PHENOMENA OF VEGETABLE LIFE.
Psychology of Flowers--Progress of Matter towards
Organization--Vital Force--Spontaneous Generation--The Vegetable
Cell--Simplest Development of Organization--The Crystal and
the Cell--Primitive Germ--Progress of Vegetation--Influence
of Light--Morphology--Germination--Production of Woody
Fibre--Leaves--Chlorophylle--Decomposition of Carbonic
Acid--Influence of Light, Heat, and Actinism on the Phenomena of
Vegetable Life--Flowers and Fruits--Etiolation--Changes in the
Sun’s Rays with the Seasons--Distribution of Plants--Electrical and
Combined Physical Powers
The variety of beautiful forms which cover the surface of this sphere,
serve, beyond the physical purposes to which we have already alluded,
to influence the mind, and give character to the inhabitants of every
locality. There are men who appear to be dead to the mild influences
of flowers; but these sweet blossoms--the stars of our earth--exert a
power as diffusive as their pervading odours.
The poet tells us of a man to whom
The primrose on the river’s brim
A yellow primrose was to him,
And it was nothing more.
But it was something more. He, perhaps, attended not to the eloquent
teaching of its pure, pale leaves: he might not have been conscious
of the mysterious singing of that lowly flower: he might, perchance,
have crushed it beneath his rude foot rather than quaff the draught of
wisdom which it secreted in its cell; but the flower still ministered
to that mere sensualist, and in its strange, tongueless manner,
reproved his passions, and kept him “a wiser and a better man,“ than if
it had pleased God to have left the world without the lovely primrose.
The psychology of flowers has found many students--than whom not one
read them more deeply than that mild spirit who sang of the Sensitive
Plant, and in wondrous music foreshadowed his own melancholy fate.[248]
That martyr to sensibility, Keats, who longed to feel the flowers
growing above him, drew the strong inspiration of his volant muse from
those delicate creations which exhibit the passage of inorganic matter
into life; and other poets will have their sensibilities awakened by
the æsthetics of flowers, and find a mirror of truth in the crystal
dew-drop which clings so lovingly to the purple violet, and draws fresh
beauties from its coloured petals.
If we examine carefully all the conditions of matter which we have
made the subject of our studies, we cannot but perceive how gradual
is the progress of the involved action of the physical forces, as we
advance from the molecule--the mere particle of matter--up to the
organic combination. At first we detect only the action of cohesion in
forming the rude mass; then we have the influence of the crystallogenic
powers giving a remarkable regularity to bodies; we next discover the
influences of heat and electrical force in determining condition, and
of chemical action as controlled by them. Yet, still we have a body
without organization. Light exerts its mysterious powers, and the same
elements assume an organized form; and, in addition to the recognized
agencies, we dimly perceive others on which vitality evidently
depends. These empyreal influences become more and more complicated
to us: ascending in the scale, they rise beyond our science; and, at
length, we find them guiding the power of intelligence, while instinct
and reason are exhibited in immediate dependence upon them.
Let it not be imagined that this view has any tendency to materialism.
The vital energy is regarded as a spiritualization, and reason as a
divine emanation; but they are connected with materialities, on which
they act, and by which they are themselves controlled. The organic
combinations, and the physical powers by which these unions of matter
are effected and retained, have a direct action over that ethereality
which is life, and the powers of life again control these more material
forces. The spirit, in whatever state, when connected with matter, is,
like Prometheus chained to his rock, in a constant struggle to escape
from its shackles, and assert the full power of its divine strength.
We have seen variety enough in the substances which make up the
inorganic part of creation; but infinitely more varied are the forms
of organization. In the vegetable world which is immediately around
us, from the green slime of our marshes to the lustrous flowers of our
gardens and the lordly trees of our forests, what an extraordinary
diversity of form is apparent! From the infusoria of an hour, to the
gigantic elephant roaming in his greatness in the forests of Siam--the
noble lion of the caves of Senegal--the mighty condor of the Andes--and
onward to man, the monarch of them all, how vast are the differences,
and yet how complete are they in their respective conditions! In the
creation we have examined, we have had conclusive evidence, that from
the combination of _atoms_ every peculiar form has been produced. In
the creation we are about to examine, we shall discover that all the
immense diversity of form, of colour, and condition which is spread
over the world in the vegetable and animal kingdoms, results from the
combination of _cells_. The atom of inorganic nature becomes a cell in
organic creation. This cell must be regarded as the compound radical of
the chemist, and by decomposing it, we destroy the essential element of
organization.
With the mysterious process by which the atom is converted into a
cell, or a compound radical, we are unacquainted; but we must regard
the cell as the organic atom. It is in vain that the chemist or the
physiologist attempts to examine this change of the inorganic elements
to an organized state; it is one of the mysteries of creation, which is
to be, in all probability, hid from our eyes, until this “mortal coil”
is shaken off, and we enjoy the full powers of that intelligence which
we are promised we shall enjoy in an immortal state.
Again and again has the attention of men been attracted to the
_generatio æquivoca_; they have sometimes thought they have discovered
a _generatio primitiva_ or _spontanea_; but a more careful examination
of these organisms has shown that an embryo existed--a real germination
has taken place.
Count Rumford[249] stated that threads of silk and wool had the power
of decomposing carbonic acid in water in the sunshine; and hence some
have referred organization to a mere chemical change produced by
luminous excitation; and we have heard of animal life resulting from
pounded siliceous matter. All such statements must be regarded as
evidences of imperfect investigation.
Dr. Carus, alluding to the experiments of Gruithuisen, Priestley, and
Ingenhousz,[250] says:--“These show, more than any other experiments,
that, in the purest water, under the influence of air, light, and heat,
beings are formed, which, oscillating as it were between the animal
and the plant, exhibit the primitive germs of both kingdoms.”[251]
Treviranus[252] repeated, and appeared to confirm these results; but in
these experiments we have no evidence that the germ did not previously
exist in the spring-water which was employed.
Some have regarded the cell as a crystal; they see the crystal forming,
by the accumulation of atoms, into a fixed form, under the influence
of an “inner life;” and, advancing but a step, they regard the cell as
the result of an increased exercise of the physical influences.[253]
We have referred crystalline form to certain magnetic conditions; and
it is evident that the atomic cell is influenced by similar forces;
but if we place a crystal in its natural fluid, though it increases in
size, it never alters in form: whereas, if we examine a cell in its
natural position, it gives indications of motion, it produces other
cells, and we have a development of organs which are in no respect
the same in form as the original. From a vesicle floating invisible
to the unaided human sense in its womb of fluid, is produced a plant
possessing strange powers, or an animal gifted with volition. The idea,
that two kinds of polarity--light on one side, and gravitation on the
other--produce the two peculiar developments of roots and branches, can
only be regarded as one of those fanciful analogies which prove more
imagination than philosophy.[254]
The conditions are, however, most curious; they deserve very attentive
study; but in examining the phenomena, the safest course is to allow
the effects as they arise to interpret to us, and not admit the love
of hypothesis to lead us into bewildering analogies; or uncertain
phenomena to betray us to hasty inferences. It is of this evil that
Bacon speaks, in his “Advancement of Learning.” He says:--
“The root of this error, as of all others, is this, that men, in
their contemplations of nature, are accustomed to make too timely a
departure, and too remote a recess from experience and particulars, and
have yielded and resigned themselves over to the fumes of their own
fancies and popular argumentations.”
Without venturing, therefore, to speculate on the origin of the
primitive cell, or unit of organic life, which involves the problem
of the metamorphosis of a rude mass--the primitive transformation
of the rudimentary atoms into organic form,--we must admit that the
highly organized plant or animal is but an aggregation of cells; their
arrangement being dependent upon certain properties peculiar to them,
and the exercise of forces such as we have been studying,--all of which
appear to act externally to the plant or animal itself.
Experiments have been brought forward, in which it appeared that, after
all organization which could by any possibility have existed, had
been destroyed by the action of fire, solutions of flint and metallic
salts, have, under the influence of electric currents, exhibited
signs of organic formations, and that, indeed, insects--a species
of acari--have been developed in them. The experiments were said to
have been made with care, and many precautions taken to shut out
all chances of any error, but not all the precautions required in a
matter of such exceeding delicacy; and we are bound not to receive the
evidence afforded as the true expression of a fact without much further
investigation. All experience,--setting aside the experiment named,--is
against the supposition that pounded or dissolved flint could by any
artificial means be awakened into life. Ova may have been conveyed
into the vessels which contained the solutions under experiment; and
in due time, although possibly quickened by electric excitation, the
animals--the most common of insects--came into existence.[255]
The rapid growth of confervæ upon water has often been brought forward
as evidence of a spontaneous generation, or the conversion of inorganic
elements into organic forms; but it has been most satisfactorily proved
that the germ must be present, otherwise no evidence of anything
like organization will be developed. All the conditions required for
the production of vegetable life appear to show, that it is quite
impossible for any kind of plant, even the very lowest in the scale, to
be formed in any other way than from an embryo in which are contained
the elements necessary for it, and the arrangements required for the
various processes which are connected with its vitality.
The earth is now covered with vegetable life, but there must have
existed a time when “darkness was upon the face of the deep,” and
organization had not yet commenced tracing its lovely net-work of cells
upon the bare surface of the ocean-buried rock. At length the mystery
of organic creation began: into this science dares not penetrate, but
it is privileged to begin its search a little beyond this point, and
we are enabled to trace the progress of organic development through a
chain of interesting results which are constantly recurring.
If we take some water, rising from a subterranean spring, and expose
it to sunshine, we shall see, after a few days, a curious formation
of bubbles, and the gradual accumulation of green matter. At first we
cannot detect any marks of organization--it appears a slimy cloud of
an irregular and undetermined form. It slowly aggregates, and forms a
sort of mat over the surface, which at the same time assumes a darker
green colour. Careful examination will now show the original corpuscles
involved in a net-work formed by slender threads, which are tubes of
circulation, and may be traced from small points which we must regard
as the compound atom, the vegetable unit. We must not forget, here,
that we have to deal with four chemical elements,--oxygen, hydrogen,
carbon, and nitrogen, which compose the world of organized forms, and
that the water affords us the two first as its constituents, gives us
carbon in the form of carbonic acid dissolved in it, and that nitrogen
is in the air surrounding it, and frequently mixed with it also.
Under the influence of sunshine, we have now seen these elements
uniting into a mysterious bond, and the result is the formation of a
cellular tissue, which possesses many of the functions of the noblest
specimens of vegetable growth. But let us examine the progress. The
bare surface of a rock rises above the waters covered over with this
green slime, a mere veil of delicate net-work, which, drying off,
leaves no perceptible trace behind it; but the basis of a mighty growth
is there, and under solar influence, in the process of time, other
changes occur.
After a period, if we examine the rock, we shall find upon its face
little coloured cups or lines with small hard discs. These, at first
sight, would not be taken for plants, but on close examination they
will be found to be lichens. These minute vegetables shed their seed
and die, and from their own remains a more numerous crop springs into
life. After a few of these changes, a sufficient depth of soil is
formed, upon which mosses begin to develope themselves, and give to
the stone a second time a faint tint of green, a mere film still, but
indicating the presence of a beautiful class of plants, which, under
the microscope, exhibit in their leaves and flowers many points of
singular elegance. These mosses, like the lichens, decaying, increase
the film of soil, and others of a larger growth supply their places,
and run themselves the same round of growth and decay. By and by,
funguses of various kinds mingle their little globes and umbrella-like
forms. Season after season plants, perish and add to the soil, which is
at the same time increased in depth by the disintegration of the rock
over which it is laid, the cohesion of particles being broken up by the
operations of vegetable life. The minute seeds of the ferns floating on
the breeze, now find a sufficient depth of earth for germination, and
their beautiful fronds, eventually, wave in loveliness to the passing
winds.
Vegetable forms of a higher and a higher order gradually succeed each
other, each series perishing in due season, and giving to the soil
additional elements for the growth of plants of their own species or
those of others. Flowering herbs find a genial home on the once bare
rock; and the primrose pale, the purple foxglove, or the gaudy poppy,
open their flowers to the joy of light. The shrub, with its hardy roots
interlaced through the soil, and binding the very stones, grows rich
in its bright greenery. Eventually the tree springs from the soil,
and where once the tempest beat on the bare cold rock, is now the
lordly and branching monarch of the forest, with its thousand leaves,
affording shelter from the storm for bird and beast.
Such are the conditions which prevail throughout nature in the progress
of vegetable growth; the green matter gathering on a pond, the mildew
accumulating on a shaded wall, being the commencement of a process
which is to end in the development of the giant trees of the forest,
and the beautifully tinted flower of nature’s most chosen spot.
We must now consider closely the phenomena connected with the growth of
an individual plant, which will illustrate the operation of physical
influences throughout the vegetable world. The process by which the
embryo, secured in the seed, is developed, is our first inquiry.
A seed is a highly carbonized body, consisting of integuments and
embryo: between these, in most seeds, lies a substance called the
_albumen_, or _perisperm_. The embryo contains the elements of the
future plant--the cotyledons, the plumule, and the radicle; the
former developing into stalk and leaves, the latter into roots. This
embryo hides the living principle, for the development of which it
is necessary that the starch and gluten undergo a chemical change,
and that an elevation of temperature is produced. The vital power
is dormant--it sleeps--in the seed until the proper conditions are
produced. It has been proved, that the powers of maintaining life
in the seed are very great; excessive cold, sufficiently intense to
freeze mercury, will not kill seed, and they resist a comparatively
high temperature. It is probable that heat only destroys seeds by
drying them too completely. The temperature at which seeds germinate is
exceedingly varied,--those belonging to our own clime will germinate
when the thermometer rises above 40° F., but the seeds of tropical
plants demand that a temperature of from 70° to 84°, or even to 90°,
be steadily applied to them. In some cases it has been found that
even boiling the seeds has been advantageous to the future process of
germination in the soil. But let us take the seed of some ordinary
plant, and trace its progress.
An apparently dead grain is placed in the soil. If the temperature is a
few degrees above the freezing point, and the soil holds a due quantity
of water, the integument of the seed imbibes moisture and swells; the
tissue is softened, and the first effort of vital force begins. The
seed has now the power of decomposing water, the oxygen combines with
some of the carbon of the seed, and is expelled as carbonic acid.
Saussure’s experiments prove this. The air above the soil in which
a horse-bean was placed to germinate, gave, before the experiment,
nitrogen 210·26, and oxygen 56·29, and after germination, nitrogen
209·41, oxygen 44·38, and carbonic acid 11·27. This part of the process
is but little removed from the merely chemical changes which we have
already considered. We find the starch of the seed changed into gum and
sugar, which affords nutritive food for the developing embryo. The seed
now lengthens downwards by the radicle, and upwards by the cotyledons,
which, as they rise above the earth, acquire a green colour. Here the
first stage of vegetable life ends, the chemically exciting process is
at an end, and a new stimulus is required to continue in full activity
the vital powers. Carbonic acid is no longer given off.
The cotyledons, which are two opposite roundish leaves, act as the
lungs; by them carbonic acid taken from the atmosphere is absorbed
and carried by a circulating process, now in full activity, through
the young plant. The carbonic acid, a compound of carbon and oxygen,
is decomposed; it is deprived of its carbon, which is retained by the
plant, and oxygen is exhaled. The plant at this period is little more
than an arrangement of cellular tissue, a very slight development
of vascular and fibrous tissue appearing as a cylinder lying in the
centre of the sheath. At this point, however, we begin more distinctly
to trace the operations of the new power; the impulses of life are
strikingly evident.
The young root is now lengthening, and absorbing from the moisture in
the soil, which always contains some soluble salts, a portion of its
nutriment, which is impelled upwards by a force--probably capillary
attraction and endosmose action combined--to the point from which the
plumule springs. Capillary force raises the fluids through the tubes
in the stalk, and conveys them to the veins in the leaves, while
the endosmose force diffuses them through the vegetable tissues.
The plumule first ascends as a little twig, and, at the same time,
by exerting a more energetic action on the carbonic acid than the
cotyledons have done, the carbon retained by them being only so much
as is necessary to form chlorophylle, or the green colouring matter of
leaves, some wood is deposited in the centre of the radicle. From this
time the process of lignification goes on through all the fabric,--the
increase, and indeed the life, of the plant depending upon the
development of a true leaf from the plumule.
It must not be imagined that the process consists, in the first place,
of a mere oxidation of the carbon in the seed,--a slow combustion by
which the spark of life is to be kindled;--the hydrogen of the water
plays an important part, and, combining also with the carbon, forms
necessary compounds, and by a secondary process gives rise again to
water by combination with oxygen in the cells of the germinating grain.
Nor must we regard the second class of phenomena as mere mechanical
processes for decomposing carbonic acid, but the result of the combined
influences of all the physical powers and life superadded.
This elongating little twig, the plumule, at length unfolds itself, and
the branch is metamorphosed into a leaf. The leaf aërates the sap it
receives, effects the decomposition of the carbonic acid, the water,
and in all probability the ammonia which it derives from the air,
and thus returns to the pores, which communicate with the pneumatic
arrangements of the plant, the necessary secretions for the formation
of bark, wood, and the various proximate principles which it contains.
After the first formation of a leaf, others successively appear, all
constructed alike, and performing similar functions. The leaf is the
principal organ to the tree; and, indeed, Linnæus divined, and Goethe
demonstrated, the beautiful fact, that the tree was developed from this
curiously-formed organ.
“Keeping in view,” says the poet-philosopher, “the observations that
have been made, there will be no difficulty in discovering the leaf in
the seed-vessel, notwithstanding the variable structure of that part
and its peculiar combinations. Thus the pod is a leaf which is folded
up and grown together at its edges, and the capsules consist of several
leaves grown together, and the compound fruit is composed of several
leaves united round a common centre, their sides being opened so as to
form a communication between them, and their edges adhering together.
This is obvious from capsules which, when ripe, split asunder, at which
time each portion is a separate pod. It is also shown by different
species of one genus, in which modifications exist of the principle
on which their fruit is formed; for instance, the capsule of _nigella
orientalis_ consists of pods assembled round a centre, and partially
united; in _nigella damascena_ their union is complete.”[256]
Professor Lindley thus explains the same view:--“Every flower, with
its peduncle and bracteolæ, being the development of a flower-bud, and
flower-buds being altogether analogous to leaf-buds, it follows as a
corollary that every flower, with its peduncle and bracteolæ, is a
metamorphosed branch.
“And, further, the flowers being abortive branches, whatever the laws
are of the arrangement of branches with respect to each other, the same
will be the laws of the flowers with respect to each other.
“In consequence of a flower and its peduncle being a branch in a
particular state, the rudimentary or metamorphosed leaves which
constitute bracteæ, floral envelopes, and sexes, are subject to exactly
the same laws of arrangement as regularly-formed leaves.”[257]
The idea that the leaf is the principal organ of the plant, and that
from it all the other organs are probably developed, is worthy the
genius of the great German poet.
Every leaf, a mystery in itself, is an individual gifted with peculiar
powers; they congregate in families, and each one ministers to the
formation of the branch on
which it hangs, and to the main trunk of the tree of which it is a
member. The tree represents a world, every part exhibiting a mutual
dependence.
“The one red leaf, the last of its clan,
That dances as often as dance it can;
Hanging so light and hanging so high,
On the topmost twig that looks up at the sky,”
is influenced by, and influences, the lowest root which pierces
the humid soil. Like whispering voices, the trembling leaves sing
rejoicingly in the breeze and summer sunshine, and they tremble alike
with agony when the equinoctial gale rends them from the parent stalk.
The influences which pervade the whole, making up the sum of vital
force, are disturbed by every movement throughout the system; a wound
on a leaf is known to disturb the whole, and an injury inflicted on the
trunk interferes with the processes which are the functions of every
individual leaf.[258]
The consideration of the physical circumstances necessary to
germination and vegetable growth, brings us acquainted with many
remarkable facts. At a temperature below the freezing point, seeds will
not germinate; at the boiling point of water, a chemical change is
produced in the grain, and its power of germinating is destroyed. Heat,
therefore, is necessary to the development of the embryo, but its power
must only be exerted within certain prescribed limits: these limits are
only constant for the same class of seeds, they vary with almost every
plant. This is apparent to every one, in the different periods required
for germination by the seeds of dissimilar vegetables.
The seed is placed in the soil; shade is always--absolute darkness
sometimes--necessary for the success of the germinating process. We
have seen that the first operation of nature is purely a chemical one,
but this manifestation of affinity is due to an exertion of force,
which is directly dependent upon solar power. The seed is buried in the
soil, when the genial showers of spring, and the increasing temperature
of the earth, furnish the required conditions for this chemistry of
life, and the plant eventually springs into sunshine. Thus we obtain
evidence that even through some depth of soil the solar power, whatever
it may be, is efficient, and that under its excitement the first spring
of life, in the germ, is effected.
The cotyledons and the plumule being formed, the plant undergoes a
remarkable change. The seed, like an animal, absorbed oxygen and
exhaled carbonic acid; the first leaves secrete carbon from carbonic
acid inspired, and send forth, as useless to the plant, an excess of
oxygen gas.
This power of decomposing carbonic acid is a vital function which
belongs to the leaves and bark. It has been stated, on the authority
of Liebig, that during the night the plant acts only as a mere bundle
of fibres,--that it allows of the circulation of carbonic acid and
its evaporation, unchanged. In his eagerness to support his chemical
hypothesis of respiration, the able chemist neglected to inquire
if this was absolutely correct. The healthy plant never ceases to
decompose carbonic acid during one moment of its existence; but during
the night, when the excitement of light is removed, and the plant
reposes, its vital powers are at their minimum of action, and a much
less quantity is decomposed than when a stimulating sun, by the action
of its rays, is compelling the exertion of every vital function.
During this process, we have another example of natural organic
chemistry. The four inorganic elements of which the vegetable kingdom
is composed--oxygen, hydrogen, nitrogen, and carbon--are absorbed as
air or moisture by the leaves and through the roots, and the great
phenomenon of vegetable life is the conversion of these to an organic
condition. Sugar and gum are constantly produced, and from these, by
combination with atmospheric nitrogen, a proteine compound is formed,
which is an essential element in the progress of development.[259]
Plants growing in the light are beautifully green, the intensity of
colouring increasing with the brilliancy of the light. Those which are
grown in the dark are etiolated, their tissues are weak and succulent,
their leaves of a pale yellow. It is, therefore, evident that the
formation of this chlorophylle--as the green colouring matter of leaves
is called--results from some action determined by the sun’s rays.
Chlorophylle is a carbonaceous compound formed in the leaves, serving,
it would appear, many purposes in the process of assimilation. In the
dark the plant still requires carbon for its further development,
and growing slowly, it removes it from the leaves, decomposing the
chlorophylle, and supports its weak existence by preying on parts of
its own structure, until at length, this being exhausted, it actually
perishes of starvation.
Plants always turn towards the light: the guiding power we know not,
but the evidence of some impulsive or attracting force is strong; and
the purpose for which they are constituted to obey it, is proved to be
the dependence of vegetable existence upon luminous power.
Light is not, however, alone sufficient to perfect the plant: another
agent is required to aid in the production of flowers and fruits,
and this power is proved to be heat--and heat, perhaps, in some
peculiar condition. Having reached that point of development when
the reproductive functions are, by another change in the chemical
operations going on within the vegetable structure, to be called forth,
it has been found that the heat rays become in a remarkable manner
effective. It has also been observed that plants bend from the red,
or calorific rays, instead of towards them, as they are found to do
to every other ray of the spectrum. From this we may argue that the
influence of these rays is to check the vegetation, and thus to ensure
the perfection of the reproductive processes.
It has already been stated that we have the means of separating, to
a considerable extent, the three principles which we discover in the
sunbeam, from each other, by the use of absorbent media.
By a peculiar yellow glass we cut off the chemical principle of the
sunbeam, and admit the passage of the _luminous rays_ only--LIGHT.
By a cobalt blue glass we obstruct the _light_, but allow the chemical
agent to pass through freely, without, indeed, any loss--ACTINISM.
By a glass coloured deep blood-red by oxide of gold we obstruct
the chemical principle and much of the light, but such a medium is
perfectly transparent to HEAT.
Therefore, this gives us the means of experimenting with either of
these principles, and of examining the parts which they respectively
play in the work of organization.
Some seeds being placed in the soil, in every respect in their natural
conditions, duly supplied with moisture, and a uniform and proper
temperature maintained, we place above the soil the three media
above named, and allow one portion to be exposed to all the ordinary
influences of the solar rays.
The result will be, that the seeds under the blue glass will germinate
long before those which are exposed to the combined influences of the
sunshine: a few of the seeds will struggle into day under the red
glass, but the process of germination is entirely checked under the
yellow glass. Here we see that the chemical radiations have quickened
the chemical changes, and accelerated the process, under the red glass,
through which rays having some peculiar chemical action pass; the
germinating process, though checked, is not entirely stopped. Whereas,
it would appear that under the influences of light which has been
deprived of chemical power, this conversion of the starch into gum and
sugar, which appears to be necessary, is entirely prevented.
If the experiment is continued, it will be found that under the
blue glass the plants grow rapidly, but weakly; and that instead of
producing leaves and wood they consist chiefly of stalk, upon which
will be seen here and there some abortive attempts to form leaves.
When the process of germination has terminated, if the young plant is
brought under the yellow glass, it grows most healthfully, and forms an
abundance of wood, the leaves having an unusually dark green colour,
from the formation of a large quantity of chlorophylle. Plants do not,
however, produce flowers with readiness under this medium; but if, at
the proper period, they are brought under the red glass, the flowering
and fruiting processes are most effectively completed.[260]
These experiments, simple as they are, prove to us the importance of
light: the _luminous principle_ of the sunbeam is exciting the vital
powers of the plant to decompose carbonic acid and form wood; and the
calorific agent, possibly under those modifications which have already
been noticed as belonging to the parathermic rays, is essential to the
production of flower and fruit.
Observations, which have been extended over many years, prove that
with the seasons these solar powers are, relatively to each other,
subject to an interesting change. In the spring, the actinic or
chemical power prevails, and during this period its agency is required
for the vitalization of the germ. As the summer comes on, the actinic
rays diminish, and those of light increase. Perhaps it would be more
strictly correct to say that the luminous intensity being increased,
the chemical power was retarded; the former expression implies a
variation in quantity, which may not be correct. We see the necessity
for this, since luminous power is required for the secretion of
the carbon, with which the woody fibre is formed, and also for the
elaboration of the proximate principles of the plant. Autumn, the
season of fruit, is characterized by an increase of the heat rays, and
a diminution of the others: this change being necessary, as science now
teaches us, for the due production of flower and fruit.
The calorific rays of the solar beam, to which the autumnal phenomena
of vegetation appear particularly to belong, are of a peculiar
character. They have been called, the Parathermic rays, and exhibit a
curious compound nature. To these rays we may refer the ripening of
fruit and grain, and the browning of the leaf before its fall. May not
the rise of the sap in spring be traced to the excitement of either
light or actinism, and its recession, in the autumn, to that power
from which the plant is found to bend, and which appears to be their
modified form of heat?
There can be no doubt that the varieties of climate and the
peculiarities of countries, as it regards their animal and vegetable
productions, are dependent on the same causes. The distribution of
species has been referred by some to specific centres of creation
around which the plants and animals have spread, without reference to
physical conditions. Although centres of creation may be admitted,
these centres themselves have been determined by the physical
fitness of each centre to the conditions of the creation, and in
like manner the migration of tribes is solely due to these physical
forces we have been considering. In every zone we find that vegetable
organization is peculiarly fitted for the considerations by which it
is surrounded. Under the equator we have the spice-bearing trees, the
nutmeg, the clove, the cinnamon, and the pepper-tree; there we have
also the odoriferous sandal, the ebony, the banyan, and the teak: we
have frankincense, and myrrh, and other incense-bearing plants; the
coffee-tree, the tea-plant, and the tamarind.
A little further north we have the apricot, the citron, the peach,
and the walnut. In Spain, Sicily, and Italy, we have the orange and
lemon-tree blooming rich with perfume, and the pomegranate and the
myrtle growing wild upon the rocks. Beyond the Alps the vegetation
again changes; instead of the cypress, the chesnut, and the cork-tree,
which prevail to the south of them, we have the hardier oak, the beech,
and the elm. Still further north, we have the Scotch and spruce fir
and larch. On the northern shores of the Baltic, and in that line of
latitude, the hazel alone appears; and beyond this the hoary alder,
the sycamore, and the mountain ash. Within the Arctic circle we find
the mezereum, the water-lilies, and the globe-flowers; and, when the
weakness of the solar radiations becomes too great even for these,
the reindeer moss still lends an aspect of gladness to the otherwise
sterile soil.
The cultivation of vegetables depends on the temperature of the clime.
The vine flourishes where the mean annual temperature ranges between
50° and 73°, and it is only cultivated profitably within 30° S. and 50°
N. of the equator. To the same limits is confined the cultivation of
maize and of olives. Cotton is grown profitably up to latitude 46° in
the Old World, but only up to 40° in the New. We have evidence derived
from photographic phenomena, that the constitution of the solar rays
varies with the latitude. The effects of the sun’s rays in France and
England in producing chemical change are infinitely more decided than,
with far greater splendour of light, they are found to be in the lands
under or near the equator. Indeed, the remarks made on the variations
in the character of the sunbeam with the changing seasons, may apply
equally to the variations in latitude.
Fungals are among the lowest forms of vegetation, but in these we have
peculiarities which appear to link them with the animal kingdom. Marcet
found that mushrooms absorbed oxygen, and disengaged carbonic acid.
In all probability this is only a chemical phenomenon of a precisely
similar character to that which we know takes place with decaying
wood. In the conversion of wood into _humus_, oxygen is absorbed, and
combining with the carbon, it is evolved as carbonic acid. Of course we
have the peculiar condition of vitality to modify the effect, and we
have, too, in this class of plants, the existence of a larger quantity
of nitrogen than is found in any other vegetating substance.
These few sketches of remarkable phenomena connected with vegetation
are intended to show merely the operations of the physical powers of
the universe, so far as we know them, upon these particular forms
of organization. During the process of germination, electricity is,
according to Pouillet, evolved; and again, in ripening fruits, there
appears to be some evidence of electrical currents. Vegetables are,
however, in the growing state, such good conductors of electricity,
that it is not, according to the laws of this force, possible that
they should accumulate it; so that the luminous phenomena stated to
have been observed cannot be due to this agency. We know, however,
that under every condition of change, whether induced by chemical or
calorific action, electricity is set in motion; and we have reasons
for believing that the excitation of light will also give rise to
electrical circulation.
The question, whether plants possess sensation, whether they have
any disposition of parts at all analogous to the nervous system of
animals, has been often put forward, but as yet the answers have been
unsatisfactory. The point is one well worthy all the attention of
the vegetable physiologist; but regarding plants as the link between
the animal and the mineral kingdom,--looking upon phyto-chemistry,
as exhibited by them, as the means employed to produce those more
complex organizations which exist in animals,--we necessarily consider
plants as mere natural machines for effecting organic arrangements,
and, as such, that they cannot possess any nervous sensibility.
Muscular contraction may be represented in many of their marvellous
arrangements; and any disturbance produced by natural or artificial
means would consequently effect a change in the operations of those
forces which combine to produce vegetable life. Indeed, the experiments
of Carlo Matteucci, already referred to, prove that an incision across
a leaf, the fracture of a branch, or the mere bruising of any part of
the plant, interferes with the exercise of that power which, under the
operation of luminous agency, decomposes carbonic acid, and effects the
assimilation of the other elements.
To recapitulate. A plant is an organized creation; it is so in virtue
of certain strange phyto-chemical operations, which are rendered active
by the solar influences involved in the great phenomena of light, and
by the excitation of caloric force mid electrical circulation. It is
a striking exemplification of the united action of certain empyreal
powers, which give rise to the combination, of inorganic principles
under such forms that they become capable of obeying the mysterious
impulses of life.
The poet has imaged the agency of external powers in various shapes
of spiritualized beauty. From the goddess Flora, and her attendant
nymphs, to the romantic enchantress who called up flowers by the light
touch of her wand, we have, in all these creations, foreshadowings of
the discovery of those powers which science has shown are essential
to vegetable life. A power from without influences the plant; but the
animal is dependent upon a higher agency which is potent within him.
The poet’s dream pleases the imaginative mind; and, associating in our
ideas all that is graceful and loveable in the female form, with that
diviner feeling which impresses the soul with the sense of some unseen
spirituality, we perceive in the goddess, the enchantress, or the
sylph, pure idealizations of the physical powers. The spirit floating
over these forms of beauty, and adorning them with all the richness
of colour--painting the rose, and giving perfume to the violet--is,
in the poet’s mind, one which ascends to nearly the highest point of
etherealization, and which becomes, indeed, to him a spirit of light;
they ride upon the zephyrs, and they float, in all the luxury of an
empyreal enjoyment, down to the earth upon a sunbeam. Such is the
work of the imagination. What is the result of the search of plodding
science after truth? The sunbeam has been torn into rays, and every ray
tasked to tell of its ministry.
Nature has answered to some of the interrogations; and, passing over
all the earth, echoed from plant to plant, we have one universal cry
proclaiming that every function of vegetable life is due to the spirits
of the sun.
The mighty Adansonia of Senegal, hoary with the mosses of five
thousand years,--the Pohon upas in their deadly valleys,--the climbing
lianas of the Guiana forests,--the contorted serpent-cactus on the
burning hills,--the oaks, which spread their branches in our tempered
climes,--the glorious flowers of the inter-tropical regions, and
those which gem our virent plains,--the reindeer lichen of northern
lands, and the confervæ of the silent pool,--the greatest and humblest
creations of the vegetable world,--all proclaim their direct
dependence upon the mysterious forces which are bound together in the
silver thread of Light.
These undulations, pulsations too refined for mortal ears, which
quicken and guide these wonderful organisms, may be indeed regarded
as sphered music for ever repeating the Divine command, “_Let there
be Light_,” by the creation of which, a dark and dreary chaos was
moulded into a star of beauty, capable of radiating brightness to other
space-wandering worlds.
FOOTNOTES:
[248] Percy Bysshe Shelley.
[249] _Experiments on the production of dephlogisticated air from water
with various substances_: by Lieut.-General Sir Benjamin, Count of
Rumford; Phil. Trans., vol. lxxvii. p. 84.
[250] _Experiments upon Vegetables, discovering their great power of
purifying the common air in the Sunshine, and of injuring it in the
Shade and at Night; to which is joined, A new method of examining the
accurate degrees of Salubrity of the Atmosphere_, by John Ingenhousz,
Councillor of the Court, and Body Physician to their Imperial and Royal
Majesties, F.R.S., &c. London: printed for P. Elmsley, in the Strand,
and H. Payne, Pall Mall, 1779.
[251] _The Kingdoms of Nature, their life and affinity_: by Dr. C. G.
Carus; Scientific Memoirs, vol. i. p. 223.
[252] In _Biologie_, by G. R. Treviranus, vol. ii. p. 302, the
following passage occurs:--“If we expose spring water to the sun in
open or even closed transparent vessels, after a few days bubbles
rise from the bottom, or from the sides of the vessel, and a green
crust is formed at the same time. Upon observing this crust through
a microscope, we discover a mass of green particles, generally of a
round or oval form, very minute, and overlaid with a transparent mucous
covering, some of them moving freely, whilst others, perfectly similar
to these, remain motionless and attached to the sides of the vessel.
This motion is sometimes greater than at others. The animalcules
frequently lie as if torpid, but soon recover their former activity.”
[253] _On the Structure of the Vegetable Cell_: by Mohl.--Scientific
Memoirs, vol. iv. p. 113. _Outlines of Structural and Physical Botany_:
by Henfrey.
[254] Dr. Carus, in the memoir already quoted, says:--“But since, in
the organization of the earth, light and air, as constituting a second
integrant part, stand opposed to gravitation, and since the plant
bears a relation, not only to gravitation, but to light also, when its
formation is complete it will necessarily present a second anatomical
system, namely, that of the spiral vessels, which have been very justly
considered, of late, as the organs that perform in plants the functions
of nerves.”
[255] Mr. Crosse’s Experiments in the Journal of the London Electrical
Society, and Mr. Weekes in the Electrical Magazine, and a communication
appended to _Explanations: a Sequel to the Vestiges of the Natural
History of Creation_.
[256] _Die Metamorphose der Pflanzen_: Goethe, sect. 78.
[257] Lindley’s _Elements of Botany_.
[258] See the very curious experiments of C. Matteucci. Traduit et
extrait du “_Cimento_.”--Archives des Sciences Physiques et Naturelles;
_Quelques Expériences sur la Respiration des Plantes_. Nov. 1846.
[259] Consult _Rural Economy_, by J. B. Boussingault; _The Chemical and
Physiological Balance of Organic Nature_, by Dumas and Boussingault;
and _Agricultural Chemistry_, by Liebig.
[260] The practical value of the discovery now described, will be best
understood from the following letter from Mr. Lawson, of Edinburgh:--
Edinburgh, 1, George the Fourth’s Bridge, Sept. 8, 1853.
MY DEAR SIR,--I am favoured with yours of the 5th, relative to my
practical experience in the effect of the chemical agency of coloured
media on the germination of seeds and the growth of plants.
I must first explain that it is our practice to test the germinating
powers of all seeds which come into our warehouses before we send
them out for sale; and, of course, it is an object to discover, with
as little delay as possible, the extent that the vital principle is
active, as the value comes to be depreciated in the ratio it is found
to be dormant. For instance, if we sow 100 seeds of any sort, and the
whole germinate, the seed will be the highest current value; but if
only 90 germinate, its value is 10 per cent. less; if 80, then its
value falls 20 per cent.
I merely give this detail to show the practical value of this test, and
the influence it exerts on the fluctuation of prices.
Our usual plan formerly was to sow the seeds to be tested in a hot-bed
or frame, and then watch the progress and note the results. It was
usually from eight to fourteen days before we were in a condition to
decide on the commercial value of the seed under trial.
My attention was, however, directed to your excellent work, “On the
Physical Phenomena of Nature,” about five years ago, and I resolved to
put your theory to a practical test. I accordingly had a case made,
the sides of which were formed of glass coloured blue or indigo, which
case I attached to a small gas stove for engendering heat; in the case
shelves were fixed in the inside, on which were placed small pots
wherein the seeds to be tested were sown.
The results were all that could be looked for: the seeds freely
germinated in from two to five days only, instead of from eight to
fourteen days as before.
I have not carried our experiments beyond the germination of seeds, so
that I cannot afford practical information as to the effect of other
rays on the after culture of plants.
I have, however, made some trials with the yellow ray in preventing the
germination of seeds, which have been successful; and I have always
found the violet ray prejudicial to the growth of the plant after
germination.--I remain, my dear Sir,
Very faithfully yours, CHARLES LAWSON.