It is now an ascertained fact that as a rule, no organised being in the
world subsists alone by the nourishment which it absorbs, either in the
form of food or of atmospheric air; it has also need of heat and light.
Light is the creator of the charming colours, the sweet perfumes, the
exquisite flavours which we gain from the vegetable kingdom. But how
these marvellous operations are accomplished, what are the rules of
the dispersion of darkness and its multiplied refractions, are not
yet thoroughly determined. Let us glance at what has been already
determined.
Plants are nourished by absorbing through their roots certain
substances in the soil, and by decomposing through their green parts
the carbonic acid gas contained in the atmosphere. They decompose this
gas into carbon, which is assimilated, and into oxygen, which they
exhale, and return to the atmosphere for the use of animals. This,
which may be called the respiration of plants, cannot be performed
without the help of the solar rays. Charles Bonnet, the well-known
philosopher of Geneva, was the first in the last century to verify this
truth. He remarked that all plants grow vertically, and stretch towards
the sun in whatever position the seed may have been planted. We have
all noticed how plants in dark places direct their stems to the place
whence a ray of light issues. He also discovered that when plunged into
water they disengage bubbles or gas under the sun’s influence. Our own
Dr Priestley took up the subject and gained another step; he burned
a light in a closed space until it went out, shewing that the oxygen
had been consumed, and that in consequence the air had become unfit
for maintaining combustion. Into the space he introduced the green
parts of a plant, and after ten days the air was so purified that the
candle would burn once more. In other words he had proved that plants
can substitute oxygen for carbonic acid gas. If some water-cress, for
instance, be grown in water, and exposed to sunlight, the presence
of the oxygen gas given off by the leaves may be demonstrated by the
rekindling of a paper the lingering spark of which is introduced into
the vessel in which the plant is contained.
Dr Ingenhousz further explained this interesting fact. He observed that
plants have the power of correcting impure air in a few hours; and that
this marvellous operation is due solely to the influence of the sun
upon plants. This influence only begins when the sun has risen some
little time above the horizon; the obscurity of night entirely suspends
the operation, as do also high buildings or the shade of trees. Towards
the close of day the production of oxygen relaxes, and entirely ceases
at sunset.
When these facts had been established, the explanation was soon
discovered: the impure gas which was absorbed and decomposed during the
day was nothing but the carbonic acid which is freely given out from
the lungs of every breathing animal, the pure gas resulting from the
decomposition being oxygen. But the diurnal respiration of most plants
is exactly the inverse of the nocturnal, for the gas which they emit
during night is the unwholesome carbonic acid. It was discovered also
that mere heat could not take the place of light in these operations.
There was another point which required elucidation; this was, the
relation that existed between the amount of carbonic acid absorbed and
of oxygen exhaled. Another Genevese citizen, De Saussure, maintained
that the latter is always the smaller quantity, and that at the same
time a portion of the oxygen retained by the plant is replaced by
nitrogen; whilst Boussingault shewed that the volume of carbonic acid
decomposed was equal to that of the oxygen produced.
There is a wonderful rapidity and energy in the performance of these
functions by the green parts of plants, as was proved by placing an
earthen vessel in the sun filled with vine-leaves. Through this a
current of carbonic acid was passed, and when it came out it was pure
oxygen. It is calculated that one single leaf of the water-lily thus
exhales during the summer about three hundred quarts of oxygen. Indeed
there are some peculiarities about aquatic plants which make them more
valuable in clearing the atmosphere than others, for during the night
they are inactive and disengage no carbonic acid, whilst they act as
others do in the daytime. It is easy to shew the direct action of the
sun on vegetable respiration by placing some leaves of the _nayas_ in a
vessel filled with water saturated with carbonic gas; as soon as this
is exposed to the sun, an infinite number of little bubbles of almost
pure oxygen will be seen rising to the surface. The shadow of a cloud
crossing the sky suffices to lessen this action, which is again resumed
with sudden activity when it has passed. By intercepting the solar rays
with a screen, the changes of quick or slow production of gas-bubbles
may be clearly observed.
So far these remarks apply only to white light, that is the mixture
of all the rays which the sun sends us; but this light is not simple;
it is composed of seven prismatic groups of colours, the properties
of which are quite distinct. This prismatic group further prolongs
and extends itself by invisible radiations. Beyond the red there are
radiations of heat; beyond the violet, chemical radiations. The first
act on the thermometer; the second determine energetic reactions in
chemical compositions. What is their influence on vegetation? Does the
solar light affect plants through its colour, its chemical properties,
or its heat?
Many experiments have been tried to solve this question, but it is
still a matter of doubt. If plants are placed in coloured glasses,
less oxygen is disengaged than under the influence of white light.
Young plants grown in comparative darkness, and consequently pale as to
colour, have been exposed to different rays of the spectrum, the effect
being that in three hours and a half they assumed a green tint under
the action of yellow light; whilst an hour longer was required for
orange, and sixteen hours for blue. It is evident from this that the
energy of solar action on plants corresponds neither with the maximum
of heat, which lies in the red rays, nor in the maximum of chemical
intensity which is at the other extremity of the spectrum, that is the
violet.
If blades of grass are put into tubes filled with water charged with
carbonic gas, and exposed to coloured rays, and the quantity of oxygen
gas disengaged is measured, it will be found that the largest quantity
is in the tubes which have been acted on by yellow and green light;
afterwards those influenced by orange and red. Just as aquatic plants
send out gaseous bubbles under white light, so do they to nearly the
same extent under orange light, but twenty times less if placed under
blue glass. These experiments would seem to prove that it is the
_luminous_ rays only, and principally the yellow and orange, that act
upon plants. To this may be added, that green light produces much the
same effect as darkness on vegetable respiration; thus explaining why
there is such a slow lingering growth under the shade of large trees or
forests, where the ground beneath is bathed in emerald light.
The sun also assists in the transpiration and constant renewal of the
moisture essential to the tissues of plants. Like the human being,
when there is no evaporation, the plant becomes dropsical, and the
leaves fall because the stem is too weak to bear their weight. This
imperious need and love which they have for light shews that the
solar rays are really the essence which gives colour. The corollas
of those flowers which grow on mountains at a great elevation have
a deeper hue than those which blow in lowlands. The sun’s rays pass
more easily through the transparent atmosphere which bathes the higher
peaks. Certain flowers vary with their altitude; thus the _Anthyllis
vulneraria_ passes from white, through pale red, to an intense purple.
Well-lighted and cleared tracts of land are much richer in colour than
those shaded by high hedges and trees; and some flowers are observed
to change during the day, owing to the direct action of the sun. The
_Hibiscus mutabilis_, for instance, blooms white in the morning and
becomes red at noon-day; the floral buds of the _Agapanthus umbellatus_
are also white at early dawn and afterwards acquire a blue tint; the
_Cheiranthus camelea_ changes from white to lemon colour and then to a
red violet. If a flower be taken as it is coming out of its sheath and
wrapped in black paper, so as to intercept the light, it remains white;
but recovers its colour when exposed to the sun. Nor are fruits any
exception to this rule; the beneficial action of daylight is necessary
to their development, and to all those principles which communicate
taste and scent to the different parts.
Another part of this interesting study relates to the _mechanical_
action which light exercises, as shewn in the sleep of flowers,
the inflection of the stems, and the inclination towards the great
luminary. Pliny speaks of the sunflower which always faces the sun
and turns round with it; a delicate sensibility which the poet Moore
has beautifully expressed in words and music. The lupine is another
instance, which indicates by its diurnal revolution the hour of the
day to the labourer. The stems of all plants as a rule turn towards
the side of the light, and bend to drink it in. This constitutes what
is known as ‘heliotropism.’ If cress be grown in darkness on moist
cotton-wool, and then placed in a room lighted on one side only, the
stems bend and incline very rapidly towards it; the higher part only
turns, the lower remaining upright. But if it be placed in a room
lighted by two windows, a fresh observation will be made. Supposing
they are on the same side, and admitting an equal amount of light, the
stem bends in the direction of the middle of the angle formed by the
rays; whilst if one window allows more light to penetrate into the room
than the other, the stem turns to it. When the two are opposite there
is no deviation from the straight line.
There are some curious facts regarding climbing-plants; their stems
generally turn from left to right round the pole used for support;
others follow a contrary direction; while to some it seems to be a
matter of indifference. Mr Darwin has concluded that light is an
influential cause. If plants of this class are placed in a room near
a window, the stem requires more time to perform the half-revolution
during which it is turned away from the light, than for that which
is towards the window. In one case the whole circle was completed in
five hours and twenty minutes; of this the half in full light only
required an hour; whilst the other could not traverse its part in less
than four hours and twenty minutes—a very striking variation. Some
Chinese ignamas, _Diascorea batatas_, in full growth were placed in a
completely darkened cave, and others in a garden; in every case those
which were in darkness lost the power of climbing round their supports;
those exposed to the sun were twisting, but as soon as they were put in
the cellar they grew with straight stems.
The sleep of plants, which certainly has a connection with light, is
another curiosity in nature. Flowers and leaves of some growths seem to
fade at particular hours, the corolla being closed, which after a state
of lethargy blows out afresh; in others, the flower falls and dies
without having closed. In the case of the convolvulus the flower is
drawn up at noon. Linnæus noted the hours in which certain plants blow
and fade, and thus composed a floral dial; but science has not yet been
able to explain these curious relations to light.
The green colouring of leaves and stems is owing to a special matter
called chlorophyll, which forms microscopical granulations contained in
their cells. These grains are more or less numerous in each cell, and
it is to their number as well as to the intensity of their colour that
the plant owes its particular shade of green. Sometimes they are found
pressed together and cover the whole internal surface of the cell;
whilst at other times they are smaller in quantity and do not touch
each other. It has recently been observed also in the latter case, that
under the influence of light the green corpuscles undergo very curious
changes of position; in certain plants they crowd to the part of the
wall of the cells exposed to the action of the sun—a phenomenon which
does not take place in darkness or under red rays only.
There might be given many other very interesting effects of light on
plants, not usually noticed. The truth is, the direct rays of the
sun exert a potent influence on every living thing, whether plant or
animal. Sunlight, fair and full upon you and upon your dwelling, might
be called the greatest blessing in nature; but on this branch of the
subject we will not at present expatiate.