as we now see it, on a certain “ideal type”; that the omnipotent
Creator, having fixed on one plan for the whole order, did not depart
from this plan; that he, therefore, made the same organ to perform
diverse functions--often of trifling importance compared with
their proper function--converted other organs into mere purposeless
rudiments, and arranged all as if they had to stand separate, and then
made them cohere? Is it not a more simple and intelligible view that
all the _Orchideæ_ owe what they have in common to descent from some
monocotyledonous plant, which, like so many other plants of the same
class, possessed fifteen organs, arranged alternately, three within
three, in five whorls; and that the now wonderfully changed structure
of the flower is due to a long course of slow modification--each
modification having been preserved which was useful to the plant,
during the incessant changes to which the organic and inorganic world
has been exposed?
SPECIAL ADAPTATIONS TO A CHANGING PURPOSE.
[Fertilization
of Orchids,
page 282.]
It has, I think, been shown that the _Orchideæ_ exhibit an almost
endless diversity of beautiful adaptations. When this or that part has
been spoken of as adapted for some special purpose, it must not be
supposed that it was originally always formed for this sole purpose.
The regular course of events seems to be, that a part which originally
served for one purpose becomes adapted by slow changes for widely
different purposes. To give an instance: in all the _Ophreæ_, the long
and nearly rigid caudicle manifestly serves for the application of
the pollen-grains to the stigma, when the pollinia are transported by
insects to another flower; and the anther opens widely in order that
the pollinium should be easily withdrawn; but, in the _Bee ophrys_, the
caudicle, by a slight increase in length and decrease in its thickness,
and by the anther opening a little more widely, becomes specially
adapted for the very different purpose of self-fertilization, through
the combined aid of the weight of the pollen-mass and the vibration
of the flower when moved by the wind. Every gradation between these
two states is possible--of which we have a partial instance in _O.
aranifera_.
Again, the elasticity of the pedicel of the pollinium in some _Vandeæ_
is adapted to free the pollen-masses from their anther-cases; but, by
a further slight modification, the elasticity of the pedicel becomes
specially adapted to shoot out the pollinium with considerable force,
so as to strike the body of the visiting insect. The great cavity in
the labellum of many _Vandeæ_ is gnawed by insects, and thus attracts
them; but in _Mormodes ignea_ it is greatly reduced in size, and serves
in chief part to keep the labellum in its new position on the summit
of the column. From the analogy of many plants we may infer that a
long, spur-like nectary is primarily adapted to secrete and hold a
store of nectar; but in many orchids it has so far lost this function
that it contains fluid only in the intercellular spaces. In those
orchids in which the nectary contains both free nectar and fluid in the
intercellular spaces, we can see how a transition from the one state
to the other could be effected, namely, by less and less nectar being
secreted from the inner membrane, with more and more retained within
the intercellular spaces. Other analogous cases could be given.
Although an organ may not have been originally formed for some special
purpose, if it now serves for this end, we are justified in saying that
it is specially adapted for it. On the same principle, if a man were to
make a machine for some special purpose, but were to use old wheels,
springs, and pulleys, only slightly altered, the whole machine, with
all its parts, might be said to be specially contrived for its present
purpose. Thus throughout nature almost every part of each living being
has probably served, in a slightly modified condition, for diverse
purposes, and has acted in the living machinery of many ancient and
distinct specific forms.
In my examination of orchids, hardly any fact has struck me so much as
the endless diversities of structure--the prodigality of resources--for
gaining the very same end, namely, the fertilization of one flower by
pollen from another plant. This fact is to a large extent intelligible
on the principle of natural selection. As all the parts of a flower are
co-ordinated, if slight variations in any one part were preserved from
being beneficial to the plant, then the other parts would generally
have to be modified in some corresponding manner. But these latter
parts might not vary at all, or they might not vary in a fitting
manner, and these other variations, whatever their nature might be,
which tended to bring all the parts into more harmonious action with
one another, would be preserved by natural selection.
AN ILLUSTRATION.
[Page 284.]
To give a simple illustration: in many orchids the ovarium (but
sometimes the foot-stalk) becomes for a period twisted, causing the
labellum to assume the position of a lower petal, so that insects can
easily visit the flower; but from slow changes in the form or position
of the petals, or from new sorts of insects visiting the flowers, it
might be advantageous to the plant that the labellum should resume
its normal position on the upper side of the flower, as is actually
the case with _Malaxis paludosa_, and some species of _Catasetum_,
etc. This change, it is obvious, might be simply effected by the
continued selection of varieties which had their ovaria less and less
twisted; but, if the plant only afforded varieties with the ovarium
more twisted, the same end could be attained by the selection of such
variations, until the flower was turned completely round on its axis.
This seems to have actually occurred with _Malaxis paludosa_, for the
labellum has acquired its present upward position by the ovarium being
twisted twice as much as is usual.
Again, we have seen that in most _Vandeæ_ there is a plain relation
between the depth of the stigmatic chamber and the length of the
pedicel, by which the pollen-masses are inserted; now, if the chamber
became slightly less deep from any change in the form of the column, or
other unknown cause, the mere shortening of the pedicel would be the
simplest corresponding change; but, if the pedicel did not happen to
vary in shortness, the slightest tendency to its becoming bowed from
elasticity, as in _Phalænopsis_, or to a backward hygrometric movement,
as in one of the _Maxillarias_, would be preserved, and the tendency
would be continually augmented by selection; thus the pedicel, as far
as its action is concerned, would be modified in the same manner as if
it had been shortened. Such processes carried on during many thousand
generations in various ways, would create an endless diversity of
co-adapted structures in the several parts of the flower for the same
general purpose. This view affords, I believe, the key which partly
solves the problem of the vast diversity of structure adapted for
closely analogous ends in many large groups of organic beings.
AS INTERESTING ON THE THEORY OF DEVELOPMENT AS ON THAT OF DIRECT
INTERPOSITION.
[Page 285.]
The more I study nature, the more I become impressed, with
ever-increasing force, that the contrivances and beautiful adaptations
slowly acquired through each part occasionally varying in a slight
degree but in many ways, with the preservation of those variations
which were beneficial to the organism under complex and ever-varying
conditions of life, transcend in an incomparable manner the
contrivances and adaptations which the most fertile imagination of man
could invent.
The use of each trifling detail of structure is far from a barren
search to those who believe in natural selection. When a naturalist
casually takes up the study of an organic being, and does not
investigate its whole life (imperfect though that study will ever be),
he naturally doubts whether each trifling point can be of any use, or,
indeed, whether it be due to any general law. Some naturalists believe
that numberless structures have been created for the sake of mere
variety and beauty--much as a workman would make different patterns.
I, for one, have often and often doubted whether this or that detail
of structure in many of the _Orchideæ_ and other plants could be of
any service; yet, if of no good, these structures could not have
been modeled by the natural preservation of useful variations; such
details can only be vaguely accounted for by the direct action of the
conditions of life, or the mysterious laws of correlated growth.
[Fertilization
of Orchids,
page 2.]
This treatise affords me also an opportunity of attempting to show that
the study of organic beings may be as interesting to an observer who is
fully convinced that the structure of each is due to secondary laws as
to one who views every trifling detail of structure as the result of
the direct interposition of the Creator.
THE SLEEP OF THE PLANTS.
[The Power
of Movement
in Plants,
page 280.]
The so-called sleep of leaves is so conspicuous a phenomenon that it
was observed as early as the time of Pliny; and since Linnæus published
his famous essay, “Somnus Plantarum,” it has been the subject of
several memoirs. Many flowers close at night, and these are likewise
said to sleep; but we are not here concerned with their movements,
for although effected by the same mechanism as in the case of young
leaves, namely, unequal growth on the opposite sides (as first proved
by Pfeffer), yet they differ essentially in being excited chiefly by
changes of temperature instead of light; and in being effected, as far
as we can judge, for a different purpose. Hardly any one supposes that
there is any real analogy between the sleep of animals and that of
plants, whether of leaves or flowers. It seems, therefore, advisable
to give a distinct name to the so-called sleep-movements of plants.
These have also generally been confounded, under the term “periodic,”
with the slight daily rise and fall of leaves, as described in the
fourth chapter; and this makes it all the more desirable to give some
distinct name to sleep-movements. Nyctitropism and nyctitropic, i. e.,
night-turning, may be applied both to leaves and flowers, and will be
occasionally used by us; but it would be best to confine the term to
leaves.
* * * * *
[Page 281.]
Leaves, when they go to sleep, move either upward or downward, or, in
the case of the leaflets of compound leaves, forward, that is, toward
the apex of the leaf, or backward, that is, toward its base; or,
again, they may rotate on their own axis without moving either upward
or downward. But in almost every case the plane of the blade is so
placed as to stand nearly or quite vertically at night. Therefore the
apex, or the base, or either lateral edge, may be directed toward the
zenith. Moreover, the upper surface of each leaf, and more especially
of each leaflet, is often brought into close contact with that of the
opposite one; and this is sometimes effected by singularly complicated
movements. This fact suggests that the upper surface requires more
protection than the lower one. For instance, the terminal leaflet in
trifolium, after turning up at night so as to stand vertically, often
continues to bend over until the upper surface is directed downward,
while the lower surface is fully exposed to the sky; and an arched roof
is thus formed over the two lateral leaflets, which have their upper
surfaces pressed closely together. Here we have the unusual case of one
of the leaflets not standing vertically, or almost vertically, at night.
Considering that leaves in assuming their nyctitropic positions often
move through an angle of 90°; that the movement is rapid in the
evening; that in some cases it is extraordinarily complicated; that
with certain seedlings, old enough to bear true leaves, the cotyledons
move vertically upward at night, while at the same time the leaflets
move vertically downward; and that in the same genus the leaves or
cotyledons of some species move upward, while those of other species
move downward--from these and other such facts, it is hardly possible
to doubt that plants must derive some great advantage from such
remarkable powers of movement.
SELF-PROTECTION DURING SLEEP.
[Page 284.]
The fact that the leaves of many plants place themselves at night in
widely different positions from what they hold during the day, but
with the one point in common, that their upper surfaces avoid facing
the zenith, often with the additional fact that they come into close
contact with opposite leaves or leaflets, clearly indicates, as it
seems to us, that the object gained is the protection of the upper
surfaces from being chilled at night by radiation. There is nothing
improbable in the upper surface needing protection more than the lower,
as the two differ in function and structure. All gardeners know that
plants suffer from radiation. It is this, and not cold winds, which
the peasants of Southern Europe fear for their olives. Seedlings are
often protected from radiation by a very thin covering of straw; and
fruit-trees on walls by a few fir-branches, or even by a fishing-net,
suspended over them. There is a variety of the gooseberry, the flowers
of which, from being produced before the leaves, are not protected by
them from radiation, and consequently often fail to yield fruit. An
excellent observer has remarked that one variety of the cherry has
the petals of its flowers much curled backward, and after a severe
frost all the stigmas were killed; while, at the same time, in another
variety with incurved petals, the stigmas were not in the least injured.
* * * * *
[Page 285.]
We are far from doubting that an additional advantage may be thus
gained; and we have observed with several plants, for instance,
_Desmodium gyrans_, that while the blade of the leaf sinks vertically
down at night, the petiole rises, so that the blade has to move through
a greater angle in order to assume its vertical position than would
otherwise have been necessary; but with the result that all the leaves
on the same plant are crowded together, as if for mutual protection.
We doubted at first whether radiation would affect in any important
manner objects so thin as are many cotyledons and leaves, and more
especially affect differently their upper and lower surfaces; for,
although the temperature of their upper surfaces would undoubtedly fall
when freely exposed to a clear sky, yet we thought that they would so
quickly acquire by conduction the temperature of the surrounding air,
that it could hardly make any sensible difference to them whether they
stood horizontally, and radiated into the open sky, or vertically, and
radiated chiefly in a lateral direction toward neighboring plants and
other objects. We endeavored, therefore, to ascertain something on
this head, by preventing the leaves of several plants from going to
sleep, and by exposing to a clear sky, when the temperature was beneath
the freezing-point, these as well as the other leaves on the same
plants, which had already assumed their nocturnal vertical position.
Our experiments show that leaves thus compelled to remain horizontal
at night suffered much more injury from frost than those which were
allowed to assume their normal vertical position. It may, however, be
said that conclusions drawn from such observations are not applicable
to sleeping plants, the inhabitants of countries where frosts do not
occur. But in every country, and at all seasons, leaves must be exposed
to nocturnal chills through radiation, which might be in some degree
injurious to them, and which they would escape by assuming a vertical
position.
* * * * *
[The Power
of Movement
in Plants,
page 403.]
Any one who had never observed continuously a sleeping plant would
naturally suppose that the leaves moved only in the evening when going
to sleep, and in the morning when awaking; but he would be quite
mistaken, for we have found no exception to the rule that leaves which
sleep continue to move during the whole twenty-four hours; they move,
however, more quickly when going to sleep and when awaking than at
other times.
INFLUENCE OF LIGHT UPON PLANTS.
[The Power
of Movement
in Plants,
page 565.]
The extreme sensitiveness of certain seedlings to light is highly
remarkable. The cotyledons of _Phalaris_ became curved toward a distant
lamp, which emitted so little light that a pencil held vertically close
to the plants did not cast any shadow which the eye could perceive on a
white card. These cotyledons, therefore, were affected by a difference
in the amount of light on their two sides, which the eye could not
distinguish. The degree of their curvature within a given time toward
a lateral light did not correspond at all strictly with the amount of
light which they received; the light not being at any time in excess.
They continued for nearly half an hour to bend toward a lateral light,
after it had been extinguished. They bend with remarkable precision
toward it, and this depends on the illumination of one whole side, or
on the obscuration of the whole opposite side. The difference in the
amount of light which plants at any time receive in comparison with
what they have shortly before received seems in all cases to be the
chief exciting cause of those movements which are influenced by light.
Thus seedlings brought out of darkness bend toward a dim lateral light,
sooner than others which had previously been exposed to daylight. We
have seen several analogous cases with the nyctitropic movements of
leaves. A striking instance was observed in the case of the periodic
movements of the cotyledons of a cassia: in the morning a pot was
placed in an obscure part of a room, and all the cotyledons rose up
closed; another pot had stood in the sunlight, and the cotyledons of
course remained expanded; both pots were now placed close together in
the middle of the room, and the cotyledons which had been exposed to
the sun immediately began to close, while the others opened; so that
the cotyledons in the two pots moved in exactly opposite directions
while exposed to the same degree of light.
We found that if seedlings, kept in a dark place, were laterally
illuminated by a small wax-taper for only two or three minutes at
intervals of about three quarters of an hour, they all became bowed
to the point where the taper had been held. We felt much surprised at
this fact, and, until we had read Wiesner’s observations, we attributed
it to the after-effects of the light; but he has shown that the same
degree of curvature in a plant may be induced in the course of an hour
by several interrupted illuminations lasting altogether for twenty
minutes as by a continuous illumination of sixty minutes. We believe
that this case, as well as our own, may be explained by the excitement
from light being due not so much to its actual amount, as to the
difference in amount from that previously received; and in our case
there were repeated alternations from complete darkness to light. In
this and in several of the above-specified respects, light seems to act
on the tissues of plants almost in the same manner as it does on the
nervous system of animals.
INFLUENCE OF GRAVITATION UPON PLANTS.
[Page 567.]
Gravitation excites plants to bend away from the center of the earth,
or toward it, or to place themselves in a transverse position with
respect to it. Although it is impossible to modify in any direct
manner the attraction of gravity, yet its influence could be moderated
indirectly, in the several ways described in the tenth chapter; and
under such circumstances the same kind of evidence as that given in the
chapter on heliotropism showed in the plainest manner that apogeotropic
and geotropic, and probably diageotropic movements, are all modified
forms of circumnutation.
Different parts of the same plant and different species are affected by
gravitation in widely different degrees and manners. Some plants and
organs exhibit hardly a trace of its action. Young seedlings, which,
as we know, circumnutate rapidly, are eminently sensitive; and we have
seen the hypocotyl of _Beta_ bending upward through 109° in three hours
and eight minutes. The after-effects of apogeotropism last for above
half an hour; and horizontally-laid hypocotyls are sometimes thus
carried temporarily beyond an upright position. The benefits derived
from geotropism, apogeotropism, and diageotropism, are generally so
manifest that they need not be specified. With the flower-peduncles
of _Oxalis_, epinasty causes them to bend down, so that the ripening
pods may be protected by the calyx from the rain. Afterward they are
carried upward by apogeotropism in combination with hyponasty, and are
thus enabled to scatter their seeds over a wider space. The capsules
and flower-heads of some plants are bowed downward through geotropism,
and they then bury themselves in the earth for the protection and slow
maturation of the seeds. This burying process is much facilitated by
the rocking movement due to circumnutation.
In the case of the radicles of several, probably of all seedling
plants, sensitiveness to gravitation is confined to the tip, which
transmits an influence to the adjoining upper part, causing it to bend
toward the center of the earth. That there is transmission of this
kind was proved in an interesting manner when horizontally extended
radicles of the bean were exposed to the attraction of gravity for an
hour or an hour and a half, and their tips were then amputated. Within
this time no trace of curvature was exhibited, and the radicles were
now placed pointing vertically downward; but an influence had already
been transmitted from the tip to the adjoining part, for it soon became
bent to one side, in the same manner as would have occurred had the
radicle remained horizontal and been still acted on by geotropism.
Radicles thus treated continued to grow out horizontally for two or
three days, until a new tip was reformed; and this was then acted on by
geotropism, and the radicle became curved perpendicularly downward.
THE POWER OF DIGESTION IN PLANTS.
[Insectivorous
Plants,
page 85.]
As we have seen that nitrogenous fluids act very differently on the
leaves of _Drosera_ from non-nitrogenous fluids, and as the leaves
remain clasped for a much longer time over various organic bodies than
over inorganic bodies, such as bits of glass, cinder, wood, etc., it
becomes an interesting inquiry whether they can only absorb matter
already in solution, or render it soluble; that is, have the power
of digestion. We shall immediately see that they certainly have this
power, and that they act on albuminous compounds in exactly the same
manner as does the gastric juice of mammals; the digested matter being
afterward absorbed. This fact, which will be clearly proved, is a
wonderful one in the physiology of plants.
* * * * *
[Page 86.]
It may be well to premise, for the sake of any reader who knows
nothing about the digestion of albuminous compounds by animals, that
this is effected by means of a ferment, pepsin, together with weak
hydrochloric acid, though almost any acid will serve. Yet neither
pepsin nor an acid by itself has any such power. We have seen that
when the glands of the disk are excited by the contact of any object,
especially of one containing nitrogenous matter, the outer tentacles
and often the blade become inflected; the leaf being thus converted
into a temporary cup or stomach. At the same time the discal glands
secrete more copiously, and the secretion becomes acid. Moreover,
they transmit some influence to the glands of the exterior tentacles,
causing them to pour forth a more copious secretion, which also becomes
acid or more acid than it was before.