
The first purpose of the spontaneous revolving movement, or, more
strictly speaking, of the continuous bowing movement directed
successively to all points of the compass, is, as Mohl has remarked, to
favour the shoot finding a support. This is admirably effected by the
revolutions carried on night and day, a wider and wider circle being
swept as the shoot increases in length. This movement likewise explains
how the plants twine; for when a revolving shoot meets with a support,
its motion is necessarily arrested at the point of contact, but the free
projecting part goes on revolving. As this continues, higher and higher
points are brought into contact with the support and are arrested; and so
onwards to the extremity; and thus the shoot winds round its support.
When the shoot follows the sun in its revolving course, it winds round
the support from right to left, the support being supposed to stand in
front of the beholder; when the shoot revolves in an opposite direction,
the line of winding is reversed. As each internode loses from age its
power of revolving, it likewise loses its power of spirally twining. If
a man swings a rope round his head, and the end hits a stick, it will
coil round the stick according to the direction of the swinging movement;
so it is with a twining plant, a line of growth travelling round the free
part of the shoot causing it to bend towards the opposite side, and this
replaces the momentum of the free end of the rope.
All the authors, except Palm and Mohl, who have discussed the spiral
twining of plants, maintain that such plants have a natural tendency to
grow spirally. Mohl believes (p. 112) that twining stems have a dull
kind of irritability, so that they bend towards any object which they
touch; but this is denied by Palm. Even before reading Mohl’s
interesting treatise, this view seemed to me so probable that I tested it
in every way that I could, but always with a negative result. I rubbed
many shoots much harder than is necessary to excite movement in any
tendril or in the foot-stalk of any leaf climber, but without any effect.
I then tied a light forked twig to a shoot of a Hop, a _Ceropegia_,
_Sphærostemma_, and _Adhatoda_, so that the fork pressed on one side
alone of the shoot and revolved with it; I purposely selected some very
slow revolvers, as it seemed most likely that these would profit most
from possessing irritability; but in no case was any effect produced.
{16} Moreover, when a shoot winds round a support, the winding movement
is always slower, as we shall immediately see, than whilst it revolves
freely and touches nothing. Hence I conclude that twining stems are not
irritable; and indeed it is not probable that they should be so, as
nature always economizes her means, and irritability would have been
superfluous. Nevertheless I do not wish to assert that they are never
irritable; for the growing axis of the leaf-climbing, but not spirally
twining, _Lophospermum scandens_ is, certainly irritable; but this case
gives me confidence that ordinary twiners do not possess any such
quality, for directly after putting a stick to the _Lophopermum_, I saw
that it behaved differently from a true twiner or any other leaf-climber.
{17}
The belief that twiners have a natural tendency to grow spirally,
probably arose from their assuming a spiral form when wound round a
support, and from the extremity, even whilst remaining free, sometimes
assuming this form. The free internodes of vigorously growing plants,
when they cease to revolve, become straight, and show no tendency to be
spiral; but when a shoot has nearly ceased to grow, or when the plant is
unhealthy, the extremity does occasionally become spiral. I have seen
this in a remarkable manner with the ends of the shoots of the
_Stauntonia_ and of the allied _Akebia_, which became wound up into a
close spire, just like a tendril; and this was apt to occur after some
small, ill-formed leaves had perished. The explanation, I believe, is,
that in such cases the lower parts of the terminal internodes very
gradually and successively lose their power of movement, whilst the
portions just above move onwards and in their turn become motionless; and
this ends in forming an irregular spire.
When a revolving shoot strikes a stick, it winds round it rather more
slowly than it revolves. For instance, a shoot of the _Ceropegia_,
revolved in 6 hrs., but took 9 hrs. 30 m. to make one complete spire
round a stick; _Aristolochia gigas_ revolved in about 5 hrs., but took 9
hrs. 15 m. to complete its spire. This, I presume, is due to the
continued disturbance of the impelling force by the arrestment of the
movement at successive points; and we shall hereafter see that even
shaking a plant retards the revolving movement. The terminal internodes
of a long, much-inclined, revolving shoot of the _Ceropegia_, after they
had wound round a stick, always slipped up it, so as to render the spire
more open than it was at first; and this was probably in part due to the
force which caused the revolutions, being now almost freed from the
constraint of gravity and allowed to act freely. With the _Wistaria_, on
the other hand, a long horizontal shoot wound itself at first into a very
close spire, which remained unchanged; but subsequently, as the shoot
twined spirally up its support, it made a much more open spire. With all
the many plants which were allowed freely to ascend a support, the
terminal internodes made at first a close spire; and this, during windy
weather, served to keep the shoots in close contact with their support;
but as the penultimate internodes grew in length, they pushed themselves
up for a considerable space (ascertained by coloured marks on the shoot
and on the support) round the stick, and the spire became more open. {18}
It follows from this latter fact that the position occupied by each leaf
with respect to the support depends on the growth of the internodes after
they have become spirally wound round it. I mention this on account of
an observation by Palm (p. 34), who states that the opposite leaves of
the Hop always stand in a row, exactly over one another, on the same side
of the supporting stick, whatever its thickness may be. My sons visited
a hop-field for me, and reported that though they generally found the
points of insertion of the leaves standing over each other for a space of
two or three feet in height, yet this never occurred up the whole length
of the pole; the points of insertion forming, as might have been
expected, an irregular spire. Any irregularity in the pole entirely
destroyed the regularity of position of the leaves. From casual
inspection, it appeared to me that the opposite leaves of _Thunbergia
alata_ were arranged in lines up the sticks round which they had twined;
accordingly, I raised a dozen plants, and gave them sticks of various
thicknesses, as well as string, to twine round; and in this case one
alone out of the dozen had its leaves arranged in a perpendicular line: I
conclude, therefore, Palm’s statement is not quite accurate.
The leaves of different twining-plants are arranged on the stem (before
it has twined) alternately, or oppositely, or in a spire. In the latter
case the line of insertion of the leaves and the course of the
revolutions coincide. This fact has been well shown by Dutrochet, {19}
who found different individuals of _Solanum dulcamara_ twining in
opposite directions, and these had their leaves in each case spirally
arranged in the same direction. A dense whorl of many leaves would
apparently be incommodious for a twining plant, and some authors assert
that none have their leaves thus arranged; but a twining _Siphomeris_ has
whorls of three leaves.
If a stick which has arrested a revolving shoot, but has not as yet been
encircled, be suddenly taken away, the shoot generally springs forward,
showing that it was pressing with some force against the stick. After a
shoot has wound round a stick, if this be withdrawn, it retains for a
time its spiral form; it then straightens itself, and again commences to
revolve. The long, much-inclined shoot of the _Ceropegia_ previously
alluded to offered some curious peculiarities. The lower and older
internodes, which continued to revolve, were incapable, on repeated
trials, of twining round a thin stick; showing that, although the power
of movement was retained, this was not sufficient to enable the plant to
twine. I then moved the stick to a greater distance, so that it was
struck by a point 2½ inches from the extremity of the penultimate
internode; and it was then neatly encircled by this part of the
penultimate and by the ultimate internode. After leaving the spirally
wound shoot for eleven hours, I quietly withdrew the stick, and in the
course of the day the curled portion straightened itself and recommenced
revolving; but the lower and not curled portion of the penultimate
internode did not move, a sort of hinge separating the moving and the
motionless part of the same internode. After a few days, however, I
found that this lower part had likewise recovered its revolving power.
These several facts show that the power of movement is not immediately
lost in the arrested portion of a revolving shoot; and that after being
temporarily lost it can be recovered. When a shoot has remained for a
considerable time round a support, it permanently retains its spiral form
even when the support is removed.
When a tall stick was placed so as to arrest the lower and rigid
internodes of the _Ceropegia_, at the distance at first of 15 and then of
21 inches from the centre of revolution, the straight shoot slowly and
gradually slid up the stick, so as to become more and more highly
inclined, but did not pass over the summit. Then, after an interval
sufficient to have allowed of a semi-revolution, the shoot suddenly
bounded from the stick and fell over to the opposite side or point of the
compass, and reassumed its previous slight inclination. It now
recommenced revolving in its usual course, so that after a
semi-revolution it again came into contact with the stick, again slid up
it, and again bounded from it and fell over to the opposite side. This
movement of the shoot had a very odd appearance, as if it were disgusted
with its failure but was resolved to try again. We shall, I think,
understand this movement by considering the former illustration of the
sapling, in which the growing surface was supposed to creep round from
the northern by the western to the southern face; and thence back again
by the eastern to the northern face, successively bowing the sapling in
all directions. Now with the _Ceropegia_, the stick being placed to the
south of the shoot and in contact with it, as soon as the circulatory
growth reached the western surface, no effect would be produced, except
that the shoot would be pressed firmly against the stick. But as soon as
growth on the southern surface began, the shoot would be slowly dragged
with a sliding movement up the stick; and then, as soon as the eastern
growth commenced, the shoot would be drawn from the stick, and its weight
coinciding with the effects of the changed surface of growth, would cause
it suddenly to fall to the opposite side, reassuming its previous slight
inclination; and the ordinary revolving movement would then go on as
before. I have described this curious case with some care, because it
first led me to understand the order in which, as I then thought, the
surfaces contracted; but in which, as we now know from Sachs and II. de
Vries, they grow for a time rapidly, thus causing the shoot to bow
towards the opposite side.
The view just given further explains, as I believe, a fact observed by
Mohl (p. 135), namely, that a revolving shoot, though it will twine round
an object as thin as a thread, cannot do so round a thick support. I
placed some long revolving shoots of a _Wistaria_ close to a post between
5 and 6 inches in diameter, but, though aided by me in many ways, they
could not wind round it. This apparently was due to the flexure of the
shoot, whilst winding round an object so gently curved as this post, not
being sufficient to hold the shoot to its place when the growing surface
crept round to the opposite surface of the shoot; so that it was
withdrawn at each revolution from its support.
When a free shoot has grown far beyond its support, it sinks downwards
from its weight, as already explained in the case of the Hop, with the
revolving extremity turned upwards. If the support be not lofty, the
shoot falls to the ground, and resting there, the extremity rises up.
Sometimes several shoots, when flexible, twine together into a cable, and
thus support one another. Single thin depending shoots, such as those of
the _Sollya Drummondii_, will turn abruptly backwards and wind up on
themselves. The greater number of the depending shoots, however, of one
twining plant, the _Hibbertia dentata_, showed but little tendency to
turn upwards. In other cases, as with the _Cryptostegia grandiflora_,
several internodes which were at first flexible and revolved, if they did
not succeed in twining round a support, become quite rigid, and
supporting themselves upright, carried on their summits the younger
revolving internodes.
Here will be a convenient place to give a Table showing the direction and
rate of movement of several twining plants, with a few appended remarks.
These plants are arranged according to Lindley’s ‘Vegetable Kingdom’ of
1853; and they have been selected from all parts of the series so as to
show that all kinds behave in a nearly uniform manner. {24}
The Rate of Revolution of various Twining Plants.
(ACOTYLEDONS.)
_Lygodium scandens_ (Polypodiaceæ) moves against the sun.
H. M.
June 18, 1st circle was made in 6 0
18, 2nd 6 15 (late in evening)
19, 3rd 5 32 (very hot day)
19, 4th 5 0 (very hot day)
20, 5th 6 0
_Lygodium articulatum_ moves against the sun.
H. M.
July 19, 1st circle was made in 16 30 (shoot very young)
20, 2nd 15 0
21, 3rd 8 0
22, 4th 10 30
(MONOCOTYLEDONS.)
_Ruscus androgynus_ (Liliaceæ), placed in the hot-house, moves against
the sun.
H. M.
May 24, 1st circle was made in 6 14 (shoot very young)
25, 2nd 2 21
25, 3rd 3 37
25, 4th 3 22
26, 5th 2 50
27, 6th 3 52
27, 7th 4 11
_Asparagus_ (unnamed species from Kew) (Liliaceæ) moves against the sun,
placed in hothouse.
H. M.
Dec. 26, 1st circle was made in 5 0
27, 2nd 5 40
_Tamus communis_ (Dioscoreaceæ). A young shoot from a tuber in a pot
placed in the greenhouse: follows the sun.
H. M.
July, 7, 1st circle was made in 3 10
7, 2nd 2 38
8, 3rd 3 5
8, 4th 2 56
8, 5th 2 30
8, 6th 2 30
_Lapagerea rosea_ (Philesiaceæ), in greenhouse, follows the sun.
H. M.
March 9, 1st circle was made in 26 15 (shoot young)
10, semicircle 8 15
11, 2nd circle 11 0
12, 3rd 15 30
13, 4th 14 15
16, 5th 8 40 when placed in the
hothouse; but the
next day the shoot
remained
stationary.
_Roxburghia viridiflora_ (Roxburghiaceæ) moves against the sun; it
completed a circle in about 24 hours.
(DICOTYLEDONS.)
_Humulus Lupulus_ (Urticaceæ) follows the sun. The plant was kept in a
room during warm weather.
H. M.
April 9, 2 circles were made in 4 16
Aug. 13, 3rd circle was 2 0
14, 4th 2 20
14, 5th 2 16
14, 6th 2 2
14, 7th 2 0
14, 8th 2 4
With the Hop a semicircle was performed, in travelling from the light, in
1 hr. 33 m.; in travelling to the light, in 1 hr. 13 m.; difference of
rate, 20 m.
_Akebia quinata_ (Lardizabalaceæ), placed in hothouse, moves against the
sun.
H. M.
March 17, 1st circle was made in 4 0 (shoot young)
18, 2nd 1 40
18, 3rd 1 30
19, 4th 1 45
_Stauntonia latifolia_ (Lardizabalaceæ), placed in hothouse, moves
against the sun.
H. M.
March 28, 1st circle was made in 3 30
29, 2nd 3 45
_Sphærostemma marmoratum_ (Schizandraceæ) follows the sun.
H. M.
August 5th, 1st circle was made in about 24 0
5th, 2nd circle was made in 18 30
_Stephania rotunda_ (Menispermaceæ) moves against the sun.
H. M.
May 27, 1st circle was made in 5 5
30, 2nd 7 6
June 2, 3rd 5 15
3, 4th 6 28
Thryallis brachystachys (Malpighiaceæ) moves against the sun: one shoot
made a circle in 12 hrs., and another in 10 hrs. 30 m.; but the next day,
which was much colder, the first shoot took 10 hrs. to perform only a
semicircle.
Hibbertia dentata (Dilleniaceæ), placed in the hothouse, followed the
sun, and made (May 18th) a circle in 7 hrs. 20 m.; on the 19th, reversed
its course, and moved against the sun, and made a circle in 7 hrs.; on
the 20th, moved against the sun one-third of a circle, and then stood
still; on the 26th, followed the sun for two-thirds of a circle, and then
returned to its starting-point, taking for this double course 11 hrs. 46
m.
_Sollya Drummondii_ (Pittosporaceæ) moves against the sun kept in
greenhouse.
H. M.
April 4, 1st circle was made in 4 25
5, 2nd 8 0 (very cold day)
6, 3rd 6 25
7, 4th 7 5
_Polygonum dumetorum_ (Polygonaceæ). This case is taken from Dutrochet
(p. 299), as I observed, no allied plant: follows the sun. Three
shoots, cut off a plant, and placed in water made circles in 3 hrs. 10
m., 5 hrs. 20 m., and 7 hrs. 15 m.
_Wistaria Chinensis_ (Leguminosæ), in greenhouse, moves against the sun.
H. M.
May 13, 1st circle was made in 3 5
13, 2nd 3 20
16, 3rd 2 5
24, 4th 3 21
25, 5th 2 37
25, 6th 2 35
_Phaseolus vulgaris_ (Leguminosæ), in greenhouse, moves against the sun.
H. M.
May, 1st circle was made in 2 0
2nd 1 55
3rd 1 55
_Dipladenia urophylla_ (Apocynaceæ) moves against the sun.
H. M.
April 18, 1st circle was made in 8 0
19, 2nd 9 15
30, 3rd 9 40
_Dipladenia crassinoda_ moves against the sun.
H. M.
May 16, 1st circle was made in 9 5
July 20, 2nd 8 0
21, 3rd 8 5
_Ceropegia Gardnerii_ (Asclepiadaceæ) moves against the sun.
H. M.
Shoot very young, 2 inches in 1st circle was 7 55
length performed in
Shoot still young 2nd 7 0
Long shoot 3rd 6 33
Long shoot 4th 5 15
Long shoot 5th 6 45
_Stephanotis floribunda_ (Asclepiadaceæ) moves against the sun and made a
circle in 6 hrs. 40 m., a second circle in about 9 hrs.
_Hoya carnosa_ (Asclepiadaceæ) made several circles in from 16 hrs. to 22
hrs. or 24 hrs.
_Ipomæa purpurea_ (Convolvulaceæ) moves against the sun. Plant placed in
room with lateral light.
1st circle was made in 2 hrs. 42 Semicircle, from the light in 1
m. hr. 14 m., to the light 1 hr. 28
m.: difference 14 m.
2nd circle was made in 2 hrs. 47 Semicircle, from the light in 1
m. hr. 17 m., to the light 1 hr. 30
m.: difference 13 m.
_Ipomæa jucunda_ (Convolvulaceæ) moves against the sun, placed in my
study, with windows facing the north-east. Weather hot.
1st circle was made in 5 hrs. 30 Semicircle, from the light in 4
m. hrs. 30 m., to the light 1 hr. 0
m.: difference 3 hrs. 30 m.
2nd circle was made in 5 hrs. 20 Semicircle, from the light in 3
m. (Late in afternoon: circle hrs. 50 m., to the light 1 hr. 30
completed at 6 hrs. 40 m. P.M.) m.: difference 2 hrs. 20 m.
We have here a remarkable instance of the power of light in retarding and
hastening the revolving movement. (_See_ ERRATA.)
_Convolvulus sepium_ (large-flowered cultivated var.) moves against the
sun. Two circles, were made each in 1 hr. 42 m.: difference in
semicircle from and to the light 14 m.
_Rivea tiliæfolia_ (Convolvulaceæ) moves against the sun, made four
revolutions in 9 hrs.; so that, on an average, each was performed in 2
hrs. 15 m.
_Plumbago rosea_ (Plumbaginaceæ) follows the sun. The shoot did not
begin to revolve until nearly a yard in height; it then made a fine
circle in 10 hrs. 45 m. During the next few days it continued to move,
but irregularly. On August 15th the shoot followed, during a period of
10 hrs. 40 m., a long and deeply zigzag course and then made a broad
ellipse. The figure apparently represented three ellipses, each of which
averaged 3 hrs. 38 m. for its completion.
_Jasminum pauciflorum_, Bentham (Jasminaceæ), moves against the sun. A
circle was made in 7 hrs. 15 m., and a second rather more quickly.
_Clerodendrum Thomsonii_ (Verbenaceæ) follows the sun.
H. M.
April 12, 1st circle was made in 5 45 (shoot very young)
14, 2nd 3 30
18, a semicircle 5 0 (directly after
the plant was
shaken on being
moved)
19, 3rd circle 3 0
20, 4th 4 20
_Tecoma jasminoides_ (Bignoniaceæ) moves against the sun.
H. M.
March 17, 1st circle was made in 6 30
19, 2nd 7 0
22, 3rd 8 30 (very cold day)
24, 4th 6 45
_Thunbergia alata_ (Acanthaceæ) moves against sun.
H. M.
April 14, 1st circle was made in 3 20
18, 2nd 2 50
18, 3rd 2 55
18, 4th 3 55 (late in
afternoon)
_Adhadota cydonæfolia_ (Acanthaceæ) follows the sun. A young shoot made
a semicircle in 24 hrs.; subsequently it made a circle in between 40 hrs.
and 48 hrs. Another shoot, however, made a circle in 26 hrs. 30 m.
_Mikania scandens_ (Compositæ) moves against the sun.
H. M.
March 14, 1st circle was made in 3 10
15, 2nd 3 0
16, 3rd 3 0
17, 4th 3 33
April 7, 5th 2 50
7, 6th 2 40 This circle was
made after a
copious watering
with cold water at
47° Fahr.
_Combretum argenteum_ (Combretaceæ) moves against the sun. Kept in
hothouse.
H. M.
Jan. 24, 1st circle was made in 2 55 Early in
morning, when
the temperature
of the house
had fallen a
little.
24, 2 circles each at an 2 20
average of
25, 4th circle was made in 2 25
_Combretum purpureum_ revolves not quite so quickly as _C. argenteum_.
_Loasa aurantiaca_ (Loasaceæ). Revolutions variable in their course: a
plant which moved against the sun.
H. M.
June 20, 1st circle was made in 2 37
20, 2nd 2 13
20, 3rd 4 0
21, 4th 2 35
22, 5th 3 26
23, 6th 3 5
Another plant which followed the sun in its revolutions.
H. M.
July 11, 1st circle was made in 1 51 Very hot day.
11, 2nd 1 46
11, 3rd 1 41
11, 4th 1 48
12, 5th 2 35
_Scyphanthus elegans_ (Loasaceæ) follows the sun.
H. M.
June 13, 1st circle was made in 1 45
13, 2nd 1 17
14, 3rd 1 36
14, 4th 1 59
14, 5th 2 3
_Siphomeris_ or _Lecontea_ (unnamed sp.) (Cinchonaceæ) follows the sun.
H. M.
May 25, semicircle was made in 10 27 (shoot extremely
young)
26, 1st circle 10 15 (shoot still young)
30, 2nd 8 55
June 2, 3rd 8 11
6, 4th 6 8
8, 5th 7 20 Taken from the
hothouse, and placed
in a room in my
house.
9, 6th 8 36
_Manettia bicolor_ (Cinchonaceæ), young plant, follows the sun.
H. M.
July 7, 1st circle was made in 6 18
8, 2nd 6 53
9, 3rd 6 30
_Lonicera brachypoda_ (Caprifoliaceæ) follows the sun, kept in a warm
room in the house.
H. M.
April, 1st circle was made in 9 10 (about)
April, 2nd circle was made in 12 20 (a distinct shoot,
very young, on same
plant)
3rd 7 30
4th 8 0 In this latter
circle, the
semicircle from the
light took 5 hrs. 23
m., and to the light
2 hrs. 37 min.:
difference 2 hrs 46
m.
_Aristolochia gigas_ (Aristolochiaceæ) moves against the sun.
H. M.
July 22, 1st circle was made in 8 0 (rather young
shoot)
23, 2nd 7 15
24, 3rd 5 0 (about)
In the foregoing Table, which includes twining plants belonging to widely
different orders, we see that the rate at which growth travels or
circulates round the axis (on which the revolving movement depends),
differs much. As long as a plant remains under the same conditions, the
rate is often remarkably uniform, as with the Hop, _Mikania_,
_Phaseolus_, &c. The Scyphanthus made one revolution in 1 hr. 17 m., and
this is the quickest rate observed by me; but we shall hereafter see a
tendril-bearing Passiflora revolving more rapidly. A shoot of the
_Akebia quinata_ made a revolution in 1 hr. 30 m., and three revolutions
at the average rate of 1 hr. 38 m.; a Convolvulus made two revolutions at
the average of 1 hr. 42 m., and _Phaseolus vulgaris_ three at the average
of 1 hr. 57 m. On the other hand, some plants take 24 hrs. for a single
revolution, and the _Adhadota_ sometimes required 48 hrs.; yet this
latter plant is an efficient twiner. Species of the same genus move at
different rates. The rate does not seem governed by the thickness of the
shoots: those of the _Sollya_ are as thin and flexible as string, but
move more slowly than the thick and fleshy shoots of the _Ruscus_, which
seem little fitted for movement of any kind. The shoots of the
_Wistaria_, which become woody, move faster than those of the herbaceous
_Ipomoea_ or _Thunbergia_.
We know that the internodes, whilst still very young, do not acquire
their proper rate of movement; hence the several shoots on the same plant
may sometimes be seen revolving at different rates. The two or three, or
even more, internodes which are first formed above the cotyledons, or
above the root-stock of a perennial plant, do not move; they can support
themselves, and nothing superfluous is granted.
A greater number of twiners revolve in a course opposed to that of the
sun, or to the hands of a watch, than in the reversed course, and,
consequently, the majority, as is well known, ascend their supports from
left to right. Occasionally, though rarely, plants of the same order
twine in opposite directions, of which Mohl (p. 125) gives a case in the
Leguminosæ, and we have in the table another in the Acanthaceæ. I have
seen no instance of two species of the same genus twining in opposite
directions, and such cases must be rare; but Fritz Müller {33} states
that although _Mikania scandens_ twines, as I have described, from left
to right, another species in South Brazil twines in an opposite
direction. It would have been an anomalous circumstance if no such cases
had occurred, for different individuals of the same species, namely, of
_Solanum dulcamara_ (Dutrochet, tom. xix. p. 299), revolve and twine in
two directions: this plant, however; is a most feeble twiner. _Loasa
aurantiaca_ (Léon, p. 351) offers a much more curious case: I raised
seventeen plants: of these eight revolved in opposition to the sun and
ascended from left to right; five followed the sun and ascended from
right to left; and four revolved and twined first in one direction, and
then reversed their course, {34} the petioles of the opposite leaves
affording a _point d’appui_ for the reversal of the spire. One of these
four plants made seven spiral turns from right to left, and five turns
from left to right. Another plant in the same family, the _Scyphanthus
elegans_, habitually twines in this same manner. I raised many plants of
it, and the stems of all took one turn, or occasionally two or even three
turns in one direction, and then, ascending for a short space straight,
reversed their course and took one or two turns in an opposite direction.
The reversal of the curvature occurred at any point in the stem, even in
the middle of an internode. Had I not seen this case, I should have
thought its occurrence most improbable. It would be hardly possible with
any plant which ascended above a few feet in height, or which lived in an
exposed situation; for the stem could be pulled away easily from its
support, with but little unwinding; nor could it have adhered at all, had
not the internodes soon become moderately rigid. With leaf-climbers, as
we shall soon see, analogous cases frequently occur; but these present no
difficulty, as the stem is secured by the clasping petioles.
In the many other revolving and twining plants observed by me, I never
but twice saw the movement reversed; once, and only for a short space, in
_Ipomoea jucunda_; but frequently with _Hibbertia dentata_. This plant
at first perplexed me much, for I continually observed its long and
flexible shoots, evidently well fitted for twining, make a whole, or
half, or quarter circle in one direction and then in an opposite
direction; consequently, when I placed the shoots near thin or thick
sticks, or perpendicularly stretched string, they seemed as if constantly
trying to ascend, but always failed. I then surrounded the plant with a
mass of branched twigs; the shoots ascended, and passed through them, but
several came out laterally, and their depending extremities seldom turned
upwards as is usual with twining plants. Finally, I surrounded a second
plant with many thin upright sticks, and placed it near the first one
with twigs; and now both had got what they liked, for they twined up the
parallel sticks, sometimes winding round one and sometimes round several;
and the shoots travelled laterally from one to the other pot; but as the
plants grew older, some of the shoots twined regularly up thin upright
sticks. Though the revolving movement was sometimes in one direction and
sometimes in the other, the twining was invariably from left to right;
{36} so that the more potent or persistent movement of revolution must
have been in opposition to the course of the sun. It would appear that
this _Hibbertia_ is adapted both to ascend by twining, and to ramble
laterally through the thick Australian scrub.
I have described the above case in some detail, because, as far as I have
seen, it is rare to find any special adaptations with twining plants, in
which respect they differ much from the more highly organized
tendril-bearers. The _Solanum dulcamara_, as we shall presently see, can
twine only round stems which are both thin and flexible. Most twining
plants are adapted to ascend supports of moderate though of different
thicknesses. Our English twiners, as far as I have seen, never twine
round trees, excepting the honeysuckle (_Lonicera periclymenum_), which I
have observed twining up a young beech-tree nearly 4½ inches in diameter.
Mohl (p. 134) found that the _Phaseolus multiflorus_ and _Ipomoea
purpurea_ could not, when placed in a room with the light entering on one
side, twine round sticks between 3 and 4 inches in diameter; for this
interfered, in a manner presently to be explained, with the revolving
movement. In the open air, however, the _Phaseolus_ twined round a
support of the above thickness, but failed in twining round one 9 inches
in diameter. Nevertheless, some twiners of the warmer temperate regions
can manage this latter degree of thickness; for I hear from Dr. Hooker
that at Kew the _Ruscus androgynus_ has ascended a column 9 inches in
diameter; and although a _Wistaria_ grown by me in a small pot tried in
vain for weeks to get round a post between 5 and 6 inches in thickness,
yet at Kew a plant ascended a trunk above 6 inches in diameter. The
tropical twiners, on the other hand, can ascend thicker trees; I hear
from Drs. Thomson and Hooker that this is the case with the _Butea
parviflora_, one of the Menispermaceæ, and with some Dalbergias and other
Leguminosæ. {37} This power would be necessary for any species which had
to ascend by twining the large trees of a tropical forest; otherwise they
would hardly ever be able to reach the light. In our temperate countries
it would be injurious to the twining plants which die down every year if
they were enabled to twine round trunks of trees, for they could not grow
tall enough in a single season to reach the summit and gain the light.
By what means certain twining plants are adapted to ascend only thin
stems, whilst others can twine round thicker ones, I do not know. It
appeared to me probable that twining plants with very long revolving
shoots would be able to ascend thick supports; accordingly I placed
_Ceropegia Gardnerii_ near a post 6 inches in diameter, but the shoots
entirely failed to wind round it; their great length and power of
movement merely aid them in finding a distant stem round which to twine.
The _Sphærostemma marmoratum_ is a vigorous tropical twiner; and as it is
a very slow revolver, I thought that this latter circumstance might help
it in ascending a thick support; but though it was able to wind round a
6-inch post, it could do this only on the same level or plane, and did
not form a spire and thus ascend.
As ferns differ so much in structure from phanerogamic plants, it may be
worth while here to show that twining ferns do not differ in their habits
from other twining plants. In _Lygodium articulatum_ the two internodes
of the stem (properly the rachis) which are first formed above the
root-stock do not move; the third from the ground revolves, but at first
very slowly. This species is a slow revolver: but _L. scandens_ made
five revolutions, each at the average rate of 5 hrs. 45 m.; and this
represents fairly well the usual rate, taking quick and slow movers,
amongst phanerogamic plants. The rate was accelerated by increased
temperature. At each stage of growth only the two upper internodes
revolved. A line painted along the convex surface of a revolving
internode becomes first lateral, then concave, then lateral and
ultimately again convex. Neither the internodes nor the petioles are
irritable when rubbed. The movement is in the usual direction, namely,
in opposition to the course of the sun; and when the stem twines round a
thin stick, it becomes twisted on its own axis in the same direction.
After the young internodes have twined round a stick, their continued
growth causes them to slip a little upwards. If the stick be soon
removed, they straighten themselves, and recommence revolving. The
extremities of the depending shoots turn upwards, and twine on
themselves. In all these respects we have complete identity with twining
phanerogamic plants; and the above enumeration may serve as a summary of
the leading characteristics of all twining plants.
The power of revolving depends on the general health and vigour of the
plant, as has been laboriously shown by Palm. But the movement of each
separate internode is so independent of the others, that cutting off an
upper one does not affect the revolutions of a lower one. When, however,
Dutrochet cut off two whole shoots of the Hop, and placed them in water,
the movement was greatly retarded; for one revolved in 20 hrs. and the
other in 23 hrs., whereas they ought to have revolved in between 2 hrs.
and 2 hrs. 30 m. Shoots of the Kidney-bean, cut off and placed in water,
were similarly retarded, but in a less degree. I have repeatedly
observed that carrying a plant from the greenhouse to my room, or from
one part to another of the greenhouse, always stopped the movement for a
time; hence I conclude that plants in a state of nature and growing in
exposed situations, would not make their revolutions during very stormy
weather. A decrease in temperature always caused a considerable
retardation in the rate of revolution; but Dutrochet (tom. xvii. pp. 994,
996) has given such precise observations on this head with respect to the
common pea that I need say nothing more. When twining plants are placed
near a window in a room, the light in some cases has a remarkable power
(as was likewise observed by Dutrochet, p. 998, with the pea) on the
revolving movement, but this differs in degree with different plants;
thus _Ipomoea jucunda_ made a complete circle in 5 hrs. 30 m.; the
semicircle from the light taking 4 hrs. 80 m., and that towards the light
only 1 hr. _Lonicera brachypoda_ revolved, in a reversed direction to
the _Ipomoea_, in 8 hrs.; the semicircle from the light taking 5 hrs. 23
m., and that to the light only 2 hrs. 37 m. From the rate of revolution
in all the plants observed by me, being nearly the same during the night
and the day, I infer that the action of the light is confined to
retarding one semicircle and accelerating the other, so as not to modify
greatly the rate of the whole revolution. This action of the light is
remarkable, when we reflect how little the leaves are developed on the
young and thin revolving internodes. It is all the more remarkable, as
botanists believe (Mohl, p. 119) that twining plants are but little
sensitive to the action of light.
I will conclude my account of twining plants by giving a few
miscellaneous and curious cases. With most twining plants all the
branches, however many there may be, go on revolving together; but,
according to Mohl (p. 4), only the lateral branches of _Tamus
elephantipes_ twine, and not the main stem. On the other hand, with a
climbing species of Asparagus, the leading shoot alone, and not the
branches, revolved and twined; but it should be stated that the plant was
not growing vigorously. My plants of _Combretum argenteum_ and _C.
purpureum_ made numerous short healthy shoots; but they showed no signs
of revolving, and I could not conceive how these plants could be
climbers; but at last _C. argenteum_ put forth from the lower part of one
of its main branches a thin shoot, 5 or 6 feet in length, differing
greatly in appearance from the previous shoots, owing to its leaves being
little developed, and this shoot revolved vigorously and twined. So that
this plant produces shoots of two kinds. With _Periploca Græca_ (Palm,
p. 43) the uppermost shoots alone twine. Polygonum convolvulus twines
only during the middle of the summer (Palm, p. 43, 94); and plants
growing vigorously in the autumn show no inclination to climb. The
majority of Asclepiadaceæ are twiners; but _Asclepias nigra_ only “in
fertiliori solo incipit scandere subvolubili caule” (Willdenow, quoted
and confirmed by Palm, p. 41). _Asclepias vincetoxicum_ does not
regularly twine, but occasionally does so (Palm, p. 42; Mohl, p. 112)
when growing under certain conditions. So it is with two species of
_Ceropegia_, as I hear from Prof. Harvey, for these plants in their
native dry South African home generally grow erect, from 6 inches to 2
feet in height,—a very few taller specimens showing some inclination to
curve; but when cultivated near Dublin, they regularly twined up sticks 5
or 6 feet in height. Most Convolvulaceæ are excellent twiners; but in
South Africa _Ipomoea argyræoides_ almost always grows erect and compact,
from about 12 to 18 inches in height, one specimen alone in Prof.
Harvey’s collection showing an evident disposition to twine. On the
other hand, seedlings raised near Dublin twined up sticks above 8 feet in
height. These facts are remarkable; for there can hardly be a doubt that
in the dryer provinces of South Africa these plants have propagated
themselves for thousands of generations in an erect condition; and yet
they have retained during this whole period the innate power of
spontaneously revolving and twining, whenever their shoots become
elongated under proper conditions of life. Most of the species of
_Phaseolus_ are twiners; but certain varieties of the _P. multiflorus_
produce (Léon, p. 681) two kinds of shoots, some upright and thick, and
others thin and twining. I have seen striking instances of this curious
case of variability in “Fulmer’s dwarf forcing-bean,” which occasionally
produced a single long twining shoot.
_Solanum dulcamara_ is one of the feeblest and poorest of twiners: it may
often be seen growing as an upright bush, and when growing in the midst
of a thicket merely scrambles up between the branches without twining;
but when, according to Dutrochet (tom. xix. p. 299), it grows near a thin
and flexible support, such as the stem of a nettle, it twines round it.
I placed sticks round several plants, and vertically stretched strings
close to others, and the strings alone were ascended by twining. The
stem twines indifferently to the right or left. Some others species of
Solanum, and of another genus, viz. _Habrothamnus_, belonging to the same
family, are described in horticultural works as twining plants, but they
seem to possess this faculty in a very feeble degree. We may suspect
that the species of these two genera have as yet only partially acquired
the habit of twining. On the other hand with _Tecoma radicans_, a member
of a family abounding with twiners and tendril-bearers, but which climbs,
like the ivy, by the aid of rootlets, we may suspect that a former habit
of twining has been lost, for the stem exhibited slight irregular
movements which could hardly be accounted for by changes in the action of
the light. There is no difficulty in understanding how a spirally
twining plant could graduate into a simple root-climber; for the young
internodes of _Bignonia Tweedyana_ and of _Hoya carnosa_ revolve and
twine, but likewise emit rootlets which adhere to any fitting surface, so
that the loss of twining would be no great disadvantage and in some
respects an advantage to these species, as they would then ascend their
supports in a more direct line. {44}
CHAPTER II.
LEAF-CLIMBERS.
Plants which climb by the aid of spontaneously revolving and sensitive
petioles—_Clematis_—_Tropæolum_—_Maurandia_, flower-peduncles moving
spontaneously and sensitive to a
touch—_Rhodochiton_—_Lophospermum_—internodes sensitive—_Solanum_,
thickening of the clasped petioles—_Fumaria_—_Adlumia_—Plants which climb
by the aid of their produced
midribs—_Gloriosa_—_Flagellaria_—_Nepenthes_—Summary on leaf-climbers.
WE now come to our second class of climbing plants, namely, those which
ascend by the aid of irritable or sensitive organs. For convenience’
sake the plants in this class have been grouped under two sub-divisions,
namely, leaf-climbers, or those which retain their leaves in a functional
condition, and tendril-bearers. But these sub-divisions graduate into
each other, as we shall see under Corydalis and the Gloriosa lily.
It has long been observed that several plants climb by the aid of their
leaves, either by their petioles (foot-stalks) or by their produced
midribs; but beyond this simple fact they have not been described. Palm
and Mohl class these plants with those which bear tendrils; but as a leaf
is generally a defined object, the present classification, though
artificial, has at least some advantages. Leaf-climbers are, moreover,
intermediate in many respects between twiners and tendril-bearers. Eight
species of _Clematis_ and seven of _Tropæolum_ were observed, in order to
see what amount of difference in the manner of climbing existed within
the same genus; and the differences are considerable.
CLEMATIS.—_C. glandulosa_.—The thin upper internodes revolve, moving
against the course of the sun, precisely like those of a true twiner, at
an average rate, judging from three revolutions, of 3 hrs. 48 m. The
leading shoot immediately twined round a stick placed near it; but, after
making an open spire of only one turn and a half, it ascended for a short
space straight, and then reversed its course and wound two turns in an
opposite direction. This was rendered possible by the straight piece
between the opposed spires having become rigid. The simple, broad, ovate
leaves of this tropical species, with their short thick petioles, seem
but ill-fitted for any movement; and whilst twining up a vertical stick,
no use is made of them. Nevertheless, if the footstalk of a young leaf
be rubbed with a thin twig a few times on any side, it will in the course
of a few hours bend to that side; afterwards becoming straight again.
The under side seemed to be the most sensitive; but the sensitiveness or
irritability is slight compared to that which we shall meet with in some
of the following species; thus, a loop of string, weighing 1.64 grain
(106.2 mg.) and hanging for some days on a young footstalk, produced a
scarcely perceptible effect. A sketch is here given of two young leaves
which had naturally caught hold of two thin branches. A forked twig
placed so as to press lightly on the under side of a young footstalk
caused it, in 12 hrs., to bend greatly, and ultimately to such an extent
that the leaf passed to the opposite side of the stem; the forked stick
having been removed, the leaf slowly recovered its former position.
[Picture: Fig. 1. Clematis glandulosa. With two young leaves clasping
two twigs, with the clasping portions thickened]
The young leaves spontaneously and gradually change their position: when
first developed the petioles are upturned and parallel to the stem; they
then slowly bend downwards, remaining for a short time at right angles to
the stem, and then become so much arched downwards that the blade of the
leaf points to the ground with its tip curled inwards, so that the whole
petiole and leaf together form a hook. They are thus enabled to catch
hold of any twig with which they may be brought into contact by the
revolving movement of the internodes. If this does not happen, they
retain their hooked shape for a considerable time, and then bending
upwards reassume their original upturned position, which is preserved
ever afterwards. The petioles which have clasped any object soon become
much thickened and strengthened, as may be seen in the drawing.
_Clematis montana_.—The long, thin petioles of the leaves, whilst young,
are sensitive, and when lightly rubbed bend to the rubbed side,
subsequently becoming straight. They are far more sensitive than the
petioles of _C. glandulosa_; for a loop of thread weighing a quarter of a
grain (16.2 mg.) caused them to bend; a loop weighing only one-eighth of
a grain (8.1 mg.) sometimes acted and sometimes did not act. The
sensitiveness extends from the blade of the leaf to the stem. I may here
state that I ascertained in all cases the weights of the string and
thread used by carefully weighing 50 inches in a chemical balance, and
then cutting off measured lengths. The main petiole carries three
leaflets; but their short, sub-petioles are not sensitive. A young,
inclined shoot (the plant being in the greenhouse) made a large circle
opposed to the course of the sun in 4 hrs. 20 m., but the next day, being
very cold, the time was 5 hrs. 10 m. A stick placed near a revolving
stem was soon struck by the petioles which stand out at right angles, and
the revolving movement was thus arrested. The petioles then began, being
excited by the contact, to slowly wind round the stick. When the stick
was thin, a petiole sometimes wound twice round it. The opposite leaf
was in no way affected. The attitude assumed by the stem after the
petiole had clasped the stick, was that of a man standing by a column,
who throws his arm horizontally round it. With respect to the stem’s
power of twining, some remarks will be made under _C. calycina_.
_Clematis Sieboldi_.—A shoot made three revolutions against the sun at an
average rate of 3 hrs. 11 m. The power of twining is like that of the
last species. Its leaves are nearly similar in structure and in
function, excepting that the sub-petioles of the lateral and terminal
leaflets are sensitive. A loop of thread, weighing one-eighth of a
grain, acted on the main petiole, but not until two or three days had
elapsed. The leaves have the remarkable habit of spontaneously
revolving, generally in vertical ellipses, in the same manner, but in a
less degree, as will be described under _C. microphylla_.
_Clematis calycina_.—The young shoots are thin and flexible: one
revolved, describing a broad oval, in 5 hrs. 30 m., and another in 6 hrs.
12 m. They followed the course of the sun; but the course, if observed
long enough, would probably be found to vary in this species, as well as
in all the others of the genus. It is a rather better twiner than the
two last species: the stem sometimes made two spiral turns round a thin
stick, if free from twigs; it then ran straight up for a space, and
reversing its course took one or two turns in an opposite direction.
This reversal of the spire occurred in all the foregoing species. The
leaves are so small compared with those of most of the other species,
that the petioles at first seem ill-adapted for clasping. Nevertheless,
the main service of the revolving movement is to bring them into contact
with surrounding objects, which are slowly but securely seized. The
young petioles, which alone are sensitive, have their ends bowed a little
downwards, so as to be in a slight degree hooked; ultimately the whole
leaf, if it catches nothing, becomes level. I gently rubbed with a thin
twig the lower surfaces of two young petioles; and in 2 hrs. 30 m. they
were slightly curved downwards; in 5 hrs., after being rubbed, the end of
one was bent completely back, parallel to the basal portion; in 4 hrs.
subsequently it became nearly straight again. To show how sensitive the
young petioles are, I may mention that I just touched the under sides of
two with a little water-colour, which when dry formed an excessively thin
and minute crust; but this sufficed in 24 hrs. to cause both to bend
downwards. Whilst the plant is young, each leaf consists of three
divided leaflets, which barely have distinct petioles, and these are not
sensitive; but when the plant is well grown, the petioles of the two
lateral and terminal leaflets are of considerable length, and become
sensitive so as to be capable of clasping an object in any direction.
When a petiole has clasped a twig, it undergoes some remarkable changes,
which may be observed with the other species, but in a less strongly
marked manner, and will here be described once for all. The clasped
petiole in the course of two or three days swells greatly, and ultimately
becomes nearly twice as thick as the opposite one which has clasped
nothing. When thin transverse slices of the two are placed under the
microscope their difference is conspicuous: the side of the petiole which
has been in contact with the support, is formed of a layer of colourless
cells with their longer axes directed from the centre, and these are very
much larger than the corresponding cells in the opposite or unchanged
petiole; the central cells, also, are in some degree enlarged, and the
whole is much indurated. The exterior surface generally becomes bright
red. But a far greater change takes place in the nature of the tissues
than that which is visible: the petiole of the unclasped leaf is flexible
and can be snapped easily, whereas the clasped one acquires an
extraordinary degree of toughness and rigidity, so that considerable
force is required to pull it into pieces. With this change, great
durability is probably acquired; at least this is the case with the
clasped petioles of _Clematis vitalba_. The meaning of these changes is
obvious, namely, that the petioles may firmly and durably support the
stem.
_Clematis microphylla_, var. _leptophylla_.—The long and thin internodes
of this Australian species revolve sometimes in one direction and
sometimes in an opposite one, describing long, narrow, irregular ellipses
or large circles. Four revolutions were completed within five minutes of
the same average rate of 1 hr. 51 m.; so that this species moves more
quickly than the others of the genus. The shoots, when placed near a
vertical stick, either twine round it, or clasp it with the basal
portions of their petioles. The leaves whilst young are nearly of the
same shape as those of _C. viticella_, and act in the same manner like a
hook, as will be described under that species. But the leaflets are more
divided, and each segment whilst young terminates in a hardish point,
which is much curved downwards and inwards; so that the whole leaf
readily catches hold of any neighbouring object. The petioles of the
young terminal leaflets are acted on by loops of thread weighing ⅛th and
even 0.0625th of a grain. The basal portion of the main petiole is much
less sensitive, but will clasp a stick against which it presses.
The leaves, whilst young, are continually and spontaneously moving
slowly. A bell-glass was placed over a shoot secured to a stick, and the
movements of the leaves were traced on it during several days. A very
irregular line was generally formed; but one day, in the course of eight
hours and three quarters, the figure clearly represented three and a half
irregular ellipses, the most perfect one of which was completed in 2 hrs.
35 m. The two opposite leaves moved independently of each other. This
movement of the leaves would aid that of the internodes in bringing the
petioles into contact with surrounding objects. I discovered this
movement too late to be enabled to observe it in the other species; but
from analogy I can hardly doubt that the leaves of at least _C.
viticella_, _C. flammula_, and _C. vitalba_ move spontaneously; and,
judging from _C. Sieboldi_, this probably is the case with _C. montana_
and _C. calycina_. I ascertained that the simple leaves of _C.
glandulosa_ exhibited no spontaneous revolving movement.
_Clematis viticella_, var. _venosa_.—In this and the two following
species the power of spirally twining is completely lost, and this seems
due to the lessened flexibility of the internodes and to the interference
caused by the large size of the leaves. But the revolving movement,
though restricted, is not lost. In our present species a young
internode, placed in front of a window, made three narrow ellipses,
transversely to the direction of the light, at an average rate of 2 hrs.
40 m. When placed so that the movements were to and from the light, the
rate was greatly accelerated in one half of the course, and retarded in
the other, as with twining plants. The ellipses were small; the longer
diameter, described by the apex of a shoot bearing a pair of not expanded
leaves, was only 4⅝ inches, and that by the apex of the penultimate
internode only 1⅛ inch. At the most favourable period of growth each
leaf would hardly be carried to and fro by the movement of the internodes
more than two or three inches, but, as above stated, it is probable that
the leaves themselves move spontaneously. The movement of the whole
shoot by the wind and by its rapid growth, would probably be almost
equally efficient as these spontaneous movements, in bringing the
petioles into contact with surrounding objects.
The leaves are of large size. Each bears three pairs of lateral leaflets
and a terminal one, all supported on rather long sub-petioles. The main
petiole bends a little angularly downwards at each point where a pair of
leaflets arises (see fig. 2), and the petiole of the terminal leaflet is
bent downwards at right angles; hence the whole petiole, with its
rectangularly bent extremity, acts as a hook. This hook, the lateral
petioles being directed a little upwards; forms an excellent grappling
apparatus, by which the leaves readily become entangled with surrounding
objects. If they catch nothing, the whole petiole ultimately grows
straight. The main petiole, the sub-petioles, and the three branches
into which each basi-lateral sub-petiole is generally subdivided, are all
sensitive. The basal portion of the main petiole, between the stem and
the first pair of leaflets, is less sensitive than the remainder; it
will, however, clasp a stick with which it is left in contact. The
inferior surface of the rectangularly bent terminal portion (carrying the
terminal leaflet), which forms the inner side of the end of the hook, is
the most sensitive part; and this portion is manifestly best adapted to
catch a distant support. To show the difference in sensibility, I gently
placed loops of string of the same weight (in one instance weighing only
0.82 of a grain or 53.14 mg.) on the several lateral sub-petioles and on
the terminal one; in a few hours the latter was bent, but after 24 hrs.
no effect was produced on the other sub-petioles. Again, a terminal
sub-petiole placed in contact with a thin stick became sensibly curved in
45 m., and in 1 hr. 10 m. moved through ninety degrees; whilst a lateral
sub-petiole did not become sensibly curved until 3 hrs. 30 m. had
elapsed. In all cases, if the sticks are taken away, the petioles
continue to move during many hours afterwards; so they do after a slight
rubbing; but they become straight again, after about a day’s interval,
that is if the flexure has not been very great or long continued.
[Picture: Fig. 2. A young leaf of Clematis viticeela]
The graduated difference in the extension of the sensitiveness in the
petioles of the above-described species deserves notice. In _C. montana_
it is confined to the main petiole, and has not spread to the
sub-petioles of the three leaflets; so it is with young plants of _C.
calycina_, but in older plants it spreads to the three sub-petioles. In
_C. viticella_ the sensitiveness has spread to the petioles of the seven
leaflets, and to the subdivisions of the basi-lateral sub-petioles. But
in this latter species it has diminished in the basal part of the main
petiole, in which alone it resided in _C. montana_; whilst it has
increased in the abruptly bent terminal portion.
_Clematis flammula_.—The rather thick, straight, and stiff shoots, whilst
growing vigorously in the spring, make small oval revolutions, following
the sun in their course. Four were made at an average rate of 3 hrs. 45
m. The longer axis of the oval, described by the extreme tip, was
directed at right angles to the line joining the opposite leaves; its
length was in one case only 1⅜, and in another case 1¾ inch; so that the
young leaves were moved a very short distance. The shoots of the same
plant observed in midsummer, when growing not so quickly, did not revolve
at all. I cut down another plant in the early summer, so that by August
1st it had formed new and moderately vigorous shoots; these, when
observed under a bell-glass, were on some days quite stationary, and on
other days moved to and fro only about the eighth of an inch.
Consequently the revolving power is much enfeebled in this species, and
under unfavourable circumstances is completely lost. The shoot must
depend for coming into contact with surrounding objects on the probable,
though not ascertained spontaneous movement of the leaves, on rapid
growth, and on movement from the wind. Hence, perhaps, it is that the
petioles have acquired a high degree of sensitiveness as a compensation
for the little power of movement in the shoots.
The petioles are bowed downwards, and have the same general hook-like
form as in _C. viticella_. The medial petiole and the lateral
sub-petioles are sensitive, especially the much bent terminal portion.
As the sensitiveness is here greater than in any other species of the
genus observed by me, and is in itself remarkable, I will give fuller
details. The petioles, when so young that they have not separated from
one another, are not sensitive; when the lamina of a leaflet has grown to
a quarter of an inch in length (that is, about one-sixth of its full
size), the sensitiveness is highest; but at this period the petioles are
relatively much more fully developed than are the blades of the leaves.
Full-grown petioles are not in the least sensitive. A thin stick placed
so as to press lightly against a petiole, having a leaflet a quarter of
an inch in length, caused the petiole to bend in 3 hrs. 15 m. In another
case a petiole curled completely round a stick in 12 hrs. These petioles
were left curled for 24 hrs., and the sticks were then removed; but they
never straightened themselves. I took a twig, thinner than the petiole
itself, and with it lightly rubbed several petioles four times up and
down; these in 1 hr. 45 m. became slightly curled; the curvature
increased during some hours and then began to decrease, but after 25 hrs.
from the time of rubbing a vestige of the curvature remained. Some other
petioles similarly rubbed twice, that is, once up and once down, became
perceptibly curved in about 2 hrs. 30 m., the terminal sub-petiole moving
more than the lateral sub-petioles; they all became straight again in
between 12 hrs. and 14 hrs. Lastly, a length of about one-eighth of an
inch of a sub-petiole, was lightly rubbed with the same twig only once;
it became slightly curved in 3 hrs., remaining so during 11 hrs., but by
the next morning was quite straight.
The following observations are more precise. After trying heavier pieces
of string and thread, I placed a loop of fine string, weighing 1.04 gr.
(67.4 mg.) on a terminal sub-petiole: in 6 hrs. 40 m. a curvature could
be seen; in 24 hrs. the petiole formed an open ring round the string; in
48 hrs. the ring had almost closed on the string, and in 72 hrs. seized
it so firmly, that some force was necessary for its withdrawal. A loop
weighing 0.52 of a grain (33.7 mg.) caused in 14 hrs. a lateral
sub-petiole just perceptibly to curve, and in 24 hrs. it moved through
ninety degrees. These observations were made during the summer: the
following were made in the spring, when the petioles apparently are more
sensitive:—A loop of thread, weighing one-eighth of a grain (8.1 mg.),
produced no effect on the lateral sub-petioles, but placed on a terminal
one, caused it, after 24 hrs., to curve moderately; the curvature, though
the loop remained suspended, was after 48 hrs. diminished, but never
disappeared; showing that the petiole had become partially accustomed to
the insufficient stimulus. This experiment was twice repeated with
nearly the same result. Lastly, a loop of thread, weighing only
one-sixteenth of a grain (4.05 mg.) was twice gently placed by a forceps
on a terminal sub-petiole (the plant being, of course, in a still and
closed room), and this weight certainly caused a flexure, which very
slowly increased until the petiole moved through nearly ninety degrees:
beyond this it did not move; nor did the petiole, the loop remaining
suspended, ever become perfectly straight again.
When we consider, on the one hand, the thickness and stiffness of the
petioles, and, on the other hand, the thinness and softness of fine
cotton thread, and what an extremely small weight one-sixteenth of a
grain (4.05 mg.) is, these facts are remarkable. But I have reason to
believe that even a less weight excites curvature when pressing over a
broader surface than that acted on by a thread. Having noticed that the
end of a suspended string which accidentally touched a petiole, caused it
to bend, I took two pieces of thin twine, 10 inches in length (weighing
1.64 gr.), and, tying them to a stick, let them hang as nearly
perpendicularly downwards as their thinness and flexuous form, after
being stretched, would permit; I then quietly placed their ends so as
just to rest on two petioles, and these certainly became curved in 36
hrs. One of the ends touched the angle between a terminal and lateral
sub-petiole, and it was in 48 hours caught between them as by a forceps.
In these cases the pressure, though spread over a wider surface than that
touched by the cotton thread, must have been excessively slight.
_Clematis vitalba_.—The plants were in pots and not healthy, so that I
dare not trust my observations, which indicate much similarity in habits
with _C. flammula_. I mention this species only because I have seen many
proofs that the petioles in a state of nature are excited to movement by
very slight pressure. For instance, I have found them embracing thin
withered blades of grass, the soft young leaves of a maple, and the
flower-peduncles of the quaking-grass or Briza. The latter are about as
thick as the hair of a man’s beard, but they were completely surrounded
and clasped. The petioles of a leaf, so young that none of the leaflets
were expanded, had partially seized a twig. Those of almost all the old
leaves, even when unattached to any object, are much convoluted; but this
is owing to their having come, whilst young, into contact during several
hours with some object subsequently removed. With none of the
above-described species, cultivated in pots and carefully observed, was
there any permanent bending of the petioles without the stimulus of
contact. In winter, the blades of the leaves of _C. vitalba_ drop off;
but the petioles (as was observed by Mohl) remain attached to the
branches, sometimes during two seasons; and, being convoluted, they
curiously resemble true tendrils, such as those possessed by the allied
genus _Naravelia_. The petioles which have clasped some object become
much more stiff, hard, and polished than those which have failed in this
their proper function.
TROPÆOLUM.—I observed _T. tricolorum_, _T. azureum_, _T. pentaphyllum_,
_T. peregrinum_, _T. elegans_, _T. tuberosum_, and a dwarf variety of, as
I believe, _T. minus_.
_Tropæolum tricolorum_, var. _grandiflorum_.—The flexible shoots, which
first rise from the tubers, are as thin as fine twine. One such shoot
revolved in a course opposed to the sun, at an average rate, judging from
three revolutions, of 1 hr. 23 m.; but no doubt the direction of the
revolving movement is variable. When the plants have grown tall and are
branched, all the many lateral shoots revolve. The stem, whilst young,
twines regularly round a thin vertical stick, and in one case I counted
eight spiral turns in the same direction; but when grown older, the stem
often runs straight up for a space, and, being arrested by the clasping
petioles, makes one or two spires in a reversed direction. Until the
plant grows to a height of two or three feet, requiring about a month
from the time when the first shoot appears above ground, no true leaves
are produced, but, in their place, filaments coloured like the stem. The
extremities of these filaments are pointed, a little flattened, and
furrowed on the upper surface. They never become developed into leaves.
As the plant grows in height new filaments are produced with slightly
enlarged tips; then others, bearing on each side of the enlarged medial
tip a rudimentary segment of a leaf; soon other segments appear, and at
last a perfect leaf is formed, with seven deep segments. So that on the
same plant we may see every step, from tendril-like clasping filaments to
perfect leaves with clasping petioles. After the plant has grown to a
considerable height, and is secured to its support by the petioles of the
true leaves, the clasping filaments on the lower part of the stem wither
and drop off; so that they perform only a temporary service.
These filaments or rudimentary leaves, as well as the petioles of the
perfect leaves, whilst young, are highly sensitive on all sides to a
touch. The slightest rub caused them to curve towards the rubbed side in
about three minutes, and one bent itself into a ring in six minutes; they
subsequently became straight. When, however, they have once completely
clasped a stick, if this is removed, they do not straighten themselves.
The most remarkable fact, and one which I have observed in no other
species of the genus, is that the filaments and the petioles of the young
leaves, if they catch no object, after standing for some days in their
original position, spontaneously and slowly oscillate a little from side
to side, and then move towards the stem and clasp it. They likewise
often become, after a time, in some degree spirally contracted. They
therefore fully deserve to be called tendrils, as they are used for
climbing, are sensitive to a touch, move spontaneously, and ultimately
contract into a spire, though an imperfect one. The present species
would have been classed amongst the tendril-bearers, had not these
characters been confined to early youth. During maturity it is a true
leaf-climber.
_Tropæolum azureum_.—An upper internode made four revolutions, following
the sun, at an average rate of 1 hr. 47 m. The stem twined spirally
round a support in the same irregular manner as that of the last species.
Rudimentary leaves or filaments do not exist. The petioles of the young
leaves are very sensitive: a single light rub with a twig caused one to
move perceptibly in 5 m., and another in 6 m. The former became bent at
right angles in 15 min., and became straight again in between 5 hrs. and
6 hrs. A loop of thread weighing ⅛th of a grain caused another petiole
to curve.
_Tropæolum pentaphyllum_.—This species has not the power of spirally
twining, which seems due, not so much to a want of flexibility in the
stem, as to continual interference from the clasping petioles. An upper
internode made three revolutions, following the sun, at an average rate
of 1 hr. 46 m. The main purpose of the revolving movement in all the
species of _Tropæolum_ manifestly is to bring the petioles into contact
with some supporting object. The petiole of a young leaf, after a slight
rub, became curved in 6 m.; another, on a cold day, in 20 m., and others
in from 8 m. to 10 m. Their curvature usually increased greatly in from
15 m. to 20 m., and they became straight again in between 5 hrs. and 6
hrs., but on one occasion in 3 hrs. When a petiole has fairly clasped a
stick, it is not able, on the removal of the stick, to straighten itself.
The free upper part of one, the base of which had already clasped a
stick, still retained the power of movement. A loop of thread weighing
⅛th of a grain caused a petiole to curve; but the stimulus was not
sufficient, the loop remaining suspended, to cause a permanent flexure.
If a much heavier loop be placed in the angle between the petiole and the
stem, it produces no effect; whereas we have seen with _Clematis montana_
that the angle between the stem and petiole is sensitive.
_Tropæolum peregrinum_.—The first-formed internodes of a young plant did
not revolve, resembling in this respect those of a twining plant. In an
older plant the four upper internodes made three irregular revolutions,
in a course opposed to the sun, at an average rate of 1 hr. 48 min. It
is remarkable that the average rate of revolution (taken, however, but
from few observations) is very nearly the same in this and the two last
species, namely, 1 hr. 47 m., 1 hr. 46 m., and 1 hr. 48 m. The present
species cannot twine spirally, which seems mainly due to the rigidity of
the stem. In a very young plant, which did not revolve, the petioles
were not sensitive. In older plants the petioles of quite young leaves,
and of leaves as much as an inch and a quarter in diameter, are
sensitive. A moderate rub caused one to curve in 10 m., and others in 20
m. They became straight again in between 5 hrs. 45 m. and 8 hrs.
Petioles which have naturally come into contact with a stick, sometimes
take two turns round it. After they have clasped a support, they become
rigid and hard. They are less sensitive to a weight than in the previous
species; for loops of string weighing 0.82 of a grain (53.14 mg.), did
not cause any curvature, but a loop of double this weight (1.64 gr.)
acted.
_Tropæolum elegans_.—I did not make many observations on this species.
The short and stiff internodes revolve irregularly, describing small oval
figures. One oval was completed in 3 hrs. A young petiole, when rubbed,
became slightly curved in 17 m.; and afterwards much more so. It was
nearly straight again in 8 hrs.
_Tropæolum tuberosum_.—On a plant nine inches in height, the internodes
did not move at all; but on an older plant they moved irregularly and
made small imperfect ovals. These movements could be detected only by
being traced on a bell-glass placed over the plant. Sometimes the shoots
stood still for hours; during some days they moved only in one direction
in a crooked line; on other days they made small irregular spires or
circles, one being completed in about 4 hrs. The extreme points reached
by the apex of the shoot were only about one or one and a half inches
asunder; yet this slight movement brought the petioles into contact with
some closely surrounding twigs, which were then clasped. With the
lessened power of spontaneously revolving, compared with that of the
previous species, the sensitiveness of the petioles is also diminished.
These, when rubbed a few times, did not become curved until half an hour
had elapsed; the curvature increased during the next two hours, and then
very slowly decreased; so that they sometimes required 24 hrs. to become
straight again. Extremely young leaves have active petioles; one with
the lamina only 0.15 of an inch in diameter, that is, about a twentieth
of the full size, firmly clasped a thin twig. But leaves grown to a
quarter of their full size can likewise act.
_Tropæolum minus_ (?).—The internodes of a variety named “dwarf crimson
Nasturtium” did not revolve, but moved in a rather irregular course
during the day to the light, and from the light at night. The petioles,
when well rubbed, showed no power of curving; nor could I see that they
ever clasped any neighbouring object. We have seen in this genus a
gradation from species such as _T. tricolorum_, which have extremely
sensitive petioles, and internodes which rapidly revolve and spirally
twine up a support, to other species such as _T. elegans_ and _T.
tuberosum_, the petioles of which are much less sensitive, and the
internodes of which have very feeble revolving powers and cannot spirally
twine round a support, to this last species, which has entirely lost or
never acquired these faculties. From the general character of the genus,
the loss of power seems the more probable alternative.
In the present species, in _T. elegans_, and probably in others, the
flower-peduncle, as soon as the seed-capsule begins to swell,
spontaneously bends abruptly downwards and becomes somewhat convoluted.
If a stick stands in the way, it is to a certain extent clasped; but, as
far as I have been able to observe, this clasping movement is independent
of the stimulus from contact.
ANTIRRHINEÆ.—In this tribe (Lindley) of the Scrophulariaceæ, at least
four of the seven included genera have leaf-climbing species.
_Maurandia Barclayana_.—A thin, slightly bowed shoot made two
revolutions, following the sun, each in 3 hrs. 17 min.; on the previous
day this same shoot revolved in an opposite direction. The shoots do not
twine spirally, but climb excellently by the aid of their young and
sensitive petioles. These petioles, when lightly rubbed, move after a
considerable interval of time, and subsequently become straight again. A
loop of thread weighing ⅛th of a grain caused them to bend.
_Maurandia semperflorens_.—This freely growing species climbs exactly
like the last, by the aid of its sensitive petioles. A young internode
made two circles, each in 1 hr. 46 min.; so that it moved almost twice as
rapidly as the last species. The internodes are not in the least
sensitive to a touch or pressure. I mention this because they are
sensitive in a closely allied genus, namely, Lophospermum. The present
species is unique in one respect. Mohl asserts (p. 45) that “the
flower-peduncles, as well as the petioles, wind like tendrils;” but he
classes as tendrils such objects as the spiral flower-stalks of the
_Vallisneria_. This remark, and the fact of the flower-peduncles being
decidedly flexuous, led me carefully to examine them. They never act as
true tendrils; I repeatedly placed thin sticks in contact with young and
old peduncles, and I allowed nine vigorous plants to grow through an
entangled mass of branches; but in no one instance did they bend round
any object. It is indeed in the highest degree improbable that this
should occur, for they are generally developed on branches which have
already securely clasped a support by the petioles of their leaves; and
when borne on a free depending branch, they are not produced by the
terminal portion of the internode which alone has the power of revolving;
so that they could be brought only by accident into contact with any
neighbouring object. Nevertheless (and this is the remarkable fact) the
flower-peduncles, whilst young, exhibit feeble revolving powers, and are
slightly sensitive to a touch. Having selected some stems which had
firmly clasped a stick by their petioles, and having placed a bell-glass
over them, I traced the movements of the young flower-peduncles. The
tracing generally formed a short and extremely irregular line, with
little loops in its course. A young peduncle 1½ inch in length was
carefully observed during a whole day, and it made four and a half
narrow, vertical, irregular, and short ellipses—each at an average rate
of about 2 hrs. 25 m. An adjoining peduncle described during the same
time similar, though fewer, ellipses. As the plant had occupied for some
time exactly the same position, these movements could not be attributed
to any change in the action of the light. Peduncles, old enough for the
coloured petals to be just visible, do not move. With respect to
irritability, {68} I rubbed two young peduncles (1½ inch in length) a few
times very lightly with a thin twig; one was rubbed on the upper, and the
other on the lower side, and they became in between 4 hrs. and 5 hrs.
distinctly bowed towards these sides; in 24 hrs. subsequently, they
straightened themselves. Next day they were rubbed on the opposite
sides, and they became perceptibly curved towards these sides. Two other
and younger peduncles (three-fourths of an inch in length) were lightly
rubbed on their adjoining sides, and they became so much curved towards
one another, that the arcs of the bows stood at nearly right angles to
their previous direction; and this was the greatest movement seen by me.
Subsequently they straightened themselves. Other peduncles, so young as
to be only three-tenths of an inch in length, became curved when rubbed.
On the other hand, peduncles above 1½ inch in length required to be
rubbed two or three times, and then became only just perceptibly bowed.
Loops of thread suspended on the peduncles produced no effect; loops of
string, however, weighing 0.82 and 1.64 of a grain sometimes caused a
slight curvature; but they were never closely clasped, as were the far
lighter loops of thread by the petioles.
In the nine vigorous plants observed by me, it is certain that neither
the slight spontaneous movements nor the slight sensitiveness of the
flower-peduncles aided the plants in climbing. If any member of the
Scrophulariaceæ had possessed tendrils produced by the modification of
flower-peduncles, I should have thought that this species of _Maurandia_
had perhaps retained a useless or rudimentary vestige of a former habit;
but this view cannot be maintained. We may suspect that, owing to the
principle of correlation, the power of movement has been transferred to
the flower-peduncles from the young internodes, and sensitiveness from
the young petioles. But to whatever cause these capacities are due, the
case is interesting; for, by a little increase in power through natural
selection, they might easily have been rendered as useful to the plant in
climbing, as are the flower-peduncles (hereafter to be described) of
Vitis or Cardiospermum.
_Rhodochiton volubile_.—A long flexible shoot swept a large circle,
following the sun, in 5 hrs. 30 m.; and, as the day became warmer, a
second circle was completed in 4 hrs. 10 m. The shoots sometimes make a
whole or a half spire round a vertical stick, they then run straight up
for a space, and afterwards turn spirally in an opposite direction. The
petioles of very young leaves about one-tenth of their full size, are
highly sensitive, and bend towards the side which is touched; but they do
not move quickly. One was perceptibly curved in 1 hr. 10 m., after being
lightly rubbed, and became considerably curved in 5 hrs. 40 m.; some
others were scarcely curved in 5 hrs. 30 m., but distinctly so in 6 hrs.
30 m. A curvature was perceptible in one petiole in between 4 hrs. 30 m.
and 5 hrs., after the suspension of a little loop of string. A loop of
fine cotton thread, weighing one sixteenth of a grain (4.05 mg.), not
only caused a petiole slowly to bend, but was ultimately so firmly
clasped that it could be withdrawn only by some little force. The
petioles, when coming into contact with a stick, take either a complete
or half a turn round it, and ultimately increase much in thickness. They
do not possess the power of spontaneously revolving.
_Lophospermum scandens_, var. _purpureum_.—Some long, moderately thin
internodes made four revolutions at an average rate of 3 hrs. 15 m. The
course pursued was very irregular, namely, an extremely narrow ellipse, a
large circle, an irregular spire or a zigzag line, and sometimes the apex
stood still. The young petioles, when brought by the revolving movement
into contact with sticks, clasped them, and soon increased considerably
in thickness. But they are not quite so sensitive to a weight as those
of the _Rhodochiton_, for loops of thread weighing one-eighth of a grain
did not always cause them to bend.
This plant presents a case not observed by me in any other leaf-climber
or twiner, {71} namely, that the young internodes of the stem are
sensitive to a touch. When a petiole of this species clasps a stick, it
draws the base of the internode against it; and then the internode itself
bends towards the stick, which is caught between the stem and the petiole
as by a pair of pincers. The internode afterwards straightens itself,
excepting the part in actual contact with the stick. Young internodes
alone are sensitive, and these are sensitive on all sides along their
whole length. I made fifteen trials by twice or thrice lightly rubbing
with a thin twig several internodes; and in about 2 hrs., but in one case
in 3 hrs., all were bent: they became straight again in about 4 hrs.
afterwards. An internode, which was rubbed as often as six or seven
times, became just perceptibly curved in 1 hr. 15 m., and in 3 hrs. the
curvature increased much; it became straight again in the course of the
succeeding night. I rubbed some internodes one day on one side, and the
next day either on the opposite side or at right angles to the first
side; and the curvature was always towards the rubbed side.
According to Palm (p. 63), the petioles of _Linaria cirrhosa_ and, to a
limited degree, those of _L. elatine_ have the power of clasping a
support.
SOLANACEÆ.—_Solanum jasminoides_.—Some of the species in this large genus
are twiners; but the present species is a true leaf-climber. A long,
nearly upright shoot made four revolutions, moving against the sun, very
regularly at an average rate of 3 hrs. 26 m. The shoots, however,
sometimes stood still. It is considered a greenhouse plant; but when
kept there, the petioles took several days to clasp a stick: in the
hothouse a stick was clasped in 7 hrs. In the greenhouse a petiole was
not affected by a loop of string, suspended during several days and
weighing 2½ grains (163 mg.); but in the hothouse one was made to curve
by a loop weighing 1.64 gr. (106.27 mg.); and, on the removal of the
string, it became straight again. Another petiole was not at all acted
on by a loop weighing only 0.82 of a grain (53.14 mg.) We have seen that
the petioles of some other leaf-climbing plants are affected by
one-thirteenth of this latter weight. In this species, and in no other
leaf-climber seen by me, a full-grown leaf is capable of clasping a
stick; but in the greenhouse the movement was so extraordinarily slow
that the act required several weeks; on each succeeding week it was clear
that the petiole had become more and more curved, until at last it firmly
clasped the stick.
[Picture: Fig. 3. Solanum jasminoides, with one of its petioles clasping
a stick]
The flexible petiole of a half or a quarter grown leaf which has clasped
an object for three or four days increases much in thickness, and after
several weeks becomes so wonderfully hard and rigid that it can hardly be
removed from its support. On comparing a thin transverse slice of such a
petiole with one from an older leaf growing close beneath, which had not
clasped anything, its diameter was found to be fully doubled, and its
structure greatly changed. In two other petioles similarly compared, and
here represented, the increase in diameter was not quite so great. In
the section of the petiole in its ordinary state (A), we see a semilunar
band of cellular tissue (not well shown in the woodcut) differing
slightly in appearance from that outside it, and including three closely
approximate groups of dark vessels. Near the upper surface of the
petiole, beneath two exterior ridges, there are two other small circular
groups of vessels. In the section of the petiole (B) which had clasped
during several weeks a stick, the two exterior ridges have become much
less prominent, and the two groups of woody vessels beneath them much
increased in diameter. The semilunar band has been converted into a
complete ring of very hard, white, woody tissue, with lines radiating
from the centre. The three groups of vessels, which, though near
together, were before distinct, are now completely blended. The upper
part of this ring of woody vessels, formed by the prolongation of the
horns of the original semilunar band, is narrower than the lower part,
and slightly less compact. This petiole after clasping the stick had
actually become thicker than the stem from which it arose; and this was
chiefly due to the increased thickness of the ring of wood. This ring
presented, both in a transverse and longitudinal section, a closely
similar structure to that of the stem. It is a singular morphological
fact that the petiole should thus acquire a structure almost identically
the same with that of the axis; and it is a still more singular
physiological fact that so great a change should have been induced by the
mere act of clasping a support. {75}
[Picture: Fig. 4. Solanum jasminoides. A. Section of the petiole in its
ordinary state. B. Section of the petiole some weeks after it had
clasped a stick, as shown in fig. 2]
FUMARIACEÆ.—_Fumaria officinalis_.—It could not have been anticipated
that so lowly a plant as this Fumaria should have been a climber. It
climbs by the aid of the main and lateral petioles of its compound
leaves; and even the much-flattened terminal portion of the petiole can
seize a support. I have seen a substance as soft as a withered blade of
grass caught. Petioles which have clasped any object ultimately become
rather thicker and more cylindrical. On lightly rubbing several petioles
with a twig, they became perceptibly curved in 1 hr. 15 m., and
subsequently straightened themselves. A stick gently placed in the angle
between two sub-petioles excited them to move, and was almost clasped in
9 hrs. A loop of thread, weighing one-eighth of a grain, caused, after
12 hrs. and before 20 hrs, had elapsed, a considerable curvature; but it
was never fairly clasped by the petiole. The young internodes are in
continual movement, which is considerable in extent, but very irregular;
a zigzag line, or a spire crossing itself; or a figure of 8 being formed.
The course during 12 hrs., when traced on a bell-glass, apparently
represented about four ellipses. The leaves themselves likewise move
spontaneously, the main petioles curving themselves in accordance with
the movements of the internodes; so that when the latter moved to one
side, the petioles moved to the same side, then, becoming straight,
reversed their curvature. The petioles, however, do not move over a wide
space, as could be seen when a shoot was securely tied to a stick. The
leaf in this case followed an irregular course, like that made by the
internodes.
_Adlumia cirrhosa_.—I raised some plants late in the summer; they formed
very fine leaves, but threw up no central stem. The first-formed leaves
were not sensitive; some of the later ones were so, but only towards
their extremities, which were thus enabled to clasp sticks. This could
be of no service to the plant, as these leaves rose from the ground; but
it showed what the future character of the plant would have been, had it
grown tall enough to climb. The tip of one of these basal leaves, whilst
young, described in 1 hr. 36 m. a narrow ellipse, open at one end, and
exactly three inches in length; a second ellipse was broader, more
irregular, and shorter, viz., only 2½ inches in length, and was completed
in 2 hrs. 2 m. From the analogy of _Fumaria_ and _Corydalis_, I have no
doubt that the internodes of Adlumia have the power of revolving.
_Corydalis claviculata_.—This plant is interesting from being in a
condition so exactly intermediate between a leaf-climber and a
tendril-bearer, that it might have been described under either head; but,
for reasons hereafter assigned, it has been classed amongst
tendril-bearers.
Besides the plants already described, _Bignonia unguis_ and its close
allies, though aided by tendrils, have clasping petioles. According to
Mohl (p. 40), _Cocculus Japonicus_ (one of the Menispermaceæ) and a fern,
the _Ophioglossum Japonicum_ (p. 39), climb by their leaf-stalks.
* * * * *
We now come to a small section of plants which climb by means of the
produced midribs or tips of their leaves.
LILIACEÆ.—_Gloriosa Plantii_.—The stem of a half-grown plant continually
moved, generally describing an irregular spire, but sometimes oval
figures with the longer axes directed in different lines. It either
followed the sun, or moved in an opposite course, and sometimes stood
still before reversing its direction. One oval was completed in 3 hrs.
40 m.; of two horseshoe-shaped figures, one was completed in 4 hrs. 35 m.
and the other in 3 hrs. The shoots, in their movements, reached points
between four and five inches asunder. The young leaves, when first
developed, stand up nearly vertically; but by the growth of the axis, and
by the spontaneous bending down of the terminal half of the leaf, they
soon become much inclined, and ultimately horizontal. The end of the
leaf forms a narrow, ribbon-like, thickened projection, which at first is
nearly straight, but by the time the leaf gets into an inclined position,
the end bends downwards into a well-formed hook. This hook is now strong
and rigid enough to catch any object, and, when caught, to anchor the
plant and stop the revolving movement. Its inner surface is sensitive,
but not in nearly so high a degree as that of the many before-described
petioles; for a loop of string, weighing 1.64 grain, produced no effect.
When the hook has caught a thin twig or even a rigid fibre, the point may
be perceived in from 1 hr. to 3 hrs. to have curled a little inwards;
and, under favourable circumstances, it curls round and permanently
seizes an object in from 8 hrs. to 10 hrs. The hook when first formed,
before the leaf has bent downwards, is but little sensitive. If it
catches hold of nothing, it remains open and sensitive for a long time;
ultimately the extremity spontaneously and slowly curls inwards, and
makes a button-like, flat, spiral coil at the end of the leaf. One leaf
was watched, and the hook remained open for thirty-three days; but during
the last week the tip had curled so much inwards that only a very thin
twig could have been inserted within it. As soon as the tip has curled
so much inwards that the hook is converted into a ring, its sensibility
is lost; but as long as it remains open some sensibility is retained.
Whilst the plant was only about six inches in height, the leaves, four or
five in number, were broader than those subsequently produced; their soft
and but little-attenuated tips were not sensitive, and did not form
hooks; nor did the stem then revolve. At this early period of growth,
the plant can support itself; its climbing powers are not required, and
consequently are not developed. So again, the leaves on the summit of a
full-grown flowering plant, which would not require to climb any higher,
were not sensitive and could not clasp a stick. We thus see how perfect
is the economy of nature.
COMMELYNACEÆ.—_Flagellaria Indica_.—From dried specimens it is manifest
that this plant climbs exactly like the _Gloriosa_. A young plant 12
inches in height, and bearing fifteen leaves, had not a single leaf as
yet produced into a hook or tendril-like filament; nor did the stem
revolve. Hence this plant acquires its climbing powers later in life
than does the _Gloriosa_ lily. According to Mohl (p. 41), _Uvularia_
(Melanthaceæ) also climbs like _Gloriosa_.
These three last-named genera are Monocotyledons; but there is one
Dicotyledon, namely _Nepenthes_, which is ranked by Mohl (p. 41) amongst
tendril-bearers; and I hear from Dr. Hooker that most of the species
climb well at Kew. This is effected by the stalk or midrib between the
leaf and the pitcher coiling round any support. The twisted part becomes
thicker; but I observed in Mr. Veitch’s hothouse that the stalk often
takes a turn when not in contact with any object, and that this twisted
part is likewise thickened. Two vigorous young plants of _N. lævis_ and
_N. distillatoria_, in my hothouse, whilst less than a foot in height,
showed no sensitiveness in their leaves, and had no power of climbing.
But when _N. lævis_ had grown to a height of 16 inches, there were signs
of these powers. The young leaves when first formed stand upright, but
soon become inclined; at this period they terminate in a stalk or
filament, with the pitcher at the extremity hardly at all developed. The
leaves now exhibited slight spontaneous movements; and when the terminal
filaments came into contact with a stick, they slowly bent round and
firmly seized it. But owing to the subsequent growth of the leaf, this
filament became after a time quite slack, though still remaining firmly
coiled round the stick. Hence it would appear that the chief use of the
coiling, at least whilst the plant is young, is to support the pitcher
with its load of secreted fluid.
* * * * *
_Summary on Leaf-climbers_.—Plants belonging to eight families are known
to have clasping petioles, and plants belonging to four families climb by
the tips of their leaves. In all the species observed by me, with one
exception, the young internodes revolve more or less regularly, in some
cases as regularly as those of a twining plant. They revolve at various
rates, in most cases rather rapidly. Some few can ascend by spirally
twining round a support. Differently from most twiners, there is a
strong tendency in the same shoot to revolve first in one and then in an
opposite direction. The object gained by the revolving movement is to
bring the petioles or the tips of the leaves into contact with
surrounding objects; and without this aid the plant would be much less
successful in climbing. With rare exceptions, the petioles are sensitive
only whilst young. They are sensitive on all sides, but in different
degrees in different plants; and in some species of _Clematis_ the
several parts of the same petiole differ much in sensitiveness. The
hooked tips of the leaves of the _Gloriosa_ are sensitive only on their
inner or inferior surfaces. The petioles are sensitive to a touch and to
excessively slight continued pressure, even from a loop of soft thread
weighing only the one-sixteenth of a grain (4.05 mg.); and there is
reason to believe that the rather thick and stiff petioles of _Clematis
flammula_ are sensitive to even much less weight if spread over a wide
surface. The petioles always bend towards the side which is pressed or
touched, at different rates in different species, sometimes within a few
minutes, but generally after a much longer period. After temporary
contact with any object, the petiole continues to bend for a considerable
time; afterwards it slowly becomes straight again, and can then re-act.
A petiole excited by an extremely slight weight sometimes bends a little,
and then becomes accustomed to the stimulus, and either bends no more or
becomes straight again, the weight still remaining suspended. Petioles
which have clasped an object for some little time cannot recover their
original position. After remaining clasped for two or three days, they
generally increase much in thickness either throughout their whole
diameter or on one side alone; they subsequently become stronger and more
woody, sometimes to a wonderful degree; and in some cases they acquire an
internal structure like that of the stem or axis.
The young internodes of the _Lophospermum_ as well as the petioles are
sensitive to a touch, and by their combined movement seize an object.
The flower-peduncles of the _Maurandia semperflorens_ revolve
spontaneously and are sensitive to a touch, yet are not used for
climbing. The leaves of at least two, and probably of most, of the
species of _Clematis_, of _Fumaria_ and _Adlumia_, spontaneously curve
from side to side, like the internodes, and are thus better adapted to
seize distant objects. The petioles of the perfect leaves of _Tropæolum
tricolorum_, as well as the tendril-like filaments of the plants whilst
young, ultimately move towards the stem or the supporting stick, which
they then clasp. These petioles and filaments also show some tendency to
contract spirally. The tips of the uncaught leaves of the _Gloriosa_, as
they grow old, contract into a flat spire or helix. These several facts
are interesting in relation to true tendrils.
With leaf climbers, as with twining plants, the first internodes which
rise from the ground do not, at least in the cases observed by me,
spontaneously revolve; nor are the petioles or tips of the first-formed
leaves sensitive. In certain species of _Clematis_, the large size of
the leaves, together with their habit of revolving, and the extreme
sensitiveness of their petioles, appear to render the revolving movement
of the internodes superfluous; and this latter power has consequently
become much enfeebled. In certain species of _Tropæolum_, both the
spontaneous movements of the internodes and the sensitiveness of the
petioles have become much enfeebled, and in one species have been
completely lost.
CHAPTER III.
TENDRIL-BEARERS.
Nature of tendrils—BIGNONIACEÆ, various species of, and their different
modes of climbing—Tendrils which avoid the light and creep into
crevices—Development of adhesive discs—Excellent adaptations for seizing
different kinds of supports.—POLEMONIACEÆ—_Cobæa scandens_ much branched
and hooked tendrils, their manner of
action—LEGUMINOSÆ—COMPOSITÆ—SMILACEÆ—_Smilax aspera_, its inefficient
tendrils—FUMARIACEÆ—_Corydalis claviculata_, its state intermediate
between that of a leaf-climber and a tendril-bearer.
BY tendrils I mean filamentary organs, sensitive to contact and used
exclusively for climbing. By this definition, spines, hooks and
rootlets, all of which are used for climbing, are excluded. True
tendrils are formed by the modification of leaves with their petioles, of
flower-peduncles, branches, {84} and perhaps stipules. Mohl, who
includes under the name of tendrils various organs having a similar
external appearance, classes them according to their homological nature,
as being modified leaves, flower-peduncles, &c. This would be an
excellent scheme; but I observe that botanists are by no means unanimous
on the homological nature of certain tendrils. Consequently I will
describe tendril-bearing plants by natural families, following Lindley’s
classification; and this will in most cases keep those of the same nature
together. The species to be described belong to ten families, and will
be given in the following order:—_Bignoniaceæ_, _Polemoniaceæ_,
_Leguminosæ_, _Compositæ_, _Smilaceæ_, _Fumariaceæ_, _Cucurbitaceæ_,
_Vitaceæ_, _Sapindaceæ_, _Passifloraceæ_. {85}
BIGNONIACEÆ.—This family contains many tendril-bearers, some twiners, and
some root-climbers. The tendrils always consist of modified leaves.
Nine species of _Bignonia_, selected by hazard, are here described, in
order to show what diversity of structure and action there may be within
the same genus, and to show what remarkable powers some tendrils possess.
The species, taken together, afford connecting links between twiners,
leaf-climbers, tendril-bearers, and root-climbers.
[Picture: Fig. 5. Bignonia. Unnamed species from Kew]
_Bignonia_ (an unnamed species from Kew, closely allied to _B. unguis_,
but with smaller and rather broader leaves).—A young shoot from a
cut-down plant made three revolutions against the sun, at an average rate
of 2 hrs. 6 m. The stem is thin and flexible; it twined round a slender
vertical stick, ascending from left to right, as perfectly and as
regularly as any true twining-plant. When thus ascending, it makes no
use of its tendrils or petioles; but when it twined round a rather thick
stick, and its petioles were brought into contact with it, these curved
round the stick, showing that they have some degree of irritability. The
petioles also exhibit a slight degree of spontaneous movement; for in one
case they certainly described minute, irregular, vertical ellipses. The
tendrils apparently curve themselves spontaneously to the same side with
the petioles; but from various causes, it was difficult to observe the
movement of either the tendrils or petioles, in this and the two
following species. The tendrils are so closely similar in all respects
to those of _B. unguis_, that one description will suffice.
_Bignonia unguis_.—The young shoots revolve, but less regularly and less
quickly than those of the last species. The stem twines imperfectly
round a vertical stick, sometimes reversing its direction, in the same
manner as described in so many leaf-climbers; and this plant though
possessing tendrils, climbs to a certain extent like a leaf-climber.
Each leaf consists of a petiole bearing a pair of leaflets, and
terminates in a tendril, which is formed by the modification of three
leaflets, and closely resembles that above figured (fig. 5). But it is a
little larger, and in a young plant was about half an inch in length. It
is curiously like the leg and foot of a small bird, with the hind toe cut
off. The straight leg or tarsus is longer than the three toes, which are
of equal length, and diverging, lie in the same plane. The toes
terminate in sharp, hard claws, much curved downwards, like those on a
bird’s foot. The petiole of the leaf is sensitive to contact; even a
small loop of thread suspended for two days caused it to bend upwards;
but the sub-petioles of the two lateral leaflets are not sensitive. The
whole tendril, namely, the tarsus and the three toes, are likewise
sensitive to contact, especially on their under surfaces. When a shoot
grows in the midst of thin branches, the tendrils are soon brought by the
revolving movement of the internodes into contact with them; and then one
toe of the tendril or more, commonly all three, bend, and after several
hours seize fast hold of the twigs, like a bird when perched. If the
tarsus of the tendril comes into contact with a twig, it goes on slowly
bending, until the whole foot is carried quite round, and the toes pass
on each side of the tarsus and seize it. In like manner, if the petiole
comes into contact with a twig, it bends round, carrying the tendril,
which then seizes its own petiole or that of the opposite leaf. The
petioles move spontaneously, and thus, when a shoot attempts to twine
round an upright stick, those on both sides after a time come into
contact with it, and are excited to bend. Ultimately the two petioles
clasp the stick in opposite directions, and the foot-like tendrils,
seizing on each other or on their own petioles, fasten the stem to the
support with surprising security. The tendrils are thus brought into
action, if the stem twines round a thin vertical stick; and in this
respect the present species differs from the last. Both species use
their tendrils in the same manner when passing through a thicket. This
plant is one of the most efficient climbers which I have observed; and it
probably could ascend a polished stem incessantly tossed by heavy storms.
To show how important vigorous health is for the action of all the parts,
I may mention that when I first examined a plant which was growing
moderately well, though not vigorously, I concluded that the tendrils
acted only like the hooks on a bramble, and that it was the most feeble
and inefficient of all climbers!
_Bignonia Tweedyana_.—This species is closely allied to the last, and
behaves in the same manner; but perhaps twines rather better round a
vertical stick. On the same plant, one branch twined in one direction
and another in an opposite direction. The internodes in one case made
two circles, each in 2 hrs. 33 m. I was enabled to observe the
spontaneous movements of the petioles better in this than in the two
preceding species: one petiole described three small vertical ellipses in
the course of 11 hrs., whilst another moved in an irregular spire. Some
little time after a stem has twined round an upright stick, and is
securely fastened to it by the clasping petioles and tendrils, it emits
aërial roots from the bases of its leaves; and these roots curve partly
round and adhere to the stick. This species of _Bignonia_, therefore,
combines four different methods of climbing generally characteristic of
distinct plants, namely, twining, leaf-climbing, tendril-climbing, and
root-climbing.
In the three foregoing species, when the foot-like tendril has caught an
object, it continues to grow and thicken, and ultimately becomes
wonderfully strong, in the same manner as the petioles of leaf-climbers.
If the tendril catches nothing, it first slowly bends downwards, and then
its power of clasping is lost. Very soon afterwards it disarticulates
itself from the petiole, and drops off like a leaf in autumn. I have
seen this process of disarticulation in no other tendrils, for these,
when they fail to catch an object, merely wither away.
_Bignonia venusta_.—The tendrils differ considerably from those of the
previous species. The lower part, or tarsus, is four times as long as
the three toes; these are of equal length and diverge equally, but do not
lie in the same plane; their tips are bluntly hooked, and the whole
tendril makes an excellent grapnel. The tarsus is sensitive on all
sides; but the three toes are sensitive only on their outer surfaces.
The sensitiveness is not much developed; for a slight rubbing with a twig
did not cause the tarsus or the toes to become curved until an hour had
elapsed, and then only in a slight degree. Subsequently they
straightened themselves. Both the tarsus and toes can seize well hold of
sticks. If the stem is secured, the tendrils are seen spontaneously to
sweep large ellipses; the two opposite tendrils moving independently of
one another. I have no doubt, from the analogy of the two following
allied species, that the petioles also move spontaneously; but they are
not irritable like those of _B. unguis_ and _B. Tweedyana_. The young
internodes sweep large circles, one being completed in 2 hrs. 15 m., and
a second in 2 hrs. 55 m. By these combined movements of the internodes,
petioles, and grapnel-like tendrils, the latter are soon brought into
contact with surrounding objects. When a shoot stands near an upright
stick, it twines regularly and spirally round it. As it ascends, it
seizes the stick with one of its tendrils, and, if the stick be thin, the
right—and left-hand tendrils are alternately used. This alternation
follows from the stem necessarily taking one twist round its own axis for
each completed circle.
The tendrils contract spirally a short time after catching any object;
those which catch nothing merely bend slowly downwards. But the whole
subject of the spiral contraction of tendrils will be discussed after all
the tendril-bearing species have been described.
_Bignonia littoralis_.—The young internodes revolve in large ellipses.
An internode bearing immature tendrils made two revolutions, each in 3
hrs. 50 m.; but when grown older with the tendrils mature, it made two
ellipses, each at the rate of 2 hrs. 44 m. This species, unlike the
preceding, is incapable of twining round a stick: this does not appear to
be due to any want of flexibility in the internodes or to the action of
the tendrils, and certainly not to any want of the revolving power; nor
can I account for the fact. Nevertheless the plant readily ascends a
thin upright stick by seizing a point above with its two opposite
tendrils, which then contract spirally. If the tendrils seize nothing,
they do not become spiral.
The species last described, ascended a vertical stick by twining spirally
and by seizing it alternately with its opposite tendrils, like a sailor
pulling himself up a rope, hand over hand; the present species pulls
itself up, like a sailor seizing with both hands together a rope above
his head.
The tendrils are similar in structure to those of the last species. They
continue growing for some time, even after they have clasped an object.
When fully grown, though borne by a young plant, they are 9 inches in
length. The three divergent toes are shorter relatively to the tarsus
than in the former species; they are blunt at their tips and but slightly
hooked; they are not quite equal in length, the middle one being rather
longer than the others. Their outer surfaces are highly sensitive; for
when lightly rubbed with a twig, they became perceptibly curved in 4 m.
and greatly curved in 7 m. In 7 hrs. they became straight again and were
ready to re-act. The tarsus, for the space of one inch close to the
toes, is sensitive, but in a rather less degree than the toes; for the
latter after a slight rubbing, became curved in about half the time.
Even the middle part of the tarsus is sensitive to prolonged contact, as
soon as the tendril has arrived at maturity. After it has grown old, the
sensitiveness is confined to the toes, and these are only able to curl
very slowly round a stick. A tendril is perfectly ready to act, as soon
as the three toes have diverged, and at this period their outer surfaces
first become irritable. The irritability spreads but little from one
part when excited to another: thus, when a stick was caught by the part
immediately beneath the three toes, these seldom clasped it, but remained
sticking straight out.
The tendrils revolve spontaneously. The movement begins before the
tendril is converted into a three-pronged grapnel by the divergence of
the toes, and before any part has become sensitive; so that the revolving
movement is useless at this early period. The movement is, also, now
slow, two ellipses being completed conjointly in 24 hrs. 18 m. A mature
tendril made an ellipse in 6 hrs.; so that it moved much more slowly than
the internodes. The ellipses which were swept, both in a vertical and
horizontal plane, were of large size. The petioles are not in the least
sensitive, but revolve like the tendrils. We thus see that the young
internodes, the petioles, and the tendrils all continue revolving
together, but at different rates. The movements of the tendrils which
rise opposite one another are quite independent. Hence, when the whole
shoot is allowed freely to revolve, nothing can be more intricate than
the course followed by the extremity of each tendril. A wide space is
thus irregularly searched for some object to be grasped.
One other curious point remains to be mentioned. In the course of a few
days after the toes have closely clasped a stick, their blunt extremities
become developed, though not invariably, into irregular disc-like balls
which have the power of adhering firmly to the wood. As similar cellular
outgrowths will be fully described under _B. capreolata_, I will here say
nothing more about them.
_Bignonia æquinoctialis_, var. _Chamberlaynii_.—The internodes, the
elongated non-sensitive petioles, and the tendrils all revolve. The stem
does not twine, but ascends a vertical stick in the same manner as the
last species. The tendrils also resemble those of the last species, but
are shorter; the three toes are more unequal in length, the two outer
ones being about one-third shorter and rather thinner than the middle
toe; but they vary in this respect. They terminate in small hard points;
and what is important, cellular adhesive discs are not developed. The
reduced size of two of the toes as well as their lessened sensitiveness,
seem to indicate a tendency to abortion; and on one of my plants the
first-formed tendrils were sometimes simple, that is, were not divided
into three toes. We are thus naturally led to the three following
species with undivided tendrils:—
_Bignonia speciosa_.—The young shoots revolve irregularly, making narrow
ellipses, spires or circles, at rates varying from 3 hrs. 30 m. to 4 hrs.
40 m.; but they show no tendency to twine. Whilst the plant is young and
does not require a support, tendrils are not developed. Those borne by a
moderately young plant were five inches in length. They revolve
spontaneously, as do the short and non-sensitive petioles. When rubbed,
they slowly bend to the rubbed side and subsequently straighten
themselves; but they are not highly sensitive. There is something
strange in their behaviour: I repeatedly placed close to them, thick and
thin, rough and smooth sticks and posts, as well as string suspended
vertically, but none of these objects were well seized. After clasping
an upright stick, they repeatedly loosed it again, and often would not
seize it at all, or their extremities did not coil closely round. I have
observed hundreds of tendrils belonging to various Cucurbitaceous,
Passifloraceous, and Leguminous plants, and never saw one behave in this
manner. When, however, my plant had grown to a height of eight or nine
feet, the tendrils acted much better. They now seized a thin, upright
stick horizontally, that is, at a point on their own level, and not some
way up the stick as in the case of all the previous species.
Nevertheless, the non-twining stem was enabled by this means to ascend
the stick.
The extremity of the tendril is almost straight and sharp. The whole
terminal portion exhibits a singular habit, which in an animal would be
called an instinct; for it continually searches for any little crevice or
hole into which to insert itself. I had two young plants; and, after
having observed this habit, I placed near them posts, which had been
bored by beetles, or had become fissured by drying. The tendrils, by
their own movement and by that of the internodes, slowly travelled over
the surface of the wood, and when the apex came to a hole or fissure it
inserted itself; in order to effect this the extremity for a length of
half or quarter of an inch, would often bend itself at right angles to
the basal part. I have watched this process between twenty and thirty
times. The same tendril would frequently withdraw from one hole and
insert its point into a second hole. I have also seen a tendril keep its
point, in one case for 20 hrs. and in another for 36 hrs., in a minute
hole, and then withdraw it. Whilst the point is thus temporarily
inserted, the opposite tendril goes on revolving.
The whole length of a tendril often fits itself closely to any surface of
wood with which it has come into contact; and I have observed one bent at
right angles, from having entered a wide and deep fissure, with its apex
abruptly re-bent and inserted into a minute lateral hole. After a
tendril has clasped a stick, it contracts spirally; if it remains
unattached it hangs straight downwards. If it has merely adapted itself
to the inequalities of a thick post, though it has clasped nothing, or if
it has inserted its apex into some little fissure, this stimulus suffices
to induce spiral contraction; but the contraction always draws the
tendril away from the post. So that in every case these movements, which
seem so nicely adapted for some purpose, were useless. On one occasion,
however, the tip became permanently jammed into a narrow fissure. I
fully expected, from the analogy of _B. capreolata_ and _B. littoralis_,
that the tips would have been developed into adhesive discs; but I could
never detect even a trace of this process. There is therefore at present
something unintelligible about the habits of this plant.
_Bignonia picta_.—This species closely resembles the last in the
structure and movements of its tendrils. I also casually examined a fine
growing plant of the allied _B. Lindleyi_, and this apparently behaved in
all respects in the same manner.
_Bignonia capreolata_.—We now come to a species having tendrils of a
different type; but first for the internodes. A young shoot made three
large revolutions, following the sun, at an average rate of 2 hrs. 23 m.
The stem is thin and flexible, and I have seen one make four regular
spiral turns round a thin upright stick, ascending of course from right
to left, and therefore in a reversed direction compared with the before
described species. Afterwards, from the interference of the tendrils, it
ascended either straight up the stick or in an irregular spire. The
tendrils are in some respects highly remarkable. In a young plant they
were about 2½ inches in length and much branched, the five chief branches
apparently representing two pairs of leaflets and a terminal one. Each
branch is, however, bifid or more commonly trifid towards the extremity,
with the points blunt yet distinctly hooked. A tendril bends to any side
which is lightly rubbed, and subsequently becomes straight again; but a
loop of thread weighing ¼th of a grain produced no effect. On two
occasions the terminal branches became slightly curved in 10 m. after
they had touched a stick; and in 30 m. the tips were curled quite round
it. The basal part is less sensitive. The tendrils revolved in an
apparently capricious manner, sometimes very slightly or not at all; at
other times they described large regular ellipses. I could detect no
spontaneous movement in the petioles of the leaves.
Whilst the tendrils are revolving more or less regularly, another
remarkable movement takes place, namely, a slow inclination from the
light towards the darkest side of the house. I repeatedly changed the
position of my plants, and some little time after the revolving movement
had ceased, the successively formed tendrils always ended by pointing to
the darkest side. When I placed a thick post near a tendril, between it
and the light, the tendril pointed in that direction. In two instances a
pair of leaves stood so that one of the two tendrils was directed towards
the light and the other to the darkest side of the house; the latter did
not move, but the opposite one bent itself first upwards and then right
over its fellow, so that the two became parallel, one above the other,
both pointing to the dark: I then turned the plant half round; and the
tendril which had turned over recovered its original position, and the
opposite one which had not before moved, now turned over to the dark
side. Lastly, on another plant, three pairs of tendrils were produced at
the same time by three shoots, and all happened to be differently
directed: I placed the pot in a box open only on one side, and obliquely
facing the light; in two days all six tendrils pointed with unerring
truth to the darkest corner of the box, though to do this each had to
bend in a different manner. Six wind-vanes could not have more truly
shown the direction of the wind, than did these branched tendrils the
course of the stream of light which entered the box. I left these
tendrils undisturbed for above 24 hrs., and then turned the pot half
round; but they had now lost their power of movement, and could not any
longer avoid the light.
When a tendril has not succeeded in clasping a support, either through
its own revolving movement or that of the shoot, or by turning towards
any object which intercepts the light, it bends vertically downwards and
then towards its own stem, which it seizes together with the supporting
stick, if there be one. A little aid is thus given in keeping the stem
secure. If the tendril seizes nothing, it does not contract spirally,
but soon withers away and drops off. If it seizes an object, all the
branches contract spirally.
I have stated that after a tendril has come into contact with a stick, it
bends round it in about half an hour; but I repeatedly observed, as in
the case of _B. speciosa_ and its allies, that it often again loosed the
stick; sometimes seizing and loosing the same stick three or four times.
Knowing that the tendrils avoided the light, I gave them a glass tube
blackened within, and a well-blackened zinc plate: the branches curled
round the tube and abruptly bent themselves round the edges of the zinc
plate; but they soon recoiled from these objects with what I can only
call disgust, and straightened themselves. I then placed a post with
extremely rugged bark close to a pair of tendrils; twice they touched it
for an hour or two, and twice they withdrew; at last one of the hooked
extremities curled round and firmly seized an excessively minute
projecting point of bark, and then the other branches spread themselves
out, following with accuracy every inequality of the surface. I
afterwards placed near the plant a post without bark but much fissured,
and the points of the tendrils crawled into all the crevices in a
beautiful manner. To my surprise, I observed that the tips of the
immature tendrils, with the branches not yet fully separated, likewise
crawled just like roots into the minutest crevices. In two or three days
after the tips had thus crawled into the crevices, or after their hooked
ends had seized minute points, the final process, now to be described,
commenced.
This process I discovered by having accidentally left a piece of wool
near a tendril; and this led me to bind a quantity of flax, moss, and
wool loosely round sticks, and to place them near tendrils. The wool
must not be dyed, for these tendrils are excessively sensitive to some
poisons. The hooked points soon caught hold of the fibres, even loosely
floating fibres, and now there was no recoiling; on the contrary, the
excitement caused the hooks to penetrate the fibrous mass and to curl
inwards, so that each hook caught firmly one or two fibres, or a small
bundle of them. The tips and the inner surfaces of the hooks now began
to swell, and in two or three days were visibly enlarged. After a few
more days the hooks were converted into whitish, irregular balls, rather
above the 0.05th of an inch (1.27 mm.) in diameter, formed of coarse
cellular tissue, which sometimes wholly enveloped and concealed the hooks
themselves. The surfaces of these balls secrete some viscid resinous
matter, to which the fibres of the flax, &c., adhere. When a fibre has
become fastened to the surface, the cellular tissue does not grow
directly beneath it, but continues to grow closely on each side; so that
when several adjoining fibres, though excessively thin, were caught, so
many crests of cellular matter, each not as thick as a human hair, grew
up between them, and these, arching over on both sides, adhered firmly
together. As the whole surface of the ball continues to grow, fresh
fibres adhere and are afterwards enveloped; so that I have seen a little
ball with between fifty and sixty fibres of flax crossing it at various
angles and all embedded more or less deeply. Every gradation in the
process could be followed—some fibres merely sticking to the surface,
others lying in more or less deep furrows, or deeply embedded, or passing
through the very centre of the cellular ball. The embedded fibres are so
closely clasped that they cannot be withdrawn. The outgrowing tissue has
so strong a tendency to unite, that two balls produced by distinct
tendrils sometimes unite and grow into a single one.
On one occasion, when a tendril had curled round a stick, half an inch in
diameter, an adhesive disc was formed; but this does not generally occur
in the case of smooth sticks or posts. If, however, the tip catches a
minute projecting point, the other branches form discs, especially if
they find crevices to crawl into. The tendrils failed to attach
themselves to a brick wall.
I infer from the adherence of the fibres to the discs or balls, that
these secrete some resinous adhesive matter; and more especially from
such fibres becoming loose if immersed in sulphuric ether. This fluid
likewise removes small, brown, glistening points which can generally be
seen on the surfaces of the older discs. If the hooked extremities of
the tendrils do not touch anything, discs, as far as I have seen, are
never formed; {102} but temporary contact during a moderate time suffices
to cause their development. I have seen eight discs formed on the same
tendril. After their development the tendrils contract spirally, and
become woody and very strong. A tendril in this state supported nearly
seven ounces, and would apparently have supported a considerably greater
weight, had not the fibres of flax to which the discs were attached
yielded.
From the facts now given, we may infer that though the tendrils of this
Bignonia can occasionally adhere to smooth cylindrical sticks and often
to rugged bark, yet that they are specially adapted to climb trees
clothed with lichens, mosses, or other such productions; and I hear from
Professor Asa Gray that the _Polypodium incanum_ abounds on the
forest-trees in the districts of North America where this species of
Bignonia grows. Finally, I may remark how singular a fact it is that a
leaf should be metamorphosed into a branched organ which turns from the
light, and which can by its extremities either crawl like roots into
crevices, or seize hold of minute projecting points, these extremities
afterwards forming cellular outgrowths which secrete an adhesive cement,
and then envelop by their continued growth the finest fibres.
_Eccremocarpus scaber_ (_Bignoniaceæ_).—Plants, though growing pretty
well in my green-house, showed no spontaneous movements in their shoots
or tendrils; but when removed to the hot-house, the young internodes
revolved at rates varying from 3 hrs. 15 m. to 1 hr. 13 m. One large
circle was swept at this latter unusually quick rate; but generally the
circles or ellipses were small, and sometimes the course pursued was
quite irregular. An internode, after making several revolutions,
sometimes stood still for 12 hrs. or 18 hrs., and then recommenced
revolving. Such strongly marked interruptions in the movements of the
internodes I have observed in hardly any other plant.
The leaves bear four leaflets, themselves subdivided, and terminate in
much-branched tendrils. The main petiole of the leaf, whilst young,
moves spontaneously, and follows nearly the same irregular course and at
about the same rate as the internodes. The movement to and from the stem
is the most conspicuous, and I have seen the chord of a curved petiole
which formed an angle of 59° with the stem, in an hour afterwards making
an angle of 106°. The two opposite petioles do not move together, and
one is sometimes so much raised as to stand close to the stem, whilst the
other is not far from horizontal. The basal part of the petiole moves
less than the distal part. The tendrils, besides being carried by the
moving petioles and internodes, themselves move spontaneously; and the
opposite tendrils occasionally move in opposite directions. By these
combined movements of the young internodes, petioles, and tendrils, a
considerable space is swept in search of a support.
In young plants the tendrils are about three inches in length: they bear
two lateral and two terminal branches; and each branch bifurcates twice,
with the tips terminating in blunt double hooks, having both points
directed to the same side. All the branches are sensitive on all sides;
and after being lightly rubbed, or after coming into contact with a
stick, bend in about 10 m. One which had become curved in 10 m. after a
light rub, continued bending for between 3 hrs. and 4 hrs., and became
straight again in 8 hrs. or 9 hrs. Tendrils, which have caught nothing,
ultimately contract into an irregular spire, as they likewise do, only
much more quickly, after clasping a support. In both cases the main
petiole bearing the leaflets, which is at first straight and inclined a
little upwards, moves downwards, with the middle part bent abruptly into
a right angle; but this is seen in _E. miniatus_ more plainly than in _E.
scaber_. The tendrils in this genus act in some respects like those of
_Bignonia capreolata_; but the whole does not move from the light, nor do
the hooked tips become enlarged into cellular discs. After the tendrils
have come into contact with a moderately thick cylindrical stick or with
rugged bark, the several branches may be seen slowly to lift themselves
up, change their positions, and again come into contact with the
supporting surface. The object of these movements is to bring the
double-hooks at the extremities of the branches, which naturally face in
all directions, into contact with the wood. I have watched a tendril,
half of which had bent itself at right angles round the sharp corner of a
square post, neatly bring every single hook into contact with both
rectangular surfaces. The appearance suggested the belief, that though
the whole tendril is not sensitive to light, yet that the tips are so,
and that they turn and twist themselves towards any dark surface.
Ultimately the branches arrange themselves very neatly to all the
irregularities of the most rugged bark, so that they resemble in their
irregular course a river with its branches, as engraved on a map. But
when a tendril has wound round a rather thick stick, the subsequent
spiral contraction generally draws it away and spoils the neat
arrangement. So it is, but not in quite so marked a manner, when a
tendril has spread itself over a large, nearly flat surface of rugged
bark. We may therefore conclude that these tendrils are not perfectly
adapted to seize moderately thick sticks or rugged bark. If a thin stick
or twig is placed near a tendril, the terminal branches wind quite round
it, and then seize their own lower branches or the main stem. The stick
is thus firmly, but not neatly, grasped. What the tendrils are really
adapted for, appears to be such objects as the thin culms of certain
grasses, or the long flexible bristles of a brush, or thin rigid leaves
such as those of the Asparagus, all of which they seize in an admirable
manner. This is due to the extremities of the branches close to the
little hooks being extremely sensitive to a touch from the thinnest
object, which they consequently curl round and clasp. When a small
brush, for instance, was placed near a tendril, the tips of each
sub-branch seized one, two, or three of the bristles; and then the spiral
contraction of the several branches brought all these little parcels
close together, so that thirty or forty bristles were drawn into a single
bundle, which afforded an excellent support.
POLEMONIACEÆ.—_Cobæa scandens_.—This is an excellently constructed
climber. The tendrils on a fine plant were eleven inches long, with the
petiole bearing two pairs of leaflets, only two and a half inches in
length. They revolve more rapidly and vigorously than those of any other
tendril-bearer observed by me, with the exception of one kind of
Passiflora. Three large, nearly circular sweeps, directed against the
sun were completed, each in 1 hr. 15 m.; and two other circles in 1 hr.
20 m. and 1 hr. 23 m. Sometimes a tendril travels in a much inclined
position, and sometimes nearly upright. The lower part moves but little
and the petiole not at all; nor do the internodes revolve; so that here
we have the tendril alone moving. On the other hand, with most of the
species of _Bignonia_ and the _Eccremocarpus_, the internodes, tendrils,
and petioles all revolved. The long, straight, tapering main stem of the
tendril of the _Cobæa_ bears alternate branches; and each branch is
several times divided, with the finer branches as thin as very thin
bristles and extremely flexible, so that they are blown about by a breath
of air; yet they are strong and highly elastic. The extremity of each
branch is a little flattened, and terminates in a minute double (though
sometimes single) hook, formed of a hard, translucent, woody substance,
and as sharp as the finest needle. On a tendril which was eleven inches
long I counted ninety-four of these beautifully constructed little hooks.
They readily catch soft wood, or gloves, or the skin of the naked hand.
With the exception of these hardened hooks, and of the basal part of the
central stem, every part of every branchlet is highly sensitive on all
sides to a slight touch, and bends in a few minutes towards the touched
side. By lightly rubbing several sub-branches on opposite sides, the
whole tendril rapidly assumed an extraordinarily crooked shape. These
movements from contact do not interfere with the ordinary revolving
movement. The branches, after becoming greatly curved from being
touched, straighten themselves at a quicker rate than in almost any other
tendril seen by me, namely, in between half an hour and an hour. After
the tendril has caught any object, spiral contraction likewise begins
after an unusually short interval of time, namely, in about twelve hours.
Before the tendril is mature, the terminal branchlets cohere, and the
hooks are curled closely inwards. At this period no part is sensitive to
a touch; but as soon as the branches diverge and the hooks stand out,
full sensitiveness is acquired. It is a singular circumstance that
immature tendrils revolve at their full velocity before they become
sensitive, but in a useless manner, as in this state they can catch
nothing. This want of perfect co-adaptation, though only for a short
time, between the structure and the functions of a climbing-plant is a
rare event. A tendril, as soon as it is ready to act, stands, together
with the supporting petiole, vertically upwards. The leaflets borne by
the petiole are at this time quite small, and the extremity of the
growing stem is bent to one side so as to be out of the way of the
revolving tendril, which sweeps large circles directly over head. The
tendrils thus revolve in a position well adapted for catching objects
standing above; and by this means the ascent of the plant is favoured.
If no object is caught, the leaf with its tendril bends downwards and
ultimately assumes a horizontal position. An open space is thus left for
the next succeeding and younger tendril to stand vertically upwards and
to revolve freely. As soon as an old tendril bends downwards, it loses
all power of movement, and contracts spirally into an entangled mass.
Although the tendrils revolve with unusual rapidity, the movement lasts
for only a short time. In a plant placed in the hot-house and growing
vigorously, a tendril revolved for not longer than 36 hours, counting
from the period when it first became sensitive; but during this period it
probably made at least 27 revolutions.
When a revolving tendril strikes against a stick, the branches quickly
bend round and clasp it. The little hooks here play an important part,
as they prevent the branches from being dragged away by the rapid
revolving movement, before they have had time to clasp the stick
securely. This is especially the case when only the extremity of a
branch has caught hold of a support. As soon as a tendril has bent a
smooth stick or a thick rugged post, or has come into contact with planed
wood (for it can adhere temporarily even to so smooth a surface as this),
the same peculiar movements may be observed as those described under
_Bignonia capreolata_ and _Eccremocarpus_. The branches repeatedly lift
themselves up and down; those which have their hooks already directed
downwards remaining in this position and securing the tendril, whilst the
others twist about until they succeed in arranging themselves in
conformity with every irregularity of the surface, and in bringing their
hooks into contact with the wood. The use of the hooks was well shown by
giving the tendrils tubes and slips of glass to catch; for these, though
temporarily seized, were invariably lost, either during the
re-arrangement of the branches or ultimately when spiral contraction
ensued.
The perfect manner in which the branches arranged themselves, creeping
like rootlets over every inequality of the surface and into any deep
crevice, is a pretty sight; for it is perhaps more effectually performed
by this than by any other species. The action is certainly more
conspicuous, as the upper surfaces of the main stem, as well as of every
branch to the extreme hooks, are angular and green, whilst the lower
surfaces are rounded and purple. I was led to infer, as in former cases,
that a less amount of light guided these movements of the branches of the
tendrils. I made many trials with black and white cards and glass tubes
to prove it, but failed from various causes; yet these trials
countenanced the belief. As a tendril consists of a leaf split into
numerous segments, there is nothing surprising in all the segments
turning their upper surfaces towards the light, as soon as the tendril is
caught and the revolving movement is arrested. But this will not account
for the whole movement, for the segments actually bend or curve to the
dark side besides turning round on their axes so that their upper
surfaces may face the light.
When the _Cobæa_ grows in the open air, the wind must aid the extremely
flexible tendrils in seizing a support, for I found that a mere breath
sufficed to cause the extreme branches to catch hold by their hooks of
twigs, which they could not have reached by the revolving movement. It
might have been thought that a tendril, thus hooked by the extremity of a
single branch, could not have fairly grasped its support. But several
times I watched cases like the following: tendril caught a thin stick by
the hooks of one of its two extreme branches; though thus held by the
tip, it still tried to revolve, bowing itself to all sides, and by this
movement the other extreme branch soon caught the stick. The first
branch then loosed itself, and, arranging its hooks, again caught hold.
After a time, from the continued movement of the tendril, the hooks of a
third branch caught hold. No other branches, as the tendril then stood,
could possibly have touched the stick. But before long the upper part of
the main stem began to contract into an open spire. It thus dragged the
shoot which bore the tendril towards the stick; and as the tendril
continually tried to revolve, a fourth branch was brought into contact.
And lastly, from the spiral contraction travelling down both the main
stem and the branches, all of them, one after another, were ultimately
brought into contact with the stick. They then wound themselves round it
and round one another, until the whole tendril was tied together in an
inextricable knot. The tendrils, though at first quite flexible, after
having clasped a support for a time, become more rigid and stronger than
they were at first. Thus the plant is secured to its support in a
perfect manner.
LEGUMINOSÆ.—_Pisum sativum_.—The common pea was the subject of a valuable
memoir by Dutrochet, {111} who discovered that the internodes and
tendrils revolve in ellipses. The ellipses are generally very narrow,
but sometimes approach to circles. I several times observed that the
longer axis slowly changed its direction, which is of importance, as the
tendril thus sweeps a wider space. Owing to this change of direction,
and likewise to the movement of the stem towards the light, the
successive irregular ellipses generally form an irregular spire. I have
thought it worth while to annex a tracing of the course pursued by the
upper internode (the movement of the tendril being neglected) of a young
plant from 8.40 A.M. to 9.15 P.M. The course was traced on a
hemispherical glass placed over the plant, and the dots with figures give
the hours of observation; each dot being joined by a straight line. No
doubt all the lines would have been curvilinear if the course had been
observed at much shorter intervals. The extremity of the petiole, from
which the young tendril arose, was two inches from the glass, so that if
a pencil two inches in length could have been affixed to the petiole, it
would have traced the annexed figure on the under side of the glass; but
it must be remembered that the figure is reduced by one-half. Neglecting
the first great sweep towards the light from the figure 1 to 2, the end
of the petiole swept a space 4 inches across in one direction, and 3
inches in another. As a full-grown tendril is considerably above two
inches in length, and as the tendril itself bends and revolves in harmony
with the internode, a considerably wider space is swept than is here
represented on a reduced scale. Dutrochet observed the completion of an
ellipse in 1 hr. 20 m.; and I saw one completed in 1 hr. 30 m. The
direction followed is variable, either with or against the sun.
[Picture: Fig. 6. Side of room with window] {113}
Dutrochet asserts that the petioles of the leaves spontaneously revolve,
as well as the young internodes and tendrils; but he does not say that he
secured the internodes; when this was done, I could never detect any
movement in the petiole, except to and from the light.
The tendrils, on the other hand, when the internodes and petioles are
secured, describe irregular spires or regular ellipses, exactly like
those made by the internodes. A young tendril, only 1⅛ of an inch in
length, revolved. Dutrochet has shown that when a plant is placed in a
room, so that the light enters laterally, the internodes travel much
quicker to the light than from it: on the other hand, he asserts that the
tendril itself moves from the light towards the dark side of the room.
With due deference to this great observer, I think he was mistaken, owing
to his not having secured the internodes. I took a young plant with
highly sensitive tendrils, and tied the petiole so that the tendril alone
could move; it completed a perfect ellipse in 1 hr. 30 m.; I then turned
the plant partly round, but this made no change in the direction of the
succeeding ellipse. The next day I watched a plant similarly secured
until the tendril (which was highly sensitive) made an ellipse in a line
exactly to and from the light; the movement was so great that the tendril
at the two ends of its elliptical course bent itself a little beneath the
horizon, thus travelling more than 180 degrees; but the curvature was
fully as great towards the light as towards the dark side of the room. I
believe Dutrochet was misled by not having secured the internodes, and by
having observed a plant of which the internodes and tendrils no longer
curved in harmony together, owing to inequality of age.
Dutrochet made no observations on the sensitiveness of the tendrils.
These, whilst young and about an inch in length with the leaflets on the
petiole only partially expanded, are highly sensitive; a single light
touch with a twig on the inferior or concave surface near the tip caused
them to bend quickly, as did occasionally a loop of thread weighing
one-seventh of a grain (9.25 mg.). The upper or convex surface is barely
or not at all sensitive. Tendrils, after bending from a touch,
straighten themselves in about two hours, and are then ready to act
again. As soon as they begin to grow old, the extremities of their two
or three pairs of branches become hooked, and they then appear to form an
excellent grappling instrument; but this is not the case. For at this
period they have generally quite lost their sensitiveness; and when
hooked on to twigs, some were not at all affected, and others required
from 18 hrs. to 24 hrs. before clasping such twigs; nevertheless, they
were able to utilise the last vestige of irritability owing to their
extremities being hooked. Ultimately the lateral branches contract
spirally, but not the middle or main stem.
_Lathyrus aphaca_.—This plant is destitute of leaves, except during a
very early age, these being replaced by tendrils, and the leaves
themselves by large stipules. It might therefore have been expected that
the tendrils would have been highly organized, but this is not so. They
are moderately long, thin, and unbranched, with their tips slightly
curved. Whilst young they are sensitive on all sides, but chiefly on the
concave side of the extremity. They have no spontaneous revolving power,
but are at first inclined upwards at an angle of about 45°, then move
into a horizontal position, and ultimately bend downwards. The young
internodes, on the other hand, revolve in ellipses, and carry with them
the tendrils. Two ellipses were completed, each in nearly 5 hrs.; their
longer axes were directed at about an angle of 45° to the axis of the
previously made ellipse.
_Lathyrus grandiflorus_.—The plants observed were young and not growing
vigorously, yet sufficiently so, I think, for my observations to be
trusted. If so, we have the rare case of neither internodes nor tendrils
revolving. The tendrils of vigorous plants are above 4 inches in length,
and are often twice divided into three branches; the tips are curved and
are sensitive on their concave sides; the lower part of the central stem
is hardly at all sensitive. Hence this plant appears to climb simply by
its tendrils being brought, through the growth of the stem, or more
efficiently by the wind, into contact with surrounding objects, which
they then clasp. I may add that the tendrils, or the internodes, or
both, of _Vicia sativa_ revolve.
COMPOSITÆ.—_Mutisia clematis_.—The immense family of the Compositæ is
well known to include very few climbing plants. We have seen in the
Table in the first chapter that _Mikania scandens_ is a regular twiner,
and F. Müller informs me that in S. Brazil there is another species which
is a leaf-climber. _Mutisia_ is the only genus in the family, as far as
I can learn, which bears tendrils: it is therefore interesting to find
that these, though rather less metamorphosed from their primordial foliar
condition than are most other tendrils, yet display all the ordinary
characteristic movements, both those that are spontaneous and those which
are excited by contact.
The long leaf bears seven or eight alternate leaflets, and terminates in
a tendril which, in a plant of considerable size, was 5 inches in length.
It consists generally of three branches; and these, although much
elongated, evidently represent the petioles and midribs of three
leaflets; for they closely resemble the same parts in an ordinary leaf,
in being rectangular on the upper surface, furrowed, and edged with
green. Moreover, the green edging of the tendrils of young plants
sometimes expands into a narrow lamina or blade. Each branch is curved a
little downwards, and is slightly hooked at the extremity.
A young upper internode revolved, judging from three revolutions, at an
average rate of 1 hr. 38 m.; it swept ellipses with the longer axes
directed at right angles to one another; but the plant, apparently,
cannot twine. The petioles and the tendrils are both in constant
movement. But their movement is slower and much less regularly
elliptical than that of the internodes. They appear to be much affected
by the light, for the whole leaf usually sinks down during the night and
rises during the day, moving, also, during the day in a crooked course to
the west. The tip of the tendril is highly sensitive on the lower
surface; and one which was just touched with a twig became perceptibly
curved in 3 m., and another in 5 m.; the upper surface is not at all
sensitive; the sides are moderately sensitive, so that two branches which
were rubbed on their inner sides converged and crossed each other. The
petiole of the leaf and the lower parts of the tendril, halfway between
the upper leaflet and the lowest branch, are not sensitive. A tendril
after curling from a touch became straight again in about 6 hrs., and was
ready to re-act; but one that had been so roughly rubbed as to have
coiled into a helix did not become perfectly straight until after 13 hrs.
The tendrils retain their sensibility to an unusually late age; for one
borne by a leaf with five or six fully developed leaves above, was still
active. If a tendril catches nothing, after a considerable interval of
time the tips of the branches curl a little inwards; but if it clasps
some object, the whole contracts spirally.
[Picture: Fig. 7. Smilax aspera]
SMILACEÆ.—_Smilax aspera_, var. _maculata_.—Aug. St.-Hilaire {118}
considers that the tendrils, which rise in pairs from the petiole, are
modified lateral leaflets; but Mohl (p. 41) ranks them as modified
stipules. These tendrils are from 1½ to 1¾ inches in length, are thin,
and have slightly curved, pointed extremities. They diverge a little
from each other, and stand at first nearly upright. When lightly rubbed
on either side, they slowly bend to that side, and subsequently become
straight again. The back or convex side when placed in contact with a
stick became just perceptibly curved in 1 hr. 20 m., but did not
completely surround it until 48 hrs. had elapsed; the concave side of
another became considerably curved in 2 hrs. and clasped a stick in 5
hrs. As the pairs of tendrils grow old, one tendril diverges more and
more from the other, and both slowly bend backwards and downwards, so
that after a time they project on the opposite side of the stem to that
from which they arise. They then still retain their sensitiveness, and
can clasp a support placed _behind_ the stem. Owing to this power, the
plant is able to ascend a thin upright stick. Ultimately the two
tendrils belonging to the same petiole, if they do not come into contact
with any object, loosely cross each other behind the stem, as at B, in
fig. 7. This movement of the tendrils towards and round the stem is, to
a certain extent, guided by their avoidance of the light; for when a
plant stood so that one of the two tendrils was compelled in thus slowly
moving to travel towards the light, and the other from the light, the
latter always moved, as I repeatedly observed, more quickly than its
fellow. The tendrils do not contract spirally in any case. Their chance
of finding a support depends on the growth of the plant, on the wind, and
on their own slow backward and downward movement, which, as we have just
seen, is guided, to a certain extent, by the avoidance of the light; for
neither the internodes nor the tendrils have any proper revolving
movement. From this latter circumstance, from the slow movements of the
tendrils after contact (though their sensitiveness is retained for an
unusual length of time), from their simple structure and shortness, this
plant is a less perfect climber than any other tendril-bearing species
observed by me. The plant whilst young and only a few inches in height,
does not produce any tendrils; and considering that it grows to only
about 8 feet in height, that the stem is zigzag and is furnished, as well
as the petioles, with spines, it is surprising that it should be provided
with tendrils, comparatively inefficient though these are. The plant
might have been left, one would have thought, to climb by the aid of its
spines alone, like our brambles. As, however, it belongs to a genus,
some of the species of which are furnished with much longer tendrils, we
may suspect that it possesses these organs solely from being descended
from progenitors more highly organized in this respect.
FUMARIACEÆ.—_Corydalis claviculata_.—According to Mohl (p. 43), the
extremities of the branched stem, as well as the leaves, are converted
into tendrils. In the specimens examined by me all the tendrils were
certainly foliar, and it is hardly credible that the same plant should
produce tendrils of a widely different homological nature. Nevertheless,
from this statement by Mohl, I have ranked this species amongst the
tendril-bearers; if classed exclusively by its foliar tendrils, it would
be doubtful whether it ought not to have been placed amongst the
leaf-climbers, with its allies, _Fumaria_ and _Adlumia_. A large
majority of its so-called tendrils still bear leaflets, though
excessively reduced in size; but some few of them may properly be
designated as tendrils, for they are completely destitute of laminæ or
blades. Consequently, we here behold a plant in an actual state of
transition from a leaf-climber to a tendril-bearer. Whilst the plant is
rather young, only the outer leaves, but when full-grown all the leaves,
have their extremities converted into more or less perfect tendrils. I
have examined specimens from one locality alone, viz. Hampshire; and it
is not improbable that plants growing under different conditions might
have their leaves a little more or less changed into true tendrils.
Whilst the plant is quite young, the first-formed leaves are not modified
in any way, but those next formed have their terminal leaflets reduced in
size, and soon all the leaves assume the structure represented in the
following drawing. This leaf bore nine leaflets; the lower ones being
much subdivided. The terminal portion of the petiole, about 1½ inch in
length (above the leaflet _f_), is thinner and more elongated than the
lower part, and may be considered as the tendril. The leaflets borne by
this part are greatly reduced in size, being, on an average, about the
tenth of an inch in length and very narrow; one small leaflet measured
one-twelfth of an inch in length and one-seventy-fifth in breadth (2.116
mm. and 0.339 mm.), so that it was almost microscopically minute. All
the reduced leaflets have branching nerves, and terminate in little
spines, like those of the fully developed leaflets. Every gradation
could be traced, until we come to branchlets (as _a_ and _d_ in the
figure) which show no vestige of a lamina or blade. Occasionally all the
terminal branchlets of the petiole are in this condition, and we then
have a true tendril.
[Picture: Fig. 8. Corydalis claviculata. Leaf-tendril of natural size]
The several terminal branches of the petiole bearing the much reduced
leaflets (_a_, _b_, _c_, _d_) are highly sensitive, for a loop of thread
weighing only the one-sixteenth of a grain (4.05 mg.) caused them to
become greatly curved in under 4 hrs. When the loop was removed, the
petioles straightened themselves in about the same time. The petiole
(_e_) was rather less sensitive; and in another specimen, in which the
corresponding petiole bore rather larger leaflets, a loop of thread
weighing one-eighth of a grain did not cause curvature until 18 hrs. had
elapsed. Loops of thread weighing one-fourth of a grain, left suspended
on the lower petioles (_f_ to _l_) during several days, produced no
effect. Yet the three petioles _f_, _g_, and _h_ were not quite
insensible, for when left in contact with a stick for a day or two they
slowly curled round it. Thus the sensibility of the petiole gradually
diminishes from the tendril-like extremity to the base. The internodes
of the stem are not at all sensitive, which makes Mohl’s statement that
they are sometimes converted into tendrils the more surprising, not to
say improbable.
The whole leaf, whilst young and sensitive, stands almost vertically
upwards, as we have seen to be the case with many tendrils. It is in
continual movement, and one that I observed swept at an average rate of
about 2 hrs. for each revolution, large, though irregular, ellipses,
which were sometimes narrow, sometimes broad, with their longer axes
directed to different points of the compass. The young internodes,
likewise revolved irregularly in ellipses or spires; so that by these
combined movements a considerable space was swept for a support. If the
terminal and attenuated portion of a petiole fails to seize any object,
it ultimately bends downwards and inwards, and soon loses all
irritability and power of movement. This bending down differs much in
nature from that which occurs with the extremities of the young leaves in
many species of _Clematis_; for these, when thus bent downwards or
hooked, first acquire their full degree of sensitiveness.
_Dicentra thalictrifolia_.—In this allied plant the metamorphosis of the
terminal leaflets is complete, and they are converted into perfect
tendrils. Whilst the plant is young, the tendrils appear like modified
branches, and a distinguished botanist thought that they were of this
nature; but in a full-grown plant there can be no doubt, as I am assured
by Dr. Hooker, that they are modified leaves. When of full size, they
are above 5 inches in length; they bifurcate twice, thrice, or even four
times; their extremities are hooked and blunt. All the branches of the
tendrils are sensitive on all sides, but the basal portion of the main
stem is only slightly so. The terminal branches when lightly rubbed with
a twig became curved in the course of from 30 m. to 42 m., and
straightened themselves in between 10 hrs. and 20 hrs. A loop of thread
weighing one-eighth of a grain plainly caused the thinner branches to
bend, as did occasionally a loop weighing one-sixteenth of a grain; but
this latter weight, though left suspended, was not sufficient to cause a
permanent flexure. The whole leaf with its tendril, as well as the young
upper internodes, revolves vigorously and quickly, though irregularly,
and thus sweeps a wide space. The figure traced on a bell-glass was
either an irregular spire or a zigzag line. The nearest approach to an
ellipse was an elongated figure of 8, with one end a little open, and
this was completed in 1 hr. 53 m. During a period of 6 hrs. 17 m.
another shoot made a complex figure, apparently representing three and a
half ellipses. When the lower part of the petiole bearing the leaflets
was securely fastened, the tendril itself described similar but much
smaller figures.
This species climbs well. The tendrils after clasping a stick become
thicker and more rigid; but the blunt hooks do not turn and adapt
themselves to the supporting surface, as is done in so perfect a manner
by some Bignoniaceæ and Cobæa. The tendrils of young plants, two or
three feet in height, are only half the length of those borne by the same
plant when grown taller, and they do not contract spirally after clasping
a support, but only become slightly flexuous. Full-sized tendrils, on
the other hand, contract spirally, with the exception of the thick basal
portion. Tendrils which have caught nothing simply bend downwards and
inwards, like the extremities of the leaves of the _Corydalis
claviculata_. But in all cases the petiole after a time is angularly and
abruptly bent downwards like that of Eccremocarpus.
CHAPTER IV.
TENDRIL-BEARERS—(_continued_).
CUCURBITACEÆ.—Homologous nature of the tendrils—_Echinocystis lobata_,
remarkable movements of the tendrils to avoid seizing the terminal
shoot—Tendrils not excited by contact with another tendril or by drops of
water—Undulatory movement of the extremity of the tendril—_Hanburya_,
adherent discs—VITACÆ—Gradation between the flower-peduncles and tendrils
of the vine—Tendrils of the Virginian Creeper turn from the light, and,
after contact, develop adhesive
discs—SAPINDACEÆ—PASSIFLORACEÆ—_Passiflora gracilis_—Rapid revolving
movement and sensitiveness of the tendrils—Not sensitive to the contact
of other tendrils or of drops of water—Spiral contraction of
tendrils—Summary on the nature and action of tendrils.
CUCURBITACEÆ.—The tendrils in this family have been ranked by competent
judges as modified leaves, stipules, or branches; or as partly a leaf and
partly a branch. De Candolle believes that the tendrils differ in their
homological nature in two of the tribes. {127a} From facts recently
adduced, Mr. Berkeley thinks that Payer’s view is the most probable,
namely, that the tendril is “a separate portion of the leaf itself;” but
much may be said in favour of the belief that it is a modified
flower-peduncle. {127b}
_Echinocystis lobata_.—Numerous observations were made on this plant
(raised from seed sent me by Prof. Asa Gray), for the spontaneous
revolving movements of the internodes and tendrils were first observed by
me in this case, and greatly perplexed me. My observations may now be
much condensed. I observed thirty-five revolutions of the internodes and
tendrils; the slowest rate was 2 hrs. and the average rate, with no great
fluctuations, 1 hr. 40 m. Sometimes I tied the internodes, so that the
tendrils alone moved; at other times I cut off the tendrils whilst very
young, so that the internodes revolved by themselves; but the rate was
not thus affected. The course generally pursued was with the sun, but
often in an opposite direction. Sometimes the movement during a short
time would either stop or be reversed; and this apparently was due to
interference from the light, as, for instance, when I placed a plant
close to a window. In one instance, an old tendril, which had nearly
ceased revolving, moved in one direction, whilst a young tendril above
moved in an opposite course. The two uppermost internodes alone revolve;
and as soon as the lower one grows old, only its upper part continues to
move. The ellipses or circles swept by the summits of the internodes are
about three inches in diameter; whilst those swept by the tips of the
tendrils, are from 15 to 16 inches in diameter. During the revolving
movement, the internodes become successively curved to all points of the
compass; in one part of their course they are often inclined, together
with the tendrils, at about 45° to the horizon, and in another part stand
vertically up. There was something in the appearance of the revolving
internodes which continually gave the false impression that their
movement was due to the weight of the long and spontaneously revolving
tendril; but, on cutting off the latter with sharp scissors, the top of
the shoot rose only a little, and went on revolving. This false
appearance is apparently due to the internodes and tendrils all curving
and moving harmoniously together.
A revolving tendril, though inclined during the greater part of its
course at an angle of about 45° (in one case of only 37°) above the
horizon, stiffened and straightened itself from tip to base in a certain
part of its course, thus becoming nearly or quite vertical. I witnessed
this repeatedly; and it occurred both when the supporting internodes were
free and when they were tied up; but was perhaps most conspicuous in the
latter case, or when the whole shoot happened to be much inclined. The
tendril forms a very acute angle with the projecting extremity of the
stem or shoot; and the stiffening always occurred as the tendril
approached, and had to pass over the shoot in its circular course. If it
had not possessed and exercised this curious power, it would infallibly
have struck against the extremity of the shoot and been arrested. As
soon as the tendril with its three branches begins to stiffen itself in
this manner and to rise from an inclined into a vertical position, the
revolving motion becomes more rapid; and as soon as the tendril has
succeeded in passing over the extremity of the shoot or point of
difficulty, its motion, coinciding with that from its weight, often
causes it to fall into its previously inclined position so quickly, that
the apex could be seen travelling like the minute hand of a gigantic
clock.
The tendrils are thin, from 7 to 9 inches in length, with a pair of short
lateral branches rising not far from the base. The tip is slightly and
permanently curved, so as to act to a limited extent as a hook. The
concave side of the tip is highly sensitive to a touch; but not so the
convex side, as was likewise observed to be the case with other species
of the family by Mohl (p. 65). I repeatedly proved this difference by
lightly rubbing four or five times the convex side of one tendril, and
only once or twice the concave side of another tendril, and the latter
alone curled inwards. In a few hours afterwards, when the tendrils which
had been rubbed on the concave side had straightened themselves, I
reversed the process of rubbing, and always with the same result. After
touching the concave side, the tip becomes sensibly curved in one or two
minutes; and subsequently, if the touch has been at all rough, it coils
itself into a helix. But the helix will, after a time, straighten
itself, and be again ready to act. A loop of thin thread only
one-sixteenth of a grain in weight caused a temporary flexure. The lower
part was repeatedly rubbed rather roughly, but no curvature ensued; yet
this part is sensitive to prolonged pressure, for when it came into
contact with a stick, it would slowly wind round it.
One of my plants bore two shoots near together, and the tendrils were
repeatedly drawn across one another, but it is a singular fact that they
did not once catch each other. It would appear as if they had become
habituated to contact of this kind, for the pressure thus caused must
have been much greater than that caused by a loop of soft thread weighing
only the one-sixteenth of a grain. I have, however, seen several
tendrils of _Bryonia dioica_ interlocked, but they subsequently released
one another. The tendrils of the Echinocystis are also habituated to
drops of water or to rain; for artificial rain made by violently flirting
a wet brush over them produced not the least effect.
The revolving movement of a tendril is not stopped by the curving of its
extremity after it has been touched. When one of the lateral branches
has firmly clasped an object, the middle branch continues to revolve.
When a stem is bent down and secured, so that the tendril depends but is
left free to move, its previous revolving movement is nearly or quite
stopped; but it soon begins to bend upwards, and as soon as it has become
horizontal the revolving movement recommences. I tried this four times;
the tendril generally rose to a horizontal position in an hour or an hour
and a half; but in one case, in which a tendril depended at an angle of
45° beneath the horizon, the uprising took two hours; in half an hour
afterwards it rose to 23° above the horizon and then recommenced
revolving. This upward movement is independent of the action of light,
for it occurred twice in the dark, and on another occasion the light came
in on one side alone. The movement no doubt is guided by opposition to
the force of gravity, as in the case of the ascent of the plumules of
germinating seeds.
A tendril does not long retain its revolving power; and as soon as this
is lost, it bends downwards and contracts spirally. After the revolving
movement has ceased, the tip still retains for a short time its
sensitiveness to contact, but this can be of little or no use to the
plant.
Though the tendril is highly flexible, and though the extremity travels,
under favourable circumstances, at about the rate of an inch in two
minutes and a quarter, yet its sensitiveness to contact is so great that
it hardly ever fails to seize a thin stick placed in its path. The
following case surprised me much: I placed a thin, smooth, cylindrical
stick (and I repeated the experiment seven times) so far from a tendril,
that its extremity could only curl half or three-quarters round the
stick; but I always found that the tip managed in the course of a few
hours to curl twice or even thrice round the stick. I at first thought
that this was due to rapid growth on the outside; but by coloured points
and measurements I proved that there had been no sensible increase of
length within the time. When a stick, flat on one side, was similarly
placed, the tip of the tendril could not curl beyond the flat surface,
but coiled itself into a helix, which, turning to one side, lay flat on
the little flat surface of wood. In one instance a portion of tendril
three-quarters of an inch in length was thus dragged on to the flat
surface by the coiling in of the helix. But the tendril thus acquires a
very insecure hold, and generally after a time slips off. In one case
alone the helix subsequently uncoiled itself, and the tip then passed
round and clasped the stick. The formation of the helix on the flat side
of the stick apparently shows us that the continued striving of the tip
to curl itself closely inwards gives the force which drags the tendril
round a smooth cylindrical stick. In this latter case, whilst the
tendril was slowly and quite insensibly crawling onwards, I observed
several times through a lens that the whole surface was not in close
contact with the stick; and I can understand the onward progress only by
supposing that the movement is slightly undulatory or vermicular, and
that the tip alternately straightens itself a little and then again curls
inwards. It thus drags itself onwards by an insensibly slow, alternate
movement, which may be compared to that of a strong man suspended by the
ends of his fingers to a horizontal pole, who works his fingers onwards
until he can grasp the pole with the palm of his hand. However this may
be, the fact is certain that a tendril which has caught a round stick
with its extreme point, can work itself onwards until it has passed twice
or even thrice round the stick, and has permanently grasped it.
_Hanburya Mexicana_.—The young internodes and tendrils of this anomalous
member of the family, revolve in the same manner and at about the same
rate as those of the _Echinocystis_. The stem does not twine, but can
ascend an upright stick by the aid of its tendrils. The concave tip of
the tendril is very sensitive; after it had become rapidly coiled into a
ring owing to a single touch, it straightened itself in 50 m. The
tendril, when in full action, stands vertically up, with the projecting
extremity of the young stem thrown a little on one side, so as to be out
of the way; but the tendril bears on the inner side, near its base, a
short rigid branch, which projects out at right angles like a spur, with
the terminal half bowed a little downwards. Hence, as the main vertical
branch revolves, the spur, from its position and rigidity, cannot pass
over the extremity of the shoot, in the same curious manner as do the
three branches of the tendril of the _Echinocystis_, namely, by
stiffening themselves at the proper point. The spur is therefore pressed
laterally against the young stem in one part of the revolving course, and
thus the sweep of the lower part of the main branch is much restricted.
A nice case of co-adaptation here comes into play: in all the other
tendrils observed by me, the several branches become sensitive at the
same period: had this been the case with the _Hanburya_, the inwardly
directed, spur-like branch, from being pressed, during the revolving
movement, against the projecting end of the shoot, would infallibly have
seized it in a useless or injurious manner. But the main branch of the
tendril, after revolving for a time in a vertical position, spontaneously
bends downwards; and in doing so, raises the spur-like branch, which
itself also curves upwards; so that by these combined movements it rises
above the projecting end of the shoot, and can now move freely without
touching the shoot; and now it first becomes sensitive.
The tips of both branches, when they come into contact with a stick,
grasp it like any ordinary tendril. But in the course of a few days, the
lower surface swells and becomes developed into a cellular layer, which
adapts itself closely to the wood, and firmly adheres to it. This layer
is analogous to the adhesive discs formed by the extremities of the
tendrils of some species of _Bignonia_ and of _Ampelopsis_; but in the
_Hanburya_ the layer is developed along the terminal inner surface,
sometimes for a length of 1¾ inches, and not at the extreme tip. The
layer is white, whilst the tendril is green, and near the tip it is
sometimes thicker than the tendril itself; it generally spreads a little
beyond the sides of the tendril, and is fringed with free elongated
cells, which have enlarged globular or retort-shaped heads. This
cellular layer apparently secretes some resinous cement; for its adhesion
to the wood was not lessened by an immersion of 24 hrs. in alcohol or
water, but was quite loosened by a similar immersion in ether or
turpentine. After a tendril has once firmly coiled itself round a stick,
it is difficult to imagine of what use the adhesive cellular layer can
be. Owing to the spiral contraction which soon ensues, the tendrils were
never able to remain, excepting in one instance, in contact with a thick
post or a nearly flat surface; if they had quickly become attached by
means of the adhesive layer, this would evidently have been of service to
the plant.
The tendrils of _Bryonia dioica_, _Cucurbita ovifera_, and _Cucumis
sativa_ are sensitive and revolve. Whether the internodes likewise
revolve I did not observe. In _Anguria Warscewiczii_, the internodes,
though thick and stiff, revolve: in this plant the lower surface of the
tendril, some time after clasping a stick, produces a coarsely cellular
layer or cushion, which adapts itself closely to the wood, like that
formed by the tendril of the _Hanburya_; but it is not in the least
adhesive. In _Zanonia Indica_, which belongs to a different tribe of the
family, the forked tendrils and the internodes revolve in periods between
2 hrs. 8 m. and 3 hrs. 35 m., moving against the sun.
VITACEÆ.—In this family and in the two following, namely, the Sapindacæ
and Passifloraceæ, the tendrils are modified flower-peduncles; and are
therefore axial in their nature. In this respect they differ from all
those previously described, with the exception, perhaps, of the
Cucurbitaceæ. The homological nature, however, of a tendril seems to
make no difference in its action.
[Picture: Fig. 9. Tendril of the Vine. A. Peduncle of tendril. B.
Longer Branch, with a scale at its base. C. Shorter branch. D.
Petiole of the opposite leaf]
_Vitis vinifera_.—The tendril is thick and of great length; one from a
vine growing out of doors and not vigorously, was 16 inches long. It
consists of a peduncle (A), bearing two branches which diverge equally
from it. One of the branches (B) has a scale at its base; it is always,
as far as I have seen, longer than the other and often bifurcates. The
branches when rubbed become curved, and subsequently straighten
themselves. After a tendril has clasped any object with its extremity,
it contracts spirally; but this does not occur (Palm, p. 56) when no
object has been seized. The tendrils move spontaneously from side to
side; and on a very hot day, one made two elliptical revolutions, at an
average rate of 2 hrs. 15 m. During these movements a coloured line,
painted along the convex surface, appeared after a time on one side, then
on the concave side, then on the opposite side, and lastly again on the
convex side. The two branches of the same tendril have independent
movements. After a tendril has spontaneously revolved for a time, it
bends from the light towards the dark: I do not state this on my own
authority, but on that of Mohl and Dutrochet. Mohl (p. 77) says that in
a vine planted against a wall the tendrils point towards it, and in a
vineyard generally more or less to the north.
The young internodes revolve spontaneously; but the movement is unusually
slight. A shoot faced a window, and I traced its course on the glass
during two perfectly calm and hot days. On one of these days it
described, in the course of ten hours, a spire, representing two and a
half ellipses. I also placed a bell-glass over a young Muscat grape in
the hot-house, and it made each day three or four very small oval
revolutions; the shoot moving less than half an inch from side to side.
Had it not made at least three revolutions whilst the sky was uniformly
overcast, I should have attributed this slight degree of movement to the
varying action of the light. The extremity of the stem is more or less
bent downwards, but it never reverses its curvature, as so generally
occurs with twining plants.
[Picture: Fig. 10. Flower-stalk of the Vine. A. Common Peduncle. B.
Flower-tendril. C. Sub-Peduncle, bearing the flower-buds. D. Petiole
of the opposite leaf]
Various authors (Palm, p. 55; Mohl, p. 45; Lindley, &c.) believe that the
tendrils of the vine are modified flower-peduncles. I here give a
drawing (fig. 10) of the ordinary state of a young flower-stalk: it
consists of the “common peduncle” (A); of the “flower-tendril” (B), which
is represented as having caught a twig; and of the “sub-peduncle” (C)
bearing the flower-buds. The whole moves spontaneously, like a true
tendril, but in a less degree; the movement, however, is greater when the
sub-peduncle (C) does not bear many flower-buds. The common peduncle (A)
has not the power of clasping a support, nor has the corresponding part
of a true tendril. The flower-tendril (B) is always longer than the
sub-peduncle (C) and has a scale at its base; it sometimes bifurcates,
and therefore corresponds in every detail with the longer scale-bearing
branch (B, fig. 9) of the true tendril. It is, however, inclined
backwards from the sub-peduncle (C), or stands at right angles with it,
and is thus adapted to aid in carrying the future bunch of grapes. When
rubbed, it curves and subsequently straightens itself; and it can, as is
shown in the drawing, securely clasp a support. I have seen an object as
soft as a young vine-leaf caught by one.
The lower and naked part of the sub-peduncle (C) is likewise slightly
sensitive to a rub, and I have seen it bent round a stick and even partly
round a leaf with which it had come into contact. That the sub-peduncle
has the same nature as the corresponding branch of an ordinary tendril,
is well shown when it bears only a few flowers; for in this case it
becomes less branched, increases in length, and gains both in
sensitiveness and in the power of spontaneous movement. I have twice
seen sub-peduncles which bore from thirty to forty flower-buds, and which
had become considerably elongated and were completely wound round sticks,
exactly like true tendrils. The whole length of another sub-peduncle,
bearing only eleven flower-buds, quickly became curved when slightly
rubbed; but even this scanty number of flowers rendered the stalk less
sensitive than the other branch, that is, the flower-tendril; for the
latter after a lighter rub became curved more quickly and in a greater
degree. I have seen a sub-peduncle thickly covered with flower-buds,
with one of its higher lateral branchlets bearing from some cause only
two buds; and this one branchlet had become much elongated and had
spontaneously caught hold of an adjoining twig; in fact, it formed a
little sub-tendril. The increasing length of the sub-peduncle (C) with
the decreasing number of the flower-buds is a good instance of the law of
compensation. In accordance with this same principle, the true tendril
as a whole is always longer than the flower-stalk; for instance, on the
same plant, the longest flower-stalk (measured from the base of the
common peduncle to the tip of the flower-tendril) was 8½ inches in
length, whilst the longest tendril was nearly double this length, namely
16 inches.
The gradations from the ordinary state of a flower-stalk, as represented
in the drawing (fig. 10), to that of a true tendril (fig. 9) are
complete. We have seen that the sub-peduncle (C), whilst still bearing
from thirty to forty flower-buds, sometimes becomes a little elongated
and partially assumes all the characters of the corresponding branch of a
true tendril. From this state we can trace every stage till we come to a
full-sized perfect tendril, bearing on the branch which corresponds with
the sub-peduncle one single flower-bud! Hence there can be no doubt that
the tendril is a modified flower-peduncle.
Another kind of gradation well deserves notice. Flower-tendrils (B, fig.
10) sometimes produce a few flower-buds. For instance, on a vine growing
against my house, there were thirteen and twenty-two flower-buds
respectively on two flower-tendrils, which still retained their
characteristic qualities of sensitiveness and spontaneous movement, but
in a somewhat lessened degree. On vines in hothouses, so many flowers
are occasionally produced on the flower-tendrils that a double bunch of
grapes is the result; and this is technically called by gardeners a
“cluster.” In this state the whole bunch of flowers presents scarcely
any resemblance to a tendril; and, judging from the facts already given,
it would probably possess little power of clasping a support, or of
spontaneous movement. Such flower-stalks closely resemble in structure
those borne by _Cissus_. This genus, belonging to the same family of the
Vitaceæ, produces well-developed tendrils and ordinary bunches of
flowers; but there are no gradations between the two states. If the
genus _Vitis_ had been unknown, the boldest believer in the modification
of species would never have surmised that the same individual plant, at
the same period of growth, would have yielded every possible gradation
between ordinary flower-stalks for the support of the flowers and fruit,
and tendrils used exclusively for climbing. But the vine clearly gives
us such a case; and it seems to me as striking and curious an instance of
transition as can well be conceived.
_Cissus discolor_.—The young shoots show no more movement than can be
accounted for by daily variations in the action of the light. The
tendrils, however, revolve with much regularity, following the sun; and,
in the plants observed by me, swept circles of about 5 inches in
diameter. Five circles were completed in the following times:—4 hrs. 45
m., 4 hrs. 50 m., 4 hrs. 45 m., 4 hrs. 30 m., and 5 hrs. The same tendril
continues to revolve during three or four days. The tendrils are from 3½
to 5 inches in length. They are formed of a long foot-stalk, bearing two
short branches, which in old plants again bifurcate. The two branches
are not of quite equal length; and as with the vine, the longer one has a
scale at its base. The tendril stands vertically upwards; the extremity
of the shoot being bent abruptly downwards, and this position is probably
of service to the plant by allowing the tendril to revolve freely and
vertically.
Both branches of the tendril, whilst young, are highly sensitive. A
touch with a pencil, so gentle as only just to move a tendril borne at
the end of a long flexible shoot, sufficed to cause it to become
perceptibly curved in four or five minutes. It became straight again in
rather above one hour. A loop of soft thread weighing one-seventh of a
grain (9.25 mg.) was thrice tried, and each time caused the tendril to
become curved in 30 or 40 m. Half this weight produced no effect. The
long foot-stalk is much less sensitive, for a slight rubbing produced no
effect, although prolonged contact with a stick caused it to bend. The
two branches are sensitive on all sides, so that they converge if touched
on their inner sides, and diverge if touched on their outer sides. If a
branch be touched at the same time with equal force on opposite sides,
both sides are equally stimulated and there is no movement. Before
examining this plant, I had observed only tendrils which are sensitive on
one side alone, and these when lightly pressed between the finger and
thumb become curved; but on thus pinching many times the tendrils of the
_Cissus_ no curvature ensued, and I falsely inferred at first that they
were not at all sensitive.
_Cissus antarcticus_.—The tendrils on a young plant were thick and
straight, with the tips a little curved. When their concave surfaces
were rubbed, and it was necessary to do this with some force, they very
slowly became curved, and subsequently straight again. They are
therefore much less sensitive than those of the last species; but they
made two revolutions, following the sun, rather more rapidly, viz., in 3
hrs. 30 m. and 4 hrs. The internodes do not revolve.
_Ampelopsis hederacea_ (_Virginian Creeper_).—The internodes apparently
do not move more than can be accounted for by the varying action of the
light. The tendrils are from 4 to 5 inches in length, with the main stem
sending off several lateral branches, which have their tips curved, as
may be seen in the upper figure (fig. 11). They exhibit no true
spontaneous revolving movement, but turn, as was long ago observed by
Andrew Knight, {145} from the light to the dark. I have seen several
tendrils move in less than 24 hours, through an angle of 180° to the dark
side of a case in which a plant was placed, but the movement is sometimes
much slower. The several lateral branches often move independently of
one another, and sometimes irregularly, without any apparent cause.
These tendrils are less sensitive to a touch than any others observed by
me. By gentle but repeated rubbing with a twig, the lateral branches,
but not the main stem, became in the course of three or four hours
slightly curved; but they seemed to have hardly any power of again
straightening themselves. The tendrils of a plant which had crawled over
a large box-tree clasped several of the branches; but I have repeatedly
seen that they will withdraw themselves after seizing a stick. When they
meet with a flat surface of wood or a wall (and this is evidently what
they are adapted for), they turn all their branches towards it, and,
spreading them widely apart, bring their hooked tips laterally into
contact with it. In effecting this, the several branches, after touching
the surface, often rise up, place themselves in a new position, and again
come down into contact with it.
In the course of about two days after a tendril has arranged its branches
so as to press on any surface, the curved tips swell, become bright red,
and form on their under-sides the well-known little discs or cushions
with which they adhere firmly. In one case the tips were slightly
swollen in 38 hrs. after coming into contact with a brick; in another
case they were considerably swollen in 48 hrs., and in an additional 24
hrs. were firmly attached to a smooth board; and lastly, the tips of a
younger tendril not only swelled but became attached to a stuccoed wall
in 42 hrs. These adhesive discs resemble, except in colour and in being
larger, those of _Bignonia capreolata_. When they were developed in
contact with a ball of tow, the fibres were separately enveloped, but not
in so effective a manner as by _B. capreolata_. Discs are never
developed, as far as I have seen, without the stimulus of at least
temporary contact with some object. {146} They are generally first
formed on one side of the curved tip, the whole of which often becomes so
much changed in appearance, that a line of the original green tissue can
be traced only along the concave surface. When, however, a tendril has
clasped a cylindrical stick, an irregular rim or disc is sometimes formed
along the inner surface at some little distance from the curved tip; this
was also observed (p. 71) by Mohl. The discs consist of enlarged cells,
with smooth projecting hemispherical surfaces, coloured red; they are at
first gorged with fluid (see section given by Mohl, p. 70), but
ultimately become woody.
As the discs soon adhere firmly to such smooth surfaces as planed or
painted wood, or to the polished leaf of the ivy, this alone renders it
probable that some cement is secreted, as has been asserted to be the
case (quoted by Mohl, p. 71) by Malpighi. I removed a number of discs
formed during the previous year from a stuccoed wall, and left them
during many hours, in warm water, diluted acetic acid and alcohol; but
the attached grains of silex were not loosened. Immersion in sulphuric
ether for 24 hrs. loosened them much, but warmed essential oils (I tried
oil of thyme and peppermint) completely released every particle of stone
in the course of a few hours. This seems to prove that some resinous
cement is secreted. The quantity, however, must be small; for when a
plant ascended a thinly whitewashed wall, the discs adhered firmly to the
whitewash; but as the cement never penetrated the thin layer, they were
easily withdrawn, together with little scales of the whitewash. It must
not be supposed that the attachment is effected exclusively by the
cement; for the cellular outgrowth completely envelopes every minute and
irregular projection, and insinuates itself into every crevice.
[Picture: Fig. 11. Ampelopsis hederacea. A. Tendril fully developed,
with a young leaf on the opposite side of the stem. B. Older tendril,
several weeks after its attachment to a wall, with the branches thickened
and spirally contracted, and with the extremities developed into discs.
The unattached branches of this tendril have withered and dropped off]
A tendril which has not become attached to any body, does not contract
spirally; and in course of a week or two shrinks into the finest thread,
withers and drops off. An attached tendril, on the other hand, contracts
spirally, and thus becomes highly elastic, so that when the main
foot-stalk is pulled the strain is distributed equally between all the
attached discs. For a few days after the attachment of the discs, the
tendril remains weak and brittle, but it rapidly increases in thickness
and acquires great strength. During the following winter it ceases to
live, but adheres firmly in a dead state both to its own stem and to the
surface of attachment. In the accompanying diagram (fig. 11.) we see the
difference between a tendril (B) some weeks after its attachment to a
wall, with one (A) from the same plant fully grown but unattached. That
the change in the nature of the tissues, as well as the spiral
contraction, are consequent on the formation of the discs, is well shown
by any lateral branches which have not become attached; for these in a
week or two wither and drop off, in the same manner as does the whole
tendril if unattached. The gain in strength and durability in a tendril
after its attachment is something wonderful. There are tendrils now
adhering to my house which are still strong, and have been exposed to the
weather in a dead state for fourteen or fifteen years. One single
lateral branchlet of a tendril, estimated to be at least ten years old,
was still elastic and supported a weight of exactly two pounds. The
whole tendril had five disc-bearing branches of equal thickness and
apparently of equal strength; so that after having been exposed during
ten years to the weather, it would probably have resisted a strain of ten
pounds!
SAPINDACEÆ.—_Cardiospermum halicacabum_.—In this family, as in the last,
the tendrils are modified flower-peduncles. In the present plant the two
lateral branches of the main flower-peduncle have been converted into a
pair of tendrils, corresponding with the single “flower-tendril” of the
common vine. The main peduncle is thin, stiff, and from 3 to 4½ inches
in length. Near the summit, above two little bracts, it divides into
three branches. The middle one divides and re-divides, and bears the
flowers; ultimately it grows half as long again as the two other modified
branches. These latter are the tendrils; they are at first thicker and
longer than the middle branch, but never become more than an inch in
length. They taper to a point and are flattened, with the lower clasping
surface destitute of hairs. At first they project straight up; but soon
diverging, spontaneously curl downwards so as to become symmetrically and
elegantly hooked, as represented in the diagram. They are now, whilst
the flower-buds are still small, ready for action.
[Picture: Fig. 12. Cardiospermum halicacabum. Upper part of the
flower-peduncle with its two tendrils]
The two or three upper internodes, whilst young, steadily revolve; those
on one plant made two circles, against the course of the sun, in 3 hrs.
12 m.; in a second plant the same course was followed, and the two
circles were completed in 3 hrs. 41 m.; in a third plant, the internodes
followed the sun and made two circles in 3 hrs. 47 m. The average rate
of these six revolutions was 1 hr. 46 m. The stem shows no tendency to
twine spirally round a support; but the allied tendril-bearing genus
_Paullinia_ is said (Mohl, p. 4) to be a twiner. The flower-peduncles,
which stand up above the end of the shoot, are carried round and round by
the revolving movement of the internodes; and when the stem is securely
tied, the long and thin flower-peduncles themselves are seen to be in
continued and sometimes rapid movement from side to side. They sweep a
wide space, but only occasionally revolve in a regular elliptical course.
By the combined movements of the internodes and peduncles, one of the two
short hooked tendrils, sooner or later, catches hold of some twig or
branch, and then it curls round and securely grasps it. These tendrils
are, however, but slightly sensitive; for by rubbing their under surface
only a slight movement is slowly produced. I hooked a tendril on to a
twig; and in 1 hr. 45 m. it was curved considerably inwards; in 2 hrs. 30
m. it formed a ring; and in from 5 to 6 hours from being first hooked, it
closely grasped the stick. A second tendril acted at nearly the same
rate; but I observed one that took 24 hours before it curled twice round
a thin twig. Tendrils which have caught nothing, spontaneously curl up
to a close helix after the interval of several days. Those which have
curled round some object, soon become a little thicker and tougher. The
long and thin main peduncle, though spontaneously moving, is not
sensitive and never clasps a support. Nor does it ever contract
spirally, {152} although a contraction of this kind apparently would have
been of service to the plant in climbing. Nevertheless it climbs pretty
well without this aid. The seed-capsules though light, are of enormous
size (hence its English name of balloon-vine), and as two or three are
carried on the same peduncle, the tendrils rising close to them may be of
service in preventing their being dashed to pieces by the wind. In the
hothouse the tendrils served simply for climbing.
The position of the tendrils alone suffices to show their homological
nature. In two instances one of two tendrils produced a flower at its
tip; this, however, did not prevent its acting properly and curling round
a twig. In a third case both lateral branches which ought to have been
modified into tendrils, produced flowers like the central branch, and had
quite lost their tendril-structure.
I have seen, but was not enabled carefully to observe, only one other
climbing Sapindaceous plant, namely, _Paullinia_. It was not in flower,
yet bore long forked tendrils. So that, _Paullinia_, with respect to its
tendrils, appears to bear the same relation to _Cardiospermum_ that
_Cissus_ does to _Vitis_.
PASSIFLORACEÆ.—After reading the discussion and facts given by Mohl (p.
47) on the nature of the tendrils in this family, no one can doubt that
they are modified flower-peduncles. The tendrils and the
flower-peduncles rise close side by side; and my son, William E. Darwin,
made sketches for me of their earliest state of development in the hybrid
_P. floribunda_. The two organs appear at first as a single papilla
which gradually divides; so that the tendril appears to be a modified
branch of the flower-peduncle. My son found one very young tendril
surmounted by traces of floral organs, exactly like those on the summit
of the true flower-peduncle at the same early age.
_Passiflora gracilis_.—This well-named, elegant, annual species differs
from the other members of the group observed by me, in the young
internodes having the power of revolving. It exceeds all the other
climbing plants which I have examined, in the rapidity of its movements,
and all tendril-bearers in the sensitiveness of the tendrils. The
internode which carries the upper active tendril and which likewise
carries one or two younger immature internodes, made three revolutions,
following the sun, at an average rate of 1 hr. 4 m.; it then made, the
day becoming very hot, three other revolutions at an average rate of
between 57 and 58 m.; so that the average of all six revolutions was 1
hr. 1 m. The apex of the tendril describes elongated ellipses, sometimes
narrow and sometimes broad, with their longer axes inclined in slightly
different directions. The plant can ascend a thin upright stick by the
aid of its tendrils; but the stem is too stiff for it to twine spirally
round it, even when not interfered with by the tendrils, these having
been successively pinched off at an early age.
When the stem is secured, the tendrils are seen to revolve in nearly the
same manner and at the same rate as the internodes. {154} The tendrils
are very thin, delicate, and straight, with the exception of the tips,
which are a little curved; they are from 7 to 9 inches in length. A
half-grown tendril is not sensitive; but when nearly full-grown they are
extremely sensitive. A single delicate touch on the concave surface of
the tip soon caused one to curve; and in 2 minutes it formed an open
helix. A loop of soft thread weighing one thirty-second of a grain (2.02
mg.) placed most gently on the tip, thrice caused distinct curvature. A
bent bit of thin platina wire weighing only fiftieth of a grain (1.23
mg.) twice produced the same effect; but this latter weight, when left
suspended, did not suffice to cause a permanent curvature. These trials
were made under a bell-glass, so that the loops of thread and wire were
not agitated by the wind. The movement after a touch is very rapid: I
took hold of the lower part of several tendrils, and then touched their
concave tips with a thin twig and watched them carefully through a lens;
the tips evidently began to bend after the following intervals—31, 25,
32, 31, 28, 39, 31, and 30 seconds; so that the movement was generally
perceptible in half a minute after a touch; but on one occasion it was
distinctly visible in 25 seconds. One of the tendrils which thus became
bent in 31 seconds, had been touched two hours previously and had coiled
into a helix; so that in this interval it had straightened itself and had
perfectly recovered its irritability.
To ascertain how often the same tendril would become curved when touched,
I kept a plant in my study, which from being cooler than the hot-house
was not very favourable for the experiment. The extremity was gently
rubbed four or five times with a thin stick, and this was done as often
as it was observed to have become nearly straight again after having been
in action; and in the course of 54 hrs. it answered to the stimulus 21
times, becoming each time hooked or spiral. On the last occasion,
however, the movement was very slight, and soon afterwards permanent
spiral contraction commenced. No trials were made during the night, so
that the tendril would perhaps have answered a greater number of times to
the stimulus; though, on the other hand, from having no rest it might
have become exhausted from so many quickly repeated efforts.
I repeated the experiment made on the _Echinocystis_, and placed several
plants of this _Passiflora_ so close together, that their tendrils were
repeatedly dragged over each other; but no curvature ensued. I likewise
repeatedly flirted small drops of water from a brush on many tendrils,
and syringed others so violently that the whole tendril was dashed about,
but they never became curved. The impact from the drops of water was
felt far more distinctly on my hand than that from the loops of thread
(weighing one thirty-second of a grain) when allowed to fall on it from a
height, and these loops, which caused the tendrils to become curved, had
been placed most gently on them. Hence it is clear, that the tendrils
either have become habituated to the touch of other tendrils and drops of
rain, or that they were from the first rendered sensitive only to
prolonged though excessively slight pressure of solid objects, with the
exclusion of that from other tendrils. To show the difference in the
kind of sensitiveness in different plants and likewise to show the force
of the syringe used, I may add that the lightest jet from it instantly
caused the leaves of a _Mimosa_ to close; whereas the loop of thread
weighing one thirty-second of a grain, when rolled into a ball and placed
gently on the glands at the bases of the leaflets of the _Mimosa_, caused
no action.
_Passiflora punctata_.—The internodes do not move, but the tendrils
revolve regularly. A half-grown and very sensitive tendril made three
revolutions, opposed to the course of the sun, in 3 hrs. 5 m., 2 hrs. 40
m. and 2 hrs. 50 m.; perhaps it might have travelled more quickly when
nearly full-grown. A plant was placed in front of a window, and, as with
twining stems, the light accelerated the movement of the tendril in one
direction and retarded it in the other; the semicircle towards the light
being performed in one instance in 15 m. less time and in a second
instance in 20 m. less time than that required by the semicircle towards
the dark end of the room. Considering the extreme tenuity of these
tendrils, the action of the light on them is remarkable. The tendrils
are long, and, as just stated, very thin, with the tip slightly curved or
hooked. The concave side is extremely sensitive to a touch—even a single
touch causing it to curl inwards; it subsequently straightened itself,
and was again ready to act. A loop of soft thread weighing one
fourteenth of a grain (4.625 mg.) caused the extreme tip to bend; another
time I tried to hang the same little loop on an inclined tendril, but
three times it slid off; yet this extraordinarily slight degree of
friction sufficed to make the tip curl. The tendril, though so
sensitive, does not move very quickly after a touch, no conspicuous
movement being observable until 5 or 10 m. had elapsed. The convex side
of the tip is not sensitive to a touch or to a suspended loop of thread.
On one occasion I observed a tendril revolving with the convex side of
the tip forwards, and in consequence it was not able to clasp a stick,
against which it scraped; whereas tendrils revolving with the concave
side forward, promptly seize any object in their path.
_Passiflora quadrangularis_.—This is a very distinct species. The
tendrils are thick, long, and stiff; they are sensitive to a touch only
on the concave surface towards the extremity. When a stick was placed so
that the middle of the tendril came into contact with it, no curvature
ensued. In the hothouse a tendril made two revolutions, each in 2 hrs.
22 m.; in a cool room one was completed in 3 hrs., and a second in 4 hrs.
The internodes do not revolve; nor do those of the hybrid _P.
floribunda_.
_Tacsonia manicata_.—Here again the internodes do not revolve. The
tendrils are moderately thin and long; one made a narrow ellipse in 5
hrs. 20 m., and the next day a broad ellipse in 5 hrs. 7 m. The
extremity being lightly rubbed on the concave surface, became just
perceptibly curved in 7 m., distinctly in 10 m., and hooked in 20 m.
We have seen that the tendrils in the last three families, namely, the
Vitaceæ, Sapindaceæ and Passifloraceæ, are modified flower-peduncles.
This is likewise the case, according to De Candolle (as quoted by Mohl),
with the tendrils of _Brunnichia_, one of the Polygonaceæ. In two or
three species of _Modecca_, one of the Papayaceæ, the tendrils, as I hear
from Prof. Oliver, occasionally bear flowers and fruit; so that they are
axial in their nature.
_The Spiral Contraction of Tendrils_.
This movement, which shortens the tendrils and renders them elastic,
commences in half a day, or in a day or two after their extremities have
caught some object. There is no such movement in any leaf-climber, with
the exception of an occasional trace of it in the petioles of _Tropæolum
tricolorum_. On the other hand, the tendrils of all tendril-bearing
plants, contract spirally after they have caught an object with the
following exceptions. Firstly, _Corydalis claviculata_, but then this
plant might be called a leaf-climber. Secondly and thirdly, _Bignonia
unguis_ with its close allies, and _Cardiospermum_; but their tendrils
are so short that their contraction could hardly occur, and would be
quite superfluous. Fourthly, _Smilaæ aspera_ offers a more marked
exception, as its tendrils are moderately long. The tendrils of
_Dicentra_, whilst the plant is young, are short and after attachment
only become slightly flexuous; in older plants they are longer and then
they contract spirally. I have seen no other exceptions to the rule that
tendrils, after clasping with their extremities a support, undergo spiral
contraction. When, however, the tendril of a plant of which the stem is
immovably fixed, catches some fixed object, it does not contract, simply
because it cannot; this, however, rarely occurs. In the common Pea the
lateral branches alone contract, and not the central stem; and with most
plants, such as the Vine, Passiflora, Bryony, the basal portion never
forms a spire.
I have said that in _Corydalis claviculata_ the end of the leaf or
tendril (for this part may be indifferently so called) does not contract
into a spire. The branchlets, however, after they have wound round thin
twigs, become deeply sinuous or zigzag. Moreover the whole end of the
petiole or tendril, if it seizes nothing, bends after a time abruptly
downwards and inwards, showing that its outer surface has gone on growing
after the inner surface has ceased to grow. That growth is the chief
cause of the spiral contraction of tendrils may be safely admitted, as
shown by the recent researches of H. de Vries. I will, however, add one
little fact in support of this conclusion.
If the short, nearly straight portion of an attached tendril of
_Passiflora gracilis_, (and, as I believe, of other tendrils,) between
the opposed spires, be examined, it will be found to be transversely
wrinkled in a conspicuous manner on the outside; and this would naturally
follow if the outer side had grown more than the inner side, this part
being at the same time forcibly prevented from becoming curved. So again
the whole outer surface of a spirally wound tendril becomes wrinkled if
it be pulled straight. Nevertheless, as the contraction travels from the
extremity of a tendril, after it has been stimulated by contact with a
support, down to the base, I cannot avoid doubting, from reasons
presently to be given, whether the whole effect ought to be attributed to
growth. An unattached tendril rolls itself up into a flat helix, as in
the case of Cardiospermum, if the contraction commences at the extremity
and is quite regular; but if the continued growth of the outer surface is
a little lateral, or if the process begins near the base, the terminal
portion cannot be rolled up within the basal portion, and the tendril
then forms a more or less open spire. A similar result follows if the
extremity has caught some object, and is thus held fast.
The tendrils of many kinds of plants, if they catch nothing, contract
after an interval of several days or weeks into a spire; but in these
cases the movement takes place after the tendril has lost its revolving
power and hangs down; it has also then partly or wholly lost its
sensibility; so that this movement can be of no use. The spiral
contraction of unattached tendrils is a much slower process than that of
attached ones. Young tendrils which have caught a support and are
spirally contracted, may constantly be seen on the same stem with the
much older unattached and uncontracted tendrils. In the _Echinocystis_ I
have seen a tendril with the two lateral branches encircling twigs and
contracted into beautiful spires, whilst the main branch which had caught
nothing remained for many days straight. In this plant I once observed a
main branch after it had caught a stick become spirally flexuous in 7
hrs., and spirally contracted in 18 hrs. Generally the tendrils of the
_Echinocystis_ begin to contract in from 12 hrs. to 24 hrs. after
catching some object; whilst unattached tendrils do not begin to contract
until two or three or even more days after all revolving movement has
ceased. A full-grown tendril of _Passiflora quadrangularis_ which had
caught a stick began in 8 hrs. to contract, and in 24 hrs. formed several
spires; a younger tendril, only two-thirds grown, showed the first trace
of contraction in two days after clasping a stick, and in two more days
formed several spires. It appears, therefore, that the contraction does
not begin until the tendril is grown to nearly its full length. Another
young tendril of about the same age and length as the last did not catch
any object; it acquired its full length in four days; in six additional
days it first became flexuous, and in two more days formed one complete
spire. This first spire was formed towards the basal end, and the
contraction steadily but slowly progressed towards the apex; but the
whole was not closely wound up into a spire until 21 days had elapsed
from the first observation, that is, until 17 days after the tendril had
grown to its full length.
The spiral contraction of tendrils is quite independent of their 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
Vitaceæ, 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 Cobæa 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. {165} 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. Léon 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.
[Picture: Fig. 13. A caught tendril of Bryonia dioica, spirally
contracted in reserved directions]
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, Cobæa, and most Passifloræ, 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 Fumariaceæ, 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 Cobæa (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 _Cobæa_ 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 æquinoctialis_ 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. {176} 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 aërial 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 Cobæa 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, {179} by the Cucurbitaceous genus _Peponopsis
adhærens_. 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 Bignoniaceæ, Vitaceæ, and Cucurbitaceæ, 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. {180} 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 {181} 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 Müller has remarked, {183} climb or scramble up
thickets in a still more simple fashion, without any special aid,
excepting that their leading shoots are generally long and flexible. It
may, however, be suspected from what follows, that these shoots in some
cases tend to avoid the light. The few hook-climbers which I have
observed, namely, _Galium aparine_, _Rubus australis_, and some climbing
Roses, exhibit no spontaneous revolving movement. If they had possessed
this power, and had been capable of twining, they would have been placed
in the class of Twiners; for some twiners are furnished with spines or
hooks, which aid them in their ascent. For instance, the Hop, which is a
twiner, has reflexed hooks as large as those of the _Galium_; some other
twiners have stiff reflexed hairs; and _Dipladenia_ has a circle of blunt
spines at the bases of its leaves. I have seen only one tendril-bearing
plant, namely, _Smilax aspera_, which is furnished with reflexed spines;
but this is the case with several branch-climbers in South Brazil and
Ceylon; and their branches graduate into true tendrils. Some few plants
apparently depend solely on their hooks for climbing, and yet do so
efficiently, as certain palms in the New and Old Worlds. Even some
climbing Roses will ascend the walls of a tall house, if covered with a
trellis. How this is effected I know not; for the young shoots of one
such Rose, when placed in a pot in a window, bent irregularly towards the
light during the day and from the light during the night, like the shoots
of any common plant; so that it is not easy to understand how they could
have got under a trellis close to the wall. {184}
_Root-climbers_.—A good many plants come under this class, and are
excellent climbers. One of the most remarkable is the _Marcgravia
umbellata_, the stem of which in the tropical forests of South America,
as I hear from Mr. Spruce, grows in a curiously flattened manner against
the trunks of trees; here and there it puts forth claspers (roots), which
adhere to the trunk, and, if the latter be slender, completely embrace
it. When this plant has climbed to the light, it produces free branches
with rounded stems, clad with sharp-pointed leaves, wonderfully different
in appearance from those borne by the stem as long as it remains
adherent. This surprising difference in the leaves, I have also observed
in a plant of _Marcgravia dubia_ in my hothouse. Root-climbers, as far
as I have seen, namely, the Ivy (_Hedera heliæ_), _Ficus repens_, and _F.
barbatus_, have no power of movement, not even from the light to the
dark. As previously stated, the _Hoya carnosa_ (Asclepiadaceæ) is a
spiral twiner, and likewise adheres by rootlets even to a flat wall. The
tendril-bearing _Bignonia Tweedyana_ emits roots, which curve half round
and adhere to thin sticks. The _Tecoma radicans_ (Bignoniaceæ), which is
closely allied to many spontaneously revolving species, climbs by
rootlets; nevertheless, its young shoots apparently move about more than
can be accounted for by the varying action of the light.
I have not closely observed many root-climbers, but can give one curious
fact. _Ficus repens_ climbs up a wall just like Ivy; and when the young
rootlets are made to press lightly on slips of glass, they emit after
about a week’s interval, as I observed several times, minute drops of
clear fluid, not in the least milky like that exuded from a wound. This
fluid is slightly viscid, but cannot be drawn out into threads. It has
the remarkable property of not soon drying; a drop, about the size of
half a pin’s head, was slightly spread out on glass, and I scattered on
it some minute grains of sand. The glass was left exposed in a drawer
during hot and dry weather, and if the fluid had been water, it would
certainly have dried in a few minutes; but it remained fluid, closely
surrounding each grain of sand, during 128 days: how much longer it would
have remained I cannot say. Some other rootlets were left in contact
with the glass for about ten days or a fortnight, and the drops of
secreted fluid were now rather larger, and so viscid that they could be
drawn out into threads. Some other rootlets were left in contact during
twenty-three days, and these were firmly cemented to the glass. Hence we
may conclude that the rootlets first secrete a slightly viscid fluid,
subsequently absorb the watery parts, (for we have seen that the fluid
will not dry by itself,) and ultimately leave a cement. When the
rootlets were torn from the glass, atoms of yellowish matter were left on
it, which were partly dissolved by a drop of bisulphide of carbon; and
this extremely volatile fluid was rendered very much less volatile by
what it had dissolved.
As the bisulphide of carbon has a strong power of softening indurated
caoutchouc, I soaked in it during a short time several rootlets of a
plant which had grown up a plaistered wall; and I then found many
extremely thin threads of transparent, not viscid, excessively elastic
matter, precisely like caoutchouc, attached to two sets of rootlets on
the same branch. These threads proceeded from the bark of the rootlet at
one end, and at the other end were firmly attached to particles of silex
or mortar from the wall. There could be no mistake in this observation,
as I played with the threads for a long time under the microscope,
drawing them out with my dissecting-needles and letting them spring back
again. Yet I looked repeatedly at other rootlets similarly treated, and
could never again discover these elastic threads. I therefore infer that
the branch in question must have been slightly moved from the wall at
some critical period, whilst the secretion was in the act of drying,
through the absorption of its watery parts. The genus _Ficus_ abounds
with caoutchouc, and we may conclude from the facts just given that this
substance, at first in solution and ultimately modified into an unelastic
cement, {187} is used by the _Ficus repens_ to cement its rootlets to any
surface which it ascends. Whether other plants, which climb by their
rootlets, emit any cement I do not know; but the rootlets of the Ivy,
placed against glass, barely adhered to it, yet secreted a little
yellowish matter. I may add, that the rootlets of the _Marcgravia dubia_
can adhere firmly to smooth painted wood.
_Vanilla aromatica_ emits aërial roots a foot in length, which point
straight down to the ground. According to Mohl (p. 49), these crawl into
crevices, and when they meet with a thin support, wind round it, as do
tendrils. A plant which I kept was young, and did not form long roots;
but on placing thin sticks in contact with them, they certainly bent a
little to that side, in the course of about a day, and adhered by their
rootlets to the wood; but they did not bend quite round the sticks, and
afterwards they re-pursued their downward course. It is probable that
these slight movements of the roots are due to the quicker growth of the
side exposed to the light, in comparison with the other side, and not
because the roots are sensitive to contact in the same manner as true
tendrils. According to Mohl, the rootlets of certain species of
_Lycopodium_ act as tendrils. {188}
_Concluding Remarks on Climbing Plants_.
Plants become climbers, in order, as it may be presumed, to reach the
light, and to expose a large surface of their leaves to its action and to
that of the free air. This is effected by climbers with wonderfully
little expenditure of organized matter, in comparison with trees, which
have to support a load of heavy branches by a massive trunk. Hence, no
doubt, it arises that there are so many climbing plants in all quarters
of the world, belonging to so many different orders. These plants have
been arranged under four classes, disregarding those which merely
scramble over bushes without any special aid. Hook-climbers are the
least efficient of all, at least in our temperate countries, and can
climb only in the midst of an entangled mass of vegetation.
Root-climbers are excellently adapted to ascend naked faces of rock or
trunks of trees; when, however, they climb trunks they are compelled to
keep much in the shade; they cannot pass from branch to branch and thus
cover the whole summit of a tree, for their rootlets require
long-continued and close contact with a steady surface in order to
adhere. The two great classes of twiners and of plants with sensitive
organs, namely, leaf-climbers and tendril-bearers taken together, far
exceed in number and in the perfection of their mechanism the climbers of
the two first classes. Those which have the power of spontaneously
revolving and of grasping objects with which they come in contact, easily
pass from branch to branch, and securely ramble over a wide, sun-lit
surface.
The divisions containing twining plants, leaf-climbers, and
tendril-bearers graduate to a certain extent into one another, and nearly
all have the same remarkable power of spontaneously revolving. Does this
gradation, it may be asked, indicate that plants belonging to one
subdivision have actually passed during the lapse of ages, or can pass,
from one state to the other? Has, for instance, any tendril-bearing
plant assumed its present structure without having previously existed as
a leaf-climber or a twiner? If we consider leaf-climbers alone, the idea
that they were primordially twiners is forcibly suggested. The
internodes of all, without exception, revolve in exactly the same manner
as twiners; some few can still twine well, and many others in an
imperfect manner. Several leaf-climbing genera are closely allied to
other genera which are simple twiners. It should also be observed, that
the possession of leaves with sensitive petioles, and with the consequent
power of clasping an object, would be of comparatively little use to a
plant, unless associated with revolving internodes, by which the leaves
are brought into contact with a support; although no doubt a scrambling
plant would be apt, as Professor Jaeger has remarked, to rest on other
plants by its leaves. On the other hand, revolving internodes, without
any other aid, suffice to give the power of climbing; so that it seems
probable that leaf-climbers were in most cases at first twiners, and
subsequently became capable of grasping a support; and this, as we shall
presently see, is a great additional advantage.
From analogous reasons, it is probable that all tendril-bearers were
primordially twiners, that is, are the descendants of plants having this
power and habit. For the internodes of the majority revolve; and, in a
few species, the flexible stem still retains the capacity of spirally
twining round an upright stick. Tendril-bearers have undergone much more
modification than leaf-climbers; hence it is not surprising that their
supposed primordial habits of revolving and twining have been more
frequently lost or modified than in the case of leaf-climbers. The three
great tendril-bearing families in which this loss has occurred in the
most marked manner, are the Cucurbitaceæ, Passifloraceæ, and Vitaceæ. In
the first, the internodes revolve; but I have heard of no twining form,
with the exception (according to Palm, p. 29. 52) of _Momordica
balsamina_, and this is only an imperfect twiner. In the two other
families I can hear of no twiners; and the internodes rarely have the
power of revolving, this power being confined to the tendrils. The
internodes, however, of _Passiflora gracilis_ have the power in a perfect
manner, and those of the common Vine in an imperfect degree: so that at
least a trace of the supposed primordial habit has been retained by some
members of all the larger tendril-bearing groups.
On the view here given, it may be asked, Why have the species which were
aboriginally twiners been converted in so many groups into leaf-climbers
or tendril-bearers? Of what advantage has this been to them? Why did
they not remain simple twiners? We can see several reasons. It might be
an advantage to a plant to acquire a thicker stem, with short internodes
bearing many or large leaves; and such stems are ill fitted for twining.
Any one who will look during windy weather at twining plants will see
that they are easily blown from their support; not so with
tendril-bearers or leaf-climbers, for they quickly and firmly grasp their
support by a much more efficient kind of movement. In those plants which
still twine, but at the same time possess tendrils or sensitive petioles,
as some species of Bignonia, Clematis, and Tropæolum, it can readily be
observed how incomparably better they grasp an upright stick than do
simple twiners. Tendrils, from possessing this power of grasping an
object, can be made long and thin; so that little organic matter is
expended in their development, and yet they sweep a wide circle in search
of a support. Tendril-bearers can, from their first growth, ascend along
the outer branches of any neighbouring bush, and they are thus always
fully exposed to the light; twiners, on the contrary, are best fitted to
ascend bare stems, and generally have to start in the shade. Within tall
and dense tropical forests, twining plants would probably succeed better
than most kinds of tendril-bearers; but the majority of twiners, at least
in our temperate regions, from the nature of their revolving movement,
cannot ascend thick trunks, whereas this can be affected by
tendril-bearers if the trunks are branched or bear twigs, and by some
species if the bark is rugged.
The advantage gained by climbing is to reach the light and free air with
as little expenditure of organic matter as possible; now, with twining
plants, the stem is much longer than is absolutely necessary; for
instance, I measured the stem of a kidney-bean, which had ascended
exactly two feet in height, and it was three feet in length: the stem of
a pea, on the other hand, which had ascended to the same height by the
aid of its tendrils, was but little longer than the height reached. That
this saving of the stem is really an advantage to climbing plants, I
infer from the species that still twine but are aided by clasping
petioles or tendrils, generally making more open spires than those made
by simple twiners. Moreover, the plants thus aided, after taking one or
two turns in one direction, generally ascend for a space straight, and
then reverse the direction of their spire. By this means they ascend to
a considerably greater height, with the same length of stem, than would
otherwise have been possible; and they do this with safety, as they
secure themselves at intervals by their clasping petioles or tendrils.
We have seen that tendrils consist of various organs in a modified state,
namely, leaves, flower-peduncles, branches, and perhaps stipules. With
respect to leaves, the evidence of their modification is ample. In young
plants of Bignonia the lower leaves often remain quite unchanged, whilst
the upper ones have their terminal leaflets converted into perfect
tendrils; in _Eccremocarpus_ I have seen a single lateral branch of a
tendril replaced by a perfect leaflet; in _Vicia sativa_, on the other
hand, leaflets are sometimes replaced by tendril-branches; and many other
such cases could be given. But he who believes in the slow modification
of species will not be content simply to ascertain the homological nature
of different kinds of tendrils; he will wish to learn, as far as is
possible, by what actual steps leaves, flower-peduncles, &c., have had
their functions wholly changed, and have come to serve merely as
prehensile organs.
In the whole group of leaf-climbers abundant evidence has been given that
an organ, still subserving the functions of a leaf, may become sensitive
to a touch, and thus grasp an adjoining object. With several
leaf-climbers the true leaves spontaneously revolve; and their petioles,
after clasping a support grow thicker and stronger. We thus see that
leaves may acquire all the leading and characteristic qualities of
tendrils, namely, sensitiveness, spontaneous movement, and subsequently
increased strength. If their blades or laminæ were to abort, they would
form true tendrils. And of this process of abortion we can follow every
step, until no trace of the original nature of the tendril is left. In
_Mutisia clematis_, the tendril, in shape and colour, closely resembles
the petiole of one of the ordinary leaves, together with the midribs of
the leaflets, but vestiges of the laminæ are still occasionally retained.
In four genera of the Fumariaceæ we can follow the whole process of
transformation. The terminal leaflets of the leaf-climbing _Fumaria
officinalis_ are not smaller than the other leaflets; those of the
leaf-climbing _Adlumia cirrhosa_ are greatly reduced; those of _Corydalis
claviculata_ (a plant which may indifferently be called a leaf-climber or
a tendril-bearer) are either reduced to microscopical dimensions or have
their blades wholly aborted, so that this plant is actually in a state of
transition; and, finally, in the _Dicentra_ the tendrils are perfectly
characterized. If, therefore, we could behold at the same time all the
progenitors of _Dicentra_, we should almost certainly see a series like
that now exhibited by the above-named three genera. In _Tropæolum
tricolorum_ we have another kind of passage; for the leaves which are
first formed on the young stems are entirely destitute of laminæ, and
must be called tendrils, whilst the later formed leaves have
well-developed laminæ. In all cases the acquirement of sensitiveness by
the mid-ribs of the leaves appears to stand in some close relation with
the abortion of their laminæ or blades.
On the view here given, leaf-climbers were primordially twiners, and
tendril-bearers (when formed of modified leaves) were primordially
leaf-climbers. The latter, therefore, are intermediate in nature between
twiners and tendril-bearers, and ought to be related to both. This is
the case: thus the several leaf-climbing species of the Antirrhineæ, of
Solanum, Cocculus, and Gloriosa, have within the same family and even
within the same genus, relatives which are twiners. In the genus
Mikania, there are leaf-climbing and twining species. The leaf-climbing
species of Clematis are very closely allied to the tendril-bearing
Naravelia. The Fumariaceæ include closely allied genera which are
leaf-climbers and tendril-bearers. Lastly, a species of Bignonia is at
the same time both a leaf-climber and a tendril-bearer; and other closely
allied species are twiners.
Tendrils of another kind consist of modified flower-peduncles. In this
case we likewise have many interesting transitional states. The common
Vine (not to mention the _Cardiospermum_) gives us every possible
gradation between a perfectly developed tendril and a flower-peduncle
covered with flowers, yet furnished with a branch, forming the
flower-tendril. When the latter itself bears a few flowers, as we know
sometimes is the case, and still retains the power of clasping a support,
we see an early condition of all those tendrils which have been formed by
the modification of flower-peduncles.
According to Mohl and others, some tendrils consist of modified branches:
I have not observed any such cases, and know nothing of their
transitional states, but these have been fully described by Fritz Müller.
The genus Lophospermum also shows us how such a transition is possible;
for its branches spontaneously revolve and are sensitive to contact.
Hence, if the leaves on some of the branches of the Lophospermum were to
abort, these branches would be converted into true tendrils. Nor is
there anything improbable in certain branches alone being thus modified,
whilst others remained unaltered; for we have seen with certain varieties
of _Phaseolus_, that some of the branches are thin, flexible, and twine,
whilst other branches on the same plant are stiff and have no such power.
If we inquire how a petiole, a branch or flower-peduncle first became
sensitive to a touch, and acquired the power of bending towards the
touched side, we get no certain answer. Nevertheless an observation by
Hofmeister {197} well deserves attention, namely, that the shoots and
leaves of all plants, whilst young, move after being shaken. Kerner also
finds, as we have seen, that the flower-peduncles of a large number of
plants, if shaken or gently rubbed bend to this side. And it is young
petioles and tendrils, whatever their homological nature may be, which
move on being touched. It thus appears that climbing plants have
utilized and perfected a widely distributed and incipient capacity, which
capacity, as far as we can see, is of no service to ordinary plants. If
we further inquire how the stems, petioles, tendrils, and
flower-peduncles of climbing plants first acquired their power of
spontaneously revolving, or, to speak more accurately, of successively
bending to all points of the compass, we are again silenced, or at most
can only remark that the power of moving, both spontaneously and from
various stimulants, is far more common with plants, than is generally
supposed to be the case by those who have not attended to the subject. I
have given one remarkable instance, namely that of the _Maurandia
semperflorens_, the young flower-peduncles of which spontaneously revolve
in very small circles, and bend when gently rubbed to the touched side;
yet this plant certainly does not profit by these two feebly developed
powers. A rigorous examination of other young plants would probably show
slight spontaneous movements in their stems, petioles or peduncles, as
well as sensitiveness to a touch. {198} We see at least that the
_Maurandia_ might, by a little augmentation of the powers which it
already possesses, come first to grasp a support by its flower-peduncles,
and then, by the abortion of some of its flowers (as with _Vitis_ or
_Cardiospermum_), acquire perfect tendrils.
There is one other interesting point which deserves notice. We have seen
that some tendrils owe their origin to modified leaves, and others to
modified flower-peduncles; so that some are foliar and others axial in
their nature. It might therefore have been expected that they would have
presented some difference in function. This is not the case. On the
contrary, they present the most complete identity in their several
characteristic powers. Tendrils of both kinds spontaneously revolve at
about the same rate. Both, when touched, bend quickly to the touched
side, and afterwards recover themselves and are able to act again. In
both the sensitiveness is either confined to one side or extends all
round the tendril. Both are either attracted or repelled by the light.
The latter property is seen in the foliar tendrils of _Bignonia
capreolata_ and in the axial tendrils of _Ampelopsis_. The tips of the
tendrils in these two plants become, after contact, enlarged into discs,
which are at first adhesive by the secretion of some cement. Tendrils of
both kinds, soon after grasping a support, contract spirally; they then
increase greatly in thickness and strength. When we add to these several
points of identity the fact that the petiole of _Solanum jasminoides_,
after it has clasped a support, assumes one of the most characteristic
features of the axis, namely, a closed ring of woody vessels, we can
hardly avoid asking, whether the difference between foliar and axial
organs can be of so fundamental a nature as is generally supposed? {199}
We have attempted to trace some of the stages in the genesis of climbing
plants. But, during the endless fluctuations of the conditions of life
to which all organic beings have been exposed, it might be expected that
some climbing plants would have lost the habit of climbing. In the cases
given of certain South African plants belonging to great twining
families, which in their native country never twine, but reassume this
habit when cultivated in England, we have a case in point. In the
leaf-climbing _Clematis flammula_, and in the tendril-bearing Vine, we
see no loss in the power of climbing, but only a remnant of the revolving
power which is indispensable to all twiners, and is so common as well as
so advantageous to most climbers. In _Tecoma radicans_, one of the
Bignoniaceæ, we see a last and doubtful trace of the power of revolving.
With respect to the abortion of tendrils, certain cultivated varieties of
_Cucurbita pepo_ have, according to Naudin, {200} either quite lost these
organs or bear semi-monstrous representatives of them. In my limited
experience, I have met with only one apparent instance of their natural
suppression, namely, in the common bean. All the other species of
_Vicia_, I believe, bear tendrils; but the bean is stiff enough to
support its own stem, and in this species, at the end of the petiole,
where, according to analogy, a tendril ought to have existed, a small
pointed filament projects, about a third of an inch in length, and which
is probably the rudiment of a tendril. This may be the more safely
inferred, as in young and unhealthy specimens of other tendril-bearing
plants similar rudiments may occasionally be observed. In the bean these
filaments are variable in shape, as is so frequently the case with
rudimentary organs; they are either cylindrical, or foliaceous, or are
deeply furrowed on the upper surface. They have not retained any vestige
of the power of revolving. It is a curious fact, that many of these
filaments, when foliaceous, have on their lower surfaces, dark-coloured
glands like those on the stipules, which excrete a sweet fluid; so that
these rudiments have been feebly utilized.
One other analogous case, though hypothetical, is worth giving. Nearly
all the species of _Lathyrus_ possesses tendrils; but _L. nissolia_ is
destitute of them. This plant has leaves, which must have struck
everyone with surprise who has noticed them, for they are quite unlike
those of all common papilionaceous plants, and resemble those of a grass.
In another species, _L. aphaca_, the tendril, which is not highly
developed (for it is unbranched, and has no spontaneous revolving-power),
replaces the leaves, the latter being replaced in function by large
stipules. Now if we suppose the tendrils of _L. aphaca_ to become
flattened and foliaceous, like the little rudimentary tendrils of the
bean, and the large stipules to become at the same time reduced in size,
from not being any longer wanted, we should have the exact counterpart of
_L. nissolia_, and its curious leaves are at once rendered intelligible
to us.
It may be added, as serving to sum up the foregoing views on the origin
of tendril-bearing plants, that _L. nissolia_ is probably descended from
a plant which was primordially a twiner; this then became a leaf-climber,
the leaves being afterwards converted by degrees into tendrils, with the
stipules greatly increased in size through the law of compensation. {202}
After a time the tendrils lost their branches and became simple; they
then lost their revolving-power (in which state they would have resembled
the tendrils of the existing _L. aphaca_), and afterwards losing their
prehensile power and becoming foliaceous would no longer be thus
designated. In this last stage (that of the existing _L. nissolia_) the
former tendrils would reassume their original function of leaves, and the
stipules which were recently much developed being no longer wanted, would
decrease in size. If species become modified in the course of ages, as
almost all naturalists now admit, we may conclude that _L. nissolia_ has
passed through a series of changes, in some degree like those here
indicated.
The most interesting point in the natural history of climbing plants is
the various kinds of movement which they display in manifest relation to
their wants. The most different organs—stems, branches,
flower-peduncles, petioles, mid-ribs of the leaf and leaflets, and
apparently aërial roots—all possess this power.
The first action of a tendril is to place itself in a proper position.
For instance, the tendril of Cobæa first rises vertically up, with its
branches divergent and with the terminal hooks turned outwards; the young
shoot at the extremity of the stem is at the same time bent to one side,
so as to be out of the way. The young leaves of Clematis, on the other
hand, prepare for action by temporarily curving themselves downwards, so
as to serve as grapnels.
Secondly, if a twining plant or a tendril gets by any accident into an
inclined position, it soon bends upwards, though secluded from the light.
The guiding stimulus no doubt is the attraction of gravity, as Andrew
Knight showed to be the case with germinating plants. If a shoot of any
ordinary plant be placed in an inclined position in a glass of water in
the dark, the extremity will, in a few hours, bend upwards; and if the
position of the shoot be then reversed, the downward-bent shoot reverses
its curvature; but if the stolen of a strawberry, which has no tendency
to grow upwards, be thus treated, it will curve downwards in the
direction of, instead of in opposition to, the force of gravity. As with
the strawberry, so it is generally with the twining shoots of the
_Hibbertia dentata_, which climbs laterally from bush to bush; for these
shoots, if placed in a position inclined downwards, show little and
sometimes no tendency to curve upwards.
Thirdly, climbing plants, like other plants, bend towards the light by a
movement closely analogous to the incurvation which causes them to
revolve, so that their revolving movement is often accelerated or
retarded in travelling to or from the light. On the other hand, in a few
instances tendrils bend towards the dark.
Fourthly, we have the spontaneous revolving movement which is independent
of any outward stimulus, but is contingent on the youth of the part, and
on vigorous health; and this again of course depends on a proper
temperature and other favourable conditions of life.
Fifthly, tendrils, whatever their homological nature may be, and the
petioles or tips of the leaves of leaf-climbers, and apparently certain
roots, all have the power of movement when touched, and bend quickly
towards the touched side. Extremely slight pressure often suffices. If
the pressure be not permanent, the part in question straightens itself
and is again ready to bend on being touched.
Sixthly, and lastly, tendrils, soon after clasping a support, but not
after a mere temporary curvature, contract spirally. If they have not
come into contact with any object, they ultimately contract spirally,
after ceasing to revolve; but in this case the movement is useless, and
occurs only after a considerable lapse of time.
With respect to the means by which these various movements are effected,
there can be little doubt from the researches of Sachs and H. de Vries,
that they are due to unequal growth; but from the reasons already
assigned, I cannot believe that this explanation applies to the rapid
movements from a delicate touch.
Finally, climbing plants are sufficiently numerous to form a conspicuous
feature in the vegetable kingdom, more especially in tropical forests.
America, which so abounds with arboreal animals, as Mr. Bates remarks,
likewise abounds according to Mohl and Palm with climbing plants; and of
the tendril-bearing plants examined by me, the highest developed kinds
are natives of this grand continent, namely, the several species of
_Bignonia_, _Eccremocarpus_, _Cobæa_, and _Ampelopsis_. But even in the
thickets of our temperate regions the number of climbing species and
individuals is considerable, as will be found by counting them. They
belong to many and widely different orders. To gain some rude idea of
their distribution in the vegetable series, I marked, from the lists
given by Mohl and Palm (adding a few myself, and a competent botanist, no
doubt, could have added many more), all those families in Lindley’s
‘Vegetable Kingdom’ which include twiners, leaf-climbers, or
tendril-bearers. Lindley divides Phanerogamic plants into fifty-nine
Alliances; of these, no less than thirty-five include climbing plants of
the above kinds, hook and root-climbers being excluded. To these a few
Cryptogamic plants must be added. When we reflect on the wide separation
of these plants in the series, and when we know that in some of the
largest, well-defined orders, such as the Compositæ, Rubiaceæ,
Scrophulariaceæ, Liliaceæ, &c., species in only two or three genera have
the power of climbing, the conclusion is forced on our minds that the
capacity of revolving, on which most climbers depend, is inherent, though
undeveloped, in almost every plant in the vegetable kingdom.
It has often been vaguely asserted that plants are distinguished from
animals by not having the power of movement. It should rather be said
that plants acquire and display this power only when it is of some
advantage to them; this being of comparatively rare occurrence, as they
are affixed to the ground, and food is brought to them by the air and
rain. We see how high in the scale of organization a plant may rise,
when we look at one of the more perfect tendril-bearers. It first places
its tendrils ready for action, as a polypus places its tentacula. If the
tendril be displaced, it is acted on by the force of gravity and rights
it self. It is acted on by the light, and bends towards or from it, or
disregards it, whichever may be most advantageous. During several days
the tendrils or internodes, or both, spontaneously revolve with a steady
motion. The tendril strikes some object, and quickly curls round and
firmly grasps it. In the course of some hours it contracts into a spire,
dragging up the stem, and forming an excellent spring. All movements now
cease. By growth the tissues soon become wonderfully strong and durable.
The tendril has done its work, and has done it in an admirable manner.
FOOTNOTES.
{iv} An English translation of the ‘Lehrbuch der Botanik’ by Professor
Sachs, has recently (1875), appeared under the title of ‘Text-Book of
Botany,’ and this is a great boon to all lovers of natural science in
England.
{1a} ‘Proc. Amer. Acad. of Arts and Sciences,’ vol. iv. Aug. 12, 1858,
p. 98.
{1b} Ludwig H. Palm, ‘Ueber das Winden der Pflanzen;’ Hugo von Mohl,
‘Ueber den Bau und des Winden der Ranken und Schlingpflanzen,’ 1827.
Palm’s Treatise was published only a few weeks before Mohl’s. See also
‘The Vegetable Cell’ (translated by Henfrey), by H. von Mohl, p. 147 to
end.
{1c} “Des Mouvements révolutife Respontanés,” &c., ‘Comptes Rendus,’
tom. xvii. (1843) p. 989; “Recherches sur la Volubilité des Tiges,” &c.,
tom. xix. (1844) p. 295.
{8} ‘Bull. Bot Soc. de France,’ tom. v. 1858, p. 356.
{9a} This whole subject has been ably discussed and explained by H. de
Vries, ‘Arbeiten des Bot. Instituts in Würzburg,’ Heft iii. pp. 331, 336.
See also Sachs (‘Text-Book of Botany,’ English translation, 1875, p.
770), who concludes “that torsion is the result of growth continuing in
the outer layers after it has ceased or begun to cease in the inner
layers.”
{9b} Professor Asa Gray has remarked to me, in a letter, that in _Thuja
occidentalis_ the twisting of the bark is very conspicuous. The twist is
generally to the right of the observer; but, in noticing about a hundred
trunks, four or five were observed to be twisted in an opposite
direction. The Spanish chestnut is often much twisted: there is an
interesting article on this subject in the ‘Scottish Farmer,’ 1865, p.
833.
{10} It is well known that the stems of many plants occasionally become
spirally twisted in a monstrous manner; and after my paper was read
before the Linnean Society, Dr. Maxwell Masters remarked to me in a
letter that “some of these cases, if not all, are dependent upon some
obstacle or resistance to their upward growth.” This conclusion agrees
with what I have said about the twisting of stems, which have twined
round rugged supports; but does not preclude the twisting being of
service to the plant by giving greater rigidity to the stem.
{12} The view that the revolving movement or nutation of the stems of
twining plants is due to growth is that advanced by Sachs and H. de
Vries; and the truth of this view is proved by their excellent
observations.
{14} The mechanism by which the end of the shoot remains hooked appears
to be a difficult and complex problem, discussed by Dr. H. de Vries
(ibid. p. 337): he concludes that “it depends on the relation between the
rapidity of torsion and the rapidity of nutation.”
{16} Dr. H. de Vries also has shown (ibid. p. 321 and 325) by a better
method than that employed by me, that the stems of twining plants are not
irritable, and that the cause of their winding up a support is exactly
what I have described.
{17} Dr. H. de Vries states (ibid. p. 322) that the stem of Cuscuta is
irritable like a tendril.
{18} See Dr. H. de Vries (ibid. p. 324) on this subject.
{19} Comptes Rendus, 1844, tom. xix. p. 295, and Annales des Sc. Nat 3rd
series, Bot., tom. ii. p. 163.
{24} I am much indebted to Dr. Hooker for having sent me many plants
from Kew; and to Mr. Veitch, of the Royal Exotic Nursery, for having
generously given me a collection of fine specimens of climbing plants.
Professor Asa Gray, Prof. Oliver, and Dr. Hooker have afforded me, as on
many previous occasions, much information and many references.
{33} Journal of the Linn. Soc. (Bot.) vol. ix. p. 344. I shall have
occasion often to quote this interesting paper, in which he corrects or
confirms various statements made by me.
{34} I raised nine plants of the hybrid _Loasa Herbertii_, and six of
these also reversed their spire in ascending a support.
{36} In another genus, namely Davilla, belonging to the same family with
Hibbertia, Fritz Müller says (ibid. p. 349) that “the stem twines
indifferently from left to right, or from right to left; and I once saw a
shoot which ascended a tree about five inches in diameter, reverse its
course in the same manner as so frequently occurs with Loasa.”
{37} Fritz Müller states (ibid. p. 349) that he saw on one occasion in
the forests of South Brazil a trunk about five feet in circumference
spirally ascended by a plant, apparently belonging to the Menispermaceæ.
He adds in his letter to me that most of the climbing plants which there
ascend thick trees, are root-climbers; some being tendril-bearers.
{44} Fritz Müller has published some interesting facts and views on the
structure of the wood of climbing plants in ‘Bot. Zeitung,’ 1866, pp. 57,
66.
{68} It appears from A. Kerner’s interesting observations, that the
flower-peduncles of a large number of plants are irritable, and bend when
they are rubbed or shaken: Die Schutzmittel des Pollens, 1873, p. 34.
{71} I have already referred to the case of the twining stem of Cuscuta,
which, according to H. de Vries (ibid. p. 322) is sensitive to a touch
like a tendril.
{75} Dr. Maxwell Masters informs me that in almost all petioles which
are cylindrical, such as those bearing peltate leaves, the woody vessels
form a closed ring; semilunar bands of vessels being confined to petioles
which are channelled along their upper surfaces. In accordance with this
statement, it may be observed that the enlarged and clasped petiole of
the _Solanum_, with its closed ring of woody vessels, has become more
cylindrical than it was in its original unclasped condition.
{84} Never having had the opportunity of examining tendrils produced by
the modification of branches, I spoke doubtfully about them in this essay
when originally published. But since then Fritz Müller has described
(Journal of Linn. Soc. vol. ix. p. 344) many striking cases in South
Brazil. In speaking of plants which climb by the aid of their branches,
more or less modified, he states that the following stages of development
can be traced: (1.) Plants supporting themselves simply by their branches
stretched out at right angles—for example, _Chiococca_. (2.) Plants
clasping a support with their unmodified branches, as with _Securidaca_.
(3.) Plants climbing by the extremities of their branches which appear
like tendrils, as is the case according to Endlicher with _Helinus_.
(4.) Plants with their branches much modified and temporarily converted
into tendrils, but which may be again transformed into branches, as with
certain Papilionaceous plants. (5.) Plants with their branches forming
true tendrils, and used exclusively for climbing—as with _Strychnos_ and
_Caulotretus_. Even the unmodified branches become much thickened when
they wind round a support. I may add that Mr. Thwaites sent me from
Ceylon a specimen of an Acacia which had climbed up the trunk of a rather
large tree, by the aid of tendril-like, curved or convoluted branchlets,
arrested in their growth and furnished with sharp recurved hooks.
{85} As far as I can make out, the history of our knowledge of tendrils
is as follows:—We have seen that Palm and von Mohl observed about the
same time the singular phenomenon of the spontaneous revolving movement
of twining-plants. Palm (p. 58), I presume, observed likewise the
revolving movement of tendrils; but I do not feel sure of this, for he
says very little on the subject. Dutrochet fully described this movement
of the tendril in the common pea. Mohl first discovered that tendrils
are sensitive to contact; but from some cause, probably from observing
too old tendrils, he was not aware how sensitive they were, and thought
that prolonged pressure was necessary to excite their movement.
Professor Asa Gray, in a paper already quoted, first noticed the extreme
sensitiveness and rapidity of the movements of the tendrils of certain
Cucurbitaceous plants.
{102} Fritz Müller states (ibid. p. 348) that in South Brazil the trifid
tendrils of Haplolophium, (one of the Bignoniaceæ) without having come
into contact with any object, terminate in smooth shining discs. These,
however, after adhering to any object, sometimes become considerably
enlarged.
{111} Comptes Rendus, tom. xvii. 1843, p. 989.
{113} Diagram showing the movement of the upper internode of the common
Pea, traced on a hemispherical glass, and transferred to paper; reduced
one-half in size (Aug. 1st)
No. H. M.
1 8 46 A.M.
2 10 0
3 11 0
4 11 37
5 12 7 P.M.
6 12 30
7 1 0
8 1 30
9 1 44
10 2 25
11 3 0
12 3 30
13 3 48
14 4 40
15 5 5
16 5 25
17 5 50
18 6 25
19 7 0
20 7 45
21 8 30
22 9 15
{118} ‘Leçons de Botanique,’ &c., 1841, p. 170.
{127a} I am indebted to Prof. Oliver for information on this head. In
the Bulletin de la Société Botanique de France, 1857, there are numerous
discussions on the nature of the tendrils in this family.
{127b} ‘Gardeners’ Chronicle,’ 1864, p. 721. From the affinity of the
Cucurbitaceæ to the Passifloraceæ, it might be argued that the tendrils
of the former are modified flower-peduncles, as is certainly the case
with those of Passion flowers. Mr. R. Holland (Hardwicke’s
‘Science-Gossip,’ 1865, p. 105) states that “a cucumber grew, a few years
ago in my own garden, where one of the short prickles upon the fruit had
grown out into a long, curled tendril.”
{145} Trans. Phil. Soc. 1812, p. 314.
{146} Dr. M’Nab remarks (Trans. Bot. Soc. Edinburgh, vol xi. p. 292)
that the tendrils of _Amp. Veitchii_ bear small globular discs before
they have came into contact with any object; and I have since observed
the same fact. These discs, however, increase greatly in size, if they
press against and adhere to any surface. The tendrils, therefore, of one
species of _Ampelopsis_ require the stimulus of contact for the first
development of their discs, whilst those of another species do not need
any such stimulus. We have seen an exactly parallel case with two
species of _Bignoniaceæ_.
{152} Fritz Müller remarks (ibid. p. 348) that a related genus,
Serjania, differs from Cardiospermum in bearing only a single tendril;
and that the common peduncle contracts spirally, when, as frequently
happens, the tendril has clasped the plant’s own stem.
{154} Prof. Asa Gray informs me that the tendrils of _P. sicyoides_
revolve even at a quicker rate than those of _P. gracilis_; four
revolutions were completed (the temperature varying from 88 degrees-92
degrees Fahr.) in the following times, 40 m., 45 m., 38½ m., and 46 m.
One half-revolution was performed in 15 m.
{165} See M. Isid. Léon in Bull. Soc. Bot. de France, tom. v. 1858, p.
650. Dr. H. de Vries points out (p. 306) that I have overlooked, in the
first edition of this essay, the following sentence by Mohl: “After a
tendril has caught a support, it begins in some days to wind into a
spire, which, since the tendril is made fast at both extremities, must of
necessity be in some places to the right, in others to the left.” But I
am not surprised that this brief sentence, without any further
explanation did not attract my attention.
{176} Sachs, however (‘Text-Book of Botany,’ Eng. Translation, 1875, p.
280), has shown that which I overlooked, namely, that the tendrils of
different species are adapted to clasp supports of different thicknesses.
He further shows that after a tendril has clasped a support it
subsequently tightens its hold.
{179} Annales des Sc. Nat. Bot. 4th series, tom. xii. p. 89.
{180} It occurred to me that the movement of notation and that from a
touch might be differently affected by anæsthetics, in the same manner as
Paul Bert has shown to be the case with the sleep-movements of Mimosa and
those from a touch. I tried the common pea and _Passiflora gracilis_,
but I succeeded only in observing that both movements were unaffected by
exposure for 1½ hrs. to a rather large dose of sulphuric ether. In this
respect they present a wonderful contrast with Drosera, owing no doubt to
the presence of absorbent glands in the latter plant.
{181} Text-Book of Botany, 1875, p. 779.
{183} Journal of Linn. Soc. vol. ix. p. 348. Professor G. Jaeger has
well remarked (‘In Sachen Darwin’s, insbesondere contra Wigand,’ 1874, p.
106) that it is highly characteristic of climbing plants to produce thin,
elongated, and flexible stems. He further remarks that plants growing
beneath other and taller species or trees, are naturally those which
would be developed into climbers; anti such plants, from stretching
towards the light, and from not being much agitated by the wind, tend to
produce long, thin and flexible shoots.
{184} Professor Asa Gray has explained, as it would appear, this
difficulty in his review (American Journal of Science, vol. xl. Sept.
1865, p. 282) of the present work. He has observed that the strong
summer shoots of the Michigan rose (_Rosa setigera_) are strongly
disposed to push into dark crevices and away from the light, so that they
would be almost sure to place themselves under a trellis. He adds that
the lateral shoots, made on the following spring emerged from the trellis
as they sought the light.
{187} Mr. Spiller has recently shown (Chemical Society, Feb. 16, 1865),
in a paper on the oxidation of india-rubber or caoutchouc, that this
substance, when exposed in a fine state of division to the air, gradually
becomes converted into brittle, resinous matter, very similar to
shell-lac.
{188} Fritz Müller informs me that he saw in the forests of South Brazil
numerous black strings, from some lines to nearly an inch in diameter,
winding spirally round the trunks of gigantic trees. At first sight he
thought that they were the stems of twining plants which were thus
ascending the trees: but he afterwards found that they were the aërial
roots of a Philodendron which grew on the branches above. These roots
therefore seem to be true twiners, though they use their powers to
descend, instead of to ascend like twining plants. The aërial roots of
some other species of Philodendron hang vertically downwards, sometimes
for a length of more than fifty feet.
{197} Quoted by Cohn, in his remarkable memoir, “Contractile Gewebe im
Pflanzenreiche,” ‘Abhandl. der Schlesischen Gesell. 1861, Heft i. s. 35.
{198} Such slight spontaneous movements, I now find, have been for some
time known to occur, for instance with the flower-stems of _Brassica
napus_ and with the leaves of many plants: Sachs’ ‘Text-Book of Botany’
1875, pp. 766, 785. Fritz Müller also has shown in relation to our
present subject (‘Jenaischen Zeitschrift,’ Bd. V. Heft 2, p. 133) that
the stems, whilst young, of an Alisma and of a Linum are continually
performing slight movements to all points of the compass, like those of
climbing plants.
{199} Mr. Herbert Spencer has recently argued (‘Principles of Biology,’
1865, p. 37 et seq.) with much force that there is no fundamental
distinction between the foliar and axial organs of plants.
{200} Annales des Sc. Nat. 4th series, Bot. tom. vi. 1856, p. 31.
{202} Moquin-Tandon (Eléments de Tératologie. 1841, p. 156) gives the
case of a monstrous bean, in which a case of compensation of this nature
was suddenly effected; for the leaves completely disappeared and the
stipules grew to an enormous size.
*** End of this LibraryBlog Digital Book "The Movements and Habits of Climbing Plants" ***
Copyright 2023 LibraryBlog. All rights reserved.