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.