[Illustration: [Flower]]
We live and the plants live. Probably neither we nor the plants spend
much time thinking about what we owe to each other.
The plants are excusable for this, for they are not great thinkers, at
least so far as we know.
But we owe so much to them, we ought to stop and think about it once in
a while. We are indebted to them not only for the food we eat, but for
the air we breathe.
We know about chlorophyll and the starch it makes, and how this starch
is stored up in potatoes and wheat and corn and rice and all sorts of
food grains and vegetables.
We know, too, how the roots suck up substances from the earth which we
need in our bodies, and how they are stored away with the starch or
sometimes by themselves. We know, in short, how all the food we eat is
made first or last by the plants. Not only do we owe our food to the
plants, but all animals do.
You see, animal cells are not able to take carbon dioxide and water and
ammonia and other gases and minerals and work them up into living cells.
The plants have to do this for them; and then the animals eat the
plants, for animal cells are able to work starch and sugar and plant
protoplasm over into animal protoplasm, which can build all sorts of
animal cells. So all the animals in the world get their food from the
plant world. If the plants were to stop living, all the animals in the
world would soon starve to death. The word “animals,” you know, means
every living thing that is not a plant; in this sense flies and bees and
oysters and caterpillars are animals as well as dogs and cats and such
large creatures. Last of all, we ourselves are animals.
So the animal world would be in a sad predicament if anything should
happen to the plants.
But there is more to thank the plants for than food. That is a pretty
large item certainly; but what do you think of having to thank them for
the air we breathe as well? Yet this we shall have to do if we begin
thanking them at all.
You know about oxygen, of course. It is one of the gases that make up
the air; and I may as well remind you that air is composed principally
of oxygen and nitrogen gases,—about four times as much nitrogen as
oxygen, but the oxygen is the most important to us. We do not use the
nitrogen in the air at all probably. It serves the purpose of diluting
the oxygen, which would be too strong for us if it were not mixed with
nitrogen. But what we do use is the oxygen.
That goes into our lungs, and some of it does not come out again. It
passes into the lung cells and from them into the blood, and is carried
by it all over our bodies to all the millions of cells.
We need a great deal of oxygen, and if the supply should be cut short we
would die.
All animals need oxygen; even the worms in the ground and the fishes and
oysters in the water must have it. So great quantities are being used up
all the time.
Now, you know, when the plants pull carbon dioxide to pieces, they keep
the carbon and return the oxygen to the air. In this way we get it to
breathe.
But there is more than this to the matter in hand. We are all the time
breathing out carbon dioxide as an impurity; so are all the millions
upon millions of animals in the world.
The air might in time contain enough carbon dioxide to kill us if there
were not some way of getting rid of it. You know what that way is.
The plants use it up. So by giving oxygen into the air and taking out
carbon dioxide, the plants keep the air fit for us and all animals to
breathe.
[Illustration: [Flower]]
But there is more than this we have to thank them for.
They shade the earth and regulate the rainfall and the water supply.
Where forests grow there are always streams of water, and the large
water courses are kept full the year round.
The Mississippi River depends upon the far-away forests for its broad
stream.
The spreading crowns of the trees shade the earth and prevent the water
which falls as rain or dew from evaporating rapidly. It collects into
streams and flows through the land, keeping the earth fresh and
beautiful.
More than this,—large forests cause the rain to fall and the dew to
collect. Their leaves condense the moisture in the air and cause it to
fall as rain or be deposited as dew.
When people recklessly cut down the forests in a country, the water
courses dry up, and even the largest rivers are affected.
When the spring rains fall over a country whose trees have been cut
away, the water rushes down the little streams all at once and causes a
terrific flood in the large rivers. It soon drains away; then the rivers
fall lower and lower until they nearly dry up. This state of affairs is
a great calamity, because the people can no longer raise crops on the
land near where the old forests stood, for it is parched and dry months
at a time.
Moreover, boats laden with coal and grain and all sorts of things can no
longer pass up and down the rivers, because the water is too low.
People ought to think of these things and not destroy too much forest
land. After awhile we shall have to go to work and plant trees instead
of cutting them down or burning them; but it takes a long time for trees
to grow, and a wiser way would be for us to take care of those we have.
You have heard a great deal about plants eating and the good they do us
by eating the carbon dioxide in the air. They take this in through their
leaves, and you remember they take in all their other food
materials—water, nitrogen compounds, sodium, potassium, magnesium, and
many other substances—through their roots.
But they do more than eat; they also breathe.
They breathe everywhere over the surface of their bodies where there are
stomata or where the skin is not too thick for the air to penetrate it.
And I must tell you they breathe just as we do,—that is, they take in
air, use the oxygen, and give off the carbon dioxide.
It seems rather inconsistent of them to take in carbon dioxide as food
and throw it off as a waste at the same time, but that does not trouble
_them_; they do not care whether they are consistent or not. And it is
true they take in carbon dioxide and give off oxygen, and take in oxygen
(in the air) and give off carbon dioxide, in one breath as it were.
You see, it is different parts of protoplasm at work that does this; one
part—that in the chlorophyll bodies—is attracting carbon dioxide,
breaking it up, and casting out oxygen. Other protoplasm in the cells
outside the chlorophyll bodies attracts and uses the oxygen, while the
carbon dioxide comes to the stomata from different parts of the plant as
a waste material, just as it comes to the cells of our lungs to be cast
out.
So plants, by breathing, make the air a little impure, but they destroy
or break up so much more carbon dioxide than they make that on the whole
they act as powerful purifiers of the air.
When we think of the great forests of the tropics, all overgrown with
luxuriant vegetation, we may remember that those tangles of vines and
trees and strange growths are our friends no less than the grass and
bushes in our dooryard.
For there is a carrier always at work bringing the pure air to us and
carrying away the impure air which we create. This carrier is the air
currents. The great winds sweep about the earth, bearing the oxygen from
the forests to the crowded cities, and sweeping away the carbon dioxide
from the cities to the fields and woods. The winds, too, stir up the
water where the water plants and fishes live, and help keep it full of
air for the things in it to breathe; the tides and currents help, so as
far down in the water as there are living things, you may be sure there
is air for them to breathe. There would not be air enough for you,
because you need so much; but for them there is plenty.
Swirling around the earth go the winds, carrying the oxygen to the
people and the carbon dioxide to the plants, for the plants are as glad
to get the carbon dioxide we breathe out as we are to get the oxygen
they give off.
And we are glad, when we come to think about it, that we are able to
give them something in return for all they give to us.
You see, we need each other,—plants and people, and the winds are
friends to us both.
[Illustration: [Flower]]