VEGETATION AND THE ATMOSPHERE
“The carbonic acid gas produced simply by the breathing of the great
human family amounts every year to about 160,000,000,000 cubic meters,
which represents 86,270,000,000 kilograms of burnt carbon. Piled up,
this carbon would form a mountain one league round at its base and
between four hundred and five hundred meters high. So much carbon is
required by man to maintain his natural heat. All of us together eat
this mountain of carbon in our food and in the course of the year
dissipate it all in the air, a breathful at a time; after which we
immediately begin the dissipation of another mountain of carbon. How
many mountains of carbon, then, since the world was created, must
mankind have exhaled into the atmosphere!
“We must take account, too, of the animals, which, collectively, those
of the land and those of the sea, use up a big mountain of combustible
matter. They are much more numerous than we; they inhabit the entire
globe, both continents and seas. What a quantity of carbon it must take
to sustain the life of our planet! And to think that it all goes forth
into the air, as a deadly gas, of which a few breaths would cause
death!
“Nor is that all. Fermentation, as in grape-juice and rising dough, and
putrefaction, as in decaying manure, produce carbonic acid gas. And it
needs only a light layer of manure to cause a cultivated field to give
forth between one hundred and two hundred cubic meters of carbonic acid
gas per day for each hectare.
“The wood, coal, and charcoal burnt in our houses, and especially the
quantities consumed in the great furnaces of factories—are not they
also returned to the atmosphere in the form of harmful gas? Just think
of the amount of carbonic acid gas vomited into the atmosphere by a
factory furnace into which coal is poured by the carload! Think also of
the volcanoes, gigantic natural chimneys which in a single eruption
throw up such quantities of gas that furnaces offer no comparison. It
is very clear: the atmosphere is constantly receiving carbonic acid gas
in torrents that defy computation. And yet animal life has nothing to
fear for the present or for the future, since the atmosphere, though
continually being poisoned with carbonic acid gas, is at the same time
always being purged of it.
“And what is the purgative agent commissioned by Providence to maintain
the salubrity of the atmosphere? It is vegetation, my friends,
vegetation, which feeds on carbonic acid gas to prevent our perishing
and turns it into the bread of life for our sustenance. This deadly
gas, which absorbs into itself all sorts of putrefaction, is the
choicest of nourishment for plant-life; and thus out of the bosom of
death the blade of grass builds up new life.
“A leaf is riddled with an infinite number of excessively minute
orifices, each encircled by two lips which give it the appearance of a
half-open mouth. They are called stomata. On a single leaf of the
linden more than a million can be counted, but so small are they as to
be quite invisible without a magnifying-glass. This picture shows you
how they look under a microscope. Well, through these orifices the
plant breathes, not pure air such as we breathe, but poisoned air,
fatal to an animal but wholesome for a plant. It inhales through its
myriads of millions of stomata the carbonic acid gas diffused through
the atmosphere; it admits this gas into the inner substance of its
leaves, and there, under the sun’s rays, a marvelous process follows.
Stimulated by the light, the leaves operate upon the deadly gas and
take from it all its carbon. They unburn (the word is not in the
dictionary, more’s the pity, for it gives the right idea)—they unburn
the burnt carbon, undo what combustion had done, separate the carbon
from the air with which it is bound up; in a word, they decompose the
carbonic acid gas.
“And do not think it any easy thing to unburn a burnt substance, to
restore to their original condition two substances united by fire.
Scientists would need all the ingenious means and powerful drugs they
possess to extract carbon from carbonic acid gas. This task, which
would tax the utmost resources of the man of science, leaves accomplish
noiselessly, without effort, even instantaneously, and with the sole
requirement that they shall have the aid of the sun.
“But if sunlight fails, the plant can do nothing with the carbonic acid
gas, the chief item in its diet. It then pines away with hunger, shoots
up as if in quest of the missing sunshine, while its bark and leaves
turn pale and lose their green color. Finally it dies. This sickly
state induced by the absence of light is called etiolation. It is
artificially produced in gardening for the purpose of obtaining
tenderer vegetables and of lessening or even entirely removing the too
strong and unpleasant taste of some plants. In this way some salad
greens are bound with a rush so that the heart, deprived of the sun’s
rays, may become tender and white; and thus, too, celery is banked up
and left to whiten, since otherwise its taste would be unbearable. If
we cover grass with a tile or hide a plant under a pot turned upside
down, we shall after a few days of this enforced darkness find the
foliage all sickly and yellow.
“When, on the other hand, the plant receives the sun’s rays without
hindrance, the carbonic acid gas is decomposed in no time, the carbon
and the air separate, and each resumes its original properties. Freed
of its carbon, the air becomes what it was before this admixture: it
becomes pure air, fit to maintain both fire and life. In this state it
is restored to the atmosphere by the stomata to be used again in
combustion and respiration. It entered the plant as a fatal gas, it
leaves it as a vivifying gas. It will return some day with a new charge
of carbon, which it will deposit in the plant, and then, restored to
purity once more, it will recommence its atmospheric round. A swarm of
bees goes and comes, from the hive to the fields and from the fields to
the hives, on one trip lightened and eager for booty and on the other
heavily laden with honey and returning to the comb on wearied wing. In
the same way air on coming to the leaves is charged with carbon from an
animal’s body, a burning fire-brand, or decaying matter; it gives it to
the plant and departs for a fresh supply.
“It is thus that the atmosphere preserves its salubrity despite the
immense torrents of carbonic acid that are cast into it. The plant
lives on deadly gas. Under the action of the sun’s light it decomposes
the gas into carbon, which it keeps for building up its own substance,
and breathable air, which it returns to the atmosphere. From this
carbon combined with other substances come wood, sugar, starch, flour,
gum, resin, oil, in fact every kind of vegetable product. Animal and
plant are of mutual assistance, the animal producing carbonic acid gas,
which nourishes the plant, and the plant changing this deadly gas into
air fit to breathe and into food. Thus our dependence on plants is
twofold: they purify the atmosphere and they give us food.”