protoplasm in cells, or movements of free bodies, such as zoospores
(Greek, _zoon_, animal, and _spora_, seed), antherozoids (Greek,
_anthos_, flower; _zoon_, animal; _eidos_, form), and sometimes even
perfect individuals, such as Desmediæ, etc., which may have the power
of temporary or permanent locomotion.
The rotation of the protoplasm of cells is attributed to causes
similar to those which produce locomotion in the simpler plants, and
these movements are strikingly like some of the movements of the
protozoa in the animal kingdom. The movements of the products of cell
contents having no cell-wall, such as zoospores and antherozoids,
are generally caused by the rapid movement of cilia (plural of the
Latin word _cilium_, an eyelid) or small filaments which cover the
surface. The locomotion of certain plants, such as Diatomaceæ, is
apparently not due to cilia.
Sensitive plants, such as the Mimosa pudica, are strongly affected
by any mechanical stimulus, and thus afford us examples of the
phenomenon named “irritability.”
The sleep of plants is most probably a case of irritability, and
differs only in degree, not in kind.
Sensitiveness in plants is affected both by light and heat. It has
been experimentally proved that sensitive plants, if kept in the
dark, lose their sensibility after a period of seven days, and
actually die after twelve days.
We know that white light is composed of light of different colors.
Light is propagated in waves, and each color is distinguished by
having a different wave-length from that of any other color. Red
light differs, for example, from violet light in the length of its
waves, and violet light differs from blue, etc.
It is, therefore, not surprising to find that the different
colored rays are capable of producing different effects. It has
been ascertained that under the influence of green light sensitive
plants die after sixteen days’ exposure, though they retain their
sensibility for twelve days.
When the plants were exposed to violet and blue light, their growth
completely ceased. They, however, retained their vitality as well as
their sensibility for three months. The effect of heat on sensitive
plants has also been ascertained.
The sensitiveness and periodical movements of Mimosa do not begin
till the temperature of the surrounding air exceeds 15° C. The
periodical movements of the lateral leaflets of the Indian telegraph
plant (Desmodium gyrans) can only occur when the temperature exceeds
22° C.
When the temperature of the air is 40° C., the leaves become stiff
in less than an hour, and at 48° C. to 50° C. rigidity takes
place within a few minutes; but when the temperature falls, the
sensitiveness may again be manifested.
A temperature of 52° C. not only causes loss of permanent motion, but
also the death of the plant.
The mechanism to which the periodic movements of plants is due is not
by any means fully known.
The particular circumstances which regulate the turgidity have not
been, so far, determined with precision.
It has, however, been clearly ascertained that this turgid state
is associated with the passage of fine threads or filaments of
protoplasm from one cell to another, and at the same time with an
accumulation of a soluble chemical compound named glucose, a kind
of sugar, in fact. This substance possesses great osmotic power;
that is, it can pass very rapidly through the flexible cell-walls
of the pulvinus forming the so-called springs. These movements are,
therefore, closely connected with the rapid absorption and expulsion
of liquid.
Contrary to the habit of most plants, the sensitive plant raises its
leaves at night and closes them by day.
The most usual kind of movement in these plants is that in which the
leaves as well as the floral envelopes assume the position they
occupied before the buds opened.
Compound leaves, such as the leaves of the Leguminosæ, or pea-family,
exhibit a simple or compound movement.
The leaves of the bean fold upward, those of the Lupinus fold
downward. In Tamarinds the leaves fold to the side. In some other
plants the common petiole of the compound leaves become raised or
depressed, while the leaflets turn downward or sidewise. This is the
case in Amorpha fruticosa and Gleditschia tracanthus.
In the well-known Mimosa pudica, which is a hothouse plant in
temperate regions, the leaflets fold together, the small stalks of
the leaflets of the compound leaves of this plant approach each
other, and the main petiole becomes depressed.
In one exceedingly sensitive species of Oxalis, the pinnate leaves
fold upward. A footfall is said to be sufficient to cause it to close
its leaves.
When these movements of leaves or leaf-organs take place at stated
hours, and when the leaves remain in the new position after the
movement has ceased until a particular period of time recur, the
closing up is called the _sleep_ of plants. This condition is
observed both in seed-leaves and true leaves, as well as in the
petals of flowers.
So far as can be made out, the object of this closing of the leaves
seems to be to prevent the chilling effect due to radiation from
being injurious to the plant. This folding up causes a smaller extent
of surface to be exposed. Radiation of heat during a clear night
goes on rapidly from all surfaces such as those of expanded leaves.
The closing of the leaves may be supposed to form a protective
covering, which prevents the heat passing away into space, and thus
saves the plant from the injurious effects of cold.
This is only true of the foliage leaves, which expand during the day
and close during the night.
The period at which the movement of closing and opening of flowers
takes place is very varied. Ordinary leaves, as has been stated,
close toward evening and open in the day. The periods of opening
and closing in the case of flowers vary considerably, being
affected, no doubt, by the visits of insects, which carry the pollen
from plant to plant belonging to the same species. By this means
flowers are fertilized, and the seeds resulting from plants that
are so fertilized are much more numerous than those resulting from
self-fertilized plants. Some plants, such as the pimpernel, close
their petals when the sky is overcast. This is doubtless to protect
the pollen from the injurious effects of rain. This kind of closing,
however, is not to be confounded with the regular and periodic
closing and opening of flowers.
The diversity in the regular and periodic opening and closing of
flowers in regard to time is so great that Linnæus was able to
arrange flowers in a list in accordance with their times of opening
and closing.
This list he named a _Horologium floræ_, or floral clock, the time of
opening or closing representing each succeeding hour.
Some closing flowers open under the influence of strong artificial
light, such, for example, as Crocus and Gentiana verna; on others,
however, such as Convolvulus, artificial light has no effect.
The closing of flowers is usually a slow process, as may easily be
observed, but there are exceptions to this.
“In Desmodium gyrans” (the Indian telegraph-plant) “the trilobate
compound leaf has a large terminal leaflet and a smaller one on each
side. When the plant is exposed to bright sunlight in a hothouse,
the end leaflet stands horizontally, and it folds downward in the
evening, but the lateral leaflets move constantly during the heat
of the day, advancing, edgewise, first toward the end leaflet, and
then returning and moving toward the base of the common petiole
alternately on each side, in a manner very well compared to the
movements of the arm of the old semaphore telegraphs.”
Such are some of the more striking movements of plants. Even in
cases where the precise advantage, as far as regards the economy of
plant life, is not fully ascertained, it can not be doubted that
such movements are advantageous. In strict accordance with the
accepted theory of evolution, no peculiarity would be continued from
generation to generation of either plants or animals, if it possessed
no essential characteristic which helped the plant or animal to hold
its own in “the struggle for existence.”
[Illustration: Cacti, Rare Flowers, and Fuci
Cacti--1 and 3, Mamillaria; 2, Echinocactus; 4, Cereus. Fuci--5,
Sargassum; 6, Agarum; 7, Thalassophyllum. The Wool Tree (Bombax) and
the Rafflesia Arnoldi]
MOVEMENT IN PLANTS
--CHARLES DARWIN
Plants become climbers in order, it may be presumed, to reach the
light and to expose a large surface of 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 in all quarters of the
world so many climbing plants belonging to so many different
orders. These plants are here classed under three heads. First,
hook-climbers, which are, at least in our temperate countries,
the least efficient of all, and can climb only in the midst of an
entangled vegetation. Secondly, root-climbers, which are excellently
adapted to ascend naked faces of rock: when they climb trees, they
are compelled to keep much in the shade; they can not pass from
branch to branch, and thus cover the whole summit of a tree, for
their rootlets can adhere only by long-continued and close contact
with a steady surface. Thirdly, the great class of spiral climbers,
with the subordinate divisions of leaf-climbers and tendril-bearers,
which together far exceed in number and in perfection of mechanism
the climbers of the two previous classes. These plants, by their
power of spontaneously revolving and grasping objects with which they
come in contact, can easily pass from branch to branch, and securely
wander over a wide and sunlit surface. I have ranked twiners, leaf
and tendril-climbers as subdivisions of one class, because they
graduate into each other, and because 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 passed, during the lapse of ages, or can pass, from one state
to the other; has, for instance, a tendril-bearing plant assumed
its present structure without having previously existed either 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; and some few can twine as well, and many others
in a more or less imperfect manner. Several leaf-climbing genera are
closely allied to other genera which are simple twiners. It should be
observed that the possession by a plant of leaves with their petioles
or tips sensitive, and with the consequent power of clasping any
object, would be of very little use, unless associated with revolving
internodes, by which the leaves could be brought into contact with
surrounding objects. On the other hand, revolving internodes, without
other aid, suffice to give the power of climbing, so that, unless we
suppose that leaf-climbers simultaneously acquired both capacities,
it seems probable that they were first twiners, and subsequently
became capable of grasping a support, which, as we shall presently
see, is a great additional advantage.
From analogous reasons, it is probable that tendril-bearing plants
were primordially twiners--that is, are the descendants of plants
having this power and habit. For the internodes of the majority
revolve, like those of twining plants; and, in a very few, the
flexible stem still retains the capacity of spirally twining
round an upright stick. With some the internodes have lost even
the revolving power. Tendril-bearers have undergone much more
modification than leaf-climbers; hence it is not surprising that
their supposed primordial revolving and twining habits have been
lost or modified more frequently than with 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 of Mormodica balsamina, and this is
only an imperfect twiner. In the other two families I can hear of no
twiners; and the internodes rarely have the power of revolving, this
power being confined to the tendrils; nevertheless, the internodes of
Passiflora gracilis have this 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 is always retained by some members of
the larger tendril-bearing groups.
On the view here given, it may be asked, Why have nearly all the
plants in so many aboriginally twining groups been converted into
leaf-climbers or tendril-bearers? Of what advantage could this have
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, we can readily
observe how incomparably more securely they grasp an upright stick
than do simple twiners. From possessing the power of movement on
contact, tendrils can be made very long and thin; so that little
organic matter is expended in their development, and yet a wide
circle is swept. Tendril-bearers can, from their first growth, ascend
along the outer branches of any neighboring bush, and thus always
keep in the full light; twiners, on the contrary, are best fitted
to ascend bare stems, and generally have to start in the shade. In
dense tropical forests, with crowded and bare stems, 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, can not ascend a thick trunk,
whereas this can be effected by tendril-bearers, if the trunks carry
many branches or twigs; and in some cases they can ascend by special
means a trunk without branches, but with a rugged bark.
The object of all climbing plants is to reach the light and free air
with as little expenditure of organic matter as possible; now, with
spirally ascending 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, ascending by its tendrils, would, on the
other hand, have been but little longer than the height gained. That
this saving of stem is really an advantage to climbing plants I infer
from observing that those that still twine, but are aided by clasping
petioles or tendrils, generally make more open spires than those made
by simple twiners. Moreover, such plants very generally, after taking
one or two turns in one direction, ascend for a space straight, and
then reverse the direction of the spire. By this means they ascend
to a considerably greater height, with the same length of stem, than
would otherwise be possible; and they can do it with safety, as they
secure themselves at intervals by their clasping petioles.
Tendrils consist of various organs in a modified state, namely,
leaves and flower-peduncles, and perhaps branches and stipules.
The position alone generally suffices to show when a tendril has
been formed from a leaf; and in Bignonia the lower leaves are often
perfect, while the upper ones terminate in a tendril in place of a
terminal leaflet; in Eccremocarpus I have seen a lateral branch of a
tendril replaced by a perfect leaflet; and 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 tendrils; he will wish
to learn, as far as possible, by what steps parts acting as leaves or
as flower-peduncles can have wholly changed their function, and have
come to serve as prehensile organs.
In the whole group of leaf-climbers abundant evidence has been
given that an organ, still subserving its proper function as a
leaf, may become sensitive to a touch, and thus grasp an adjoining
object. In several leaf-climbers true leaves spontaneously revolve;
and their petioles, after clasping a support, grow thicker and
stronger. We thus see that true leaves may acquire all the leading
and characteristic qualities of tendrils, namely, sensitiveness,
spontaneous movement, and subsequent thickening and induration. If
their blades or laminæ were to abort, they would form true tendrils.
And of this process of abortion we have seen every stage; for in an
ordinary tendril, as in that of the pea, we can discover no trace
of its primordial nature; in Mutisia clematis, the tendril in shape
and color closely resembles a petiole with the denuded midribs of
its leaflets; and occasionally vestiges of laminæ are retained
or reappear. Lastly, in four genera in the same family of the
Fumariaceæ we see the whole gradation; for 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 the Corydalis claviculata (a plant which may be
indifferently called a leaf-climber or tendril-bearer) are either
reduced to microscopical dimensions or have their blades quite
aborted, so that this plant is in an actual state of transition; and,
finally, in the Dicentra the tendrils are perfectly characterized.
Hence, if we were to see at the same time all the progenitors of the
Dicentra, we should almost certainly behold a series like that now
exhibited by the above-named four genera. In Tropæolum tricolorum we
have another kind of passage; for the leaves which are first formed
on the young plant are entirely destitute of laminæ, and must be
called tendrils, while the later formed leaves have well-developed
laminæ. In all cases, in the several kinds of leaf-climbers and of
tendril-bearers, the acquirement of sensitiveness by the midribs
of the leaves apparently stands in the closest relation with the
abortion of their laminæ or blades.
On the view here given, leaf-climbers were primordially twiners, and
tendril-bearers (of the modified leaf division) were primordially
leaf-climbers. Hence leaf-climbers 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, of Cocculus, of Gloriosa are related to the other genera
in the same family, or even to other species in the same genus, which
are true climbers. On the other hand, 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, one species of Bignonia is both a
leaf-climber and a tendril-bearer, and other closely allied species
are twiners.
Tendrils of the second great division consist of modified
flower-peduncles. In this case likewise we have many interesting
transitional states. The common vine (not to mention the
Cardiospermum) gives us every possible grade from finely developed
tendrils to a bunch of flower-buds, bearing the single usual lateral
flower-tendril. And when the latter itself bears some flowers, as we
know is not rarely the case, and yet retains the power of clasping a
support, we see the primordial state of all these tendrils which have
been formed by the modification of flower-peduncles.
According to Mohl and others, some tendrils consist of modified
branches. I have seen no such case, and, therefore, of course, know
nothing of any transitional states, if such occur. But Lophospermum,
at least, shows us that such a transition is possible; for its
branches spontaneously revolve, and are sensitive to contact. Hence,
if the leaves of some of the branches were to abort, they would be
converted into true tendrils. Nor is it so improbable as may at first
appear that certain branches alone should become modified, the others
remaining unaltered; for with certain varieties of Phaseolus some of
the branches are thin and flexible and twine, while other branches on
the same plant are stiff and have no such power.
If we inquire how the petiole of a leaf, or the peduncle of a
flower, or a branch first becomes sensitive and acquires the power
of bending toward the touched side, we get no certain answer.
Nevertheless, an observation by Hofmeister well deserves attention,
namely, that the shoots and leaves of all plants, while young, move
after being shaken; and it is almost invariably young petioles and
young tendrils, whether of modified leaves or flower-peduncles,
which move on being touched; so that it would appear as if these
plants had 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 movement,
both spontaneous and from various stimuli, is far more common with
plants, as we shall presently see, than is generally supposed to
be the case by those who have not attended to the subject. There
is, however, one remarkable case of the Maurandia semperflorens, in
which the young flower-peduncles spontaneously revolve in very small
circles, and bend themselves, when gently rubbed, to the touched
side; yet this plant certainly profits in no way by these two feebly
developed powers. A rigorous examination of other young plants would
probably show some slight spontaneous movement in the peduncles
and petioles, as well as that sensitiveness to shaking observed by
Hofmeister. We see at least in the Maurandia a plant which might,
by a little augmentation of qualities which it already possesses,
come first to grasp a support by its flower-peduncles (as with Vitis
or Cardiospermum) and then, by the abortion of some of its flowers,
acquire perfect tendrils.