[Illustration: [Leaves]]
Chlorophyll is plant green.
That is what the word means.
We are so used to seeing green leaves that we think very little about
it.
It probably never has occurred to most of us that the green coloring
matter of plants can be of much importance. Yet it is one of the most
important things in the world.
Like many other things, it is not what it seems. It is not merely a dye
as one might suppose, but much more than that.
We cannot really see what it is without a microscope, and when we look
at a piece of green leaf through the microscope we are surprised to find
the leaf is not green at all.
It is colorless like glass, but in the cells just behind the skin cells
we see little roundish green bodies packed away. These are the
chlorophyll grains, and when there are a great many of them close
together they show through the skin and make the whole plant green.
The skin protects them, you see, and yet it is transparent and allows
the light to get to them, which is a matter of great importance to the
chlorophyll grains, for they are hard workers, but cannot do a single
thing without sunlight.
[Illustration: [Cells]]
Chlorophyll grains lie just behind the skin cells in all parts of the
plant that look green. The cells they lie in are often long with their
short ends towards the skin. Leaves contain several layers of
chlorophyll cells. The inner ones are not long like the outer ones, and
do not contain so many chlorophyll grains. In the illustration, _a_, _a_
represent the upper and lower skin and _b_ the cells containing
chlorophyll. The under side of a leaf usually has fewer chlorophyll
grains in its cells, for the light is not so bright there, and
chlorophyll needs plenty of light.
Sometimes the cells in the middle of a leaf, that is, halfway between
the upper and lower surfaces, have no chlorophyll at all.
Now what _do_ you suppose is the work the chlorophyll grains have to do?
You never could guess, so I may as well tell you at once. If it is not
making sugar, it is something very like it. To begin at the beginning,
which is a long way from sugar, but which will certainly bring us to it,
I must tell you that these little round green chlorophyll people have a
strong attraction for carbon dioxide, which you know is a gas and is
always found in the air. You know, too, we breathe it out as an
impurity. Probably you did not know it had anything to do with sugar,
but it has a very great deal to do with it.
The chlorophyll grains attract carbon dioxide as strongly as a magnet
attracts bits of iron. The carbon dioxide in the air goes through the
pores in the leaf skin, right through everything to the cell where the
chlorophyll lies. You know carbon dioxide is made of carbon and oxygen.
The plant needs a great deal of carbon, for nearly all its hard parts
are made of it. Wood for one thing is nearly all carbon.
As soon as carbon dioxide comes where chlorophyll is, the chlorophyll,
which of course is chiefly made of protoplasm, tears it to pieces. It
pulls the carbon away from the oxygen and the oxygen rushes out through
the pores back into the air. But the carbon stays behind.
You see oxygen is a gas and carbon is a solid. When carbon and oxygen
unite in a certain way, they make another gas, our carbon dioxide.
It is very queer that carbon should have the form of a gas when united
with oxygen, and I cannot explain it here. You must just remember that
it is so.
When the oxygen flies away into the air again and leaves the carbon
behind, the work of the chlorophyll has but just begun. Raw carbon is of
no use whatever,—no more use than carbon dioxide, which we know is good
for nothing to the plant or else the chlorophyll would not tear it to
pieces.
But if the chlorophyll can only get a little water, something worth
while will happen. This it can always do, as the roots take good care to
send it plenty.
Water, you know, is made of two gases, hydrogen and oxygen, united
together.
Here, you see, gases unite and make a liquid. Well, chlorophyll has a
way of its own of uniting the carbon it took away from the carbon
dioxide with the hydrogen and oxygen it gets from the water and forming
a solid, which the plant cannot live without.
Now what do you suppose this new solid is? Probably you _never_ could
guess.
It is _starch_, just _starch_!
Chlorophyll makes starch out of carbon, hydrogen, and oxygen.
Sometimes it makes sugar and oil out of them, but its work is most
generally starch-making.
The carbon, you remember, it gets from the carbon dioxide of the air,
and the hydrogen and oxygen from the water the roots send it.
[Illustration: [Leaves]]
The strangest thing about all this is, chlorophyll is the only thing
that _can_ make starch.
Perhaps you do not think starch worth making such a fuss about. But wait
a moment.
There is more to starch than you ever dreamed of. Really and truly, if
it were not for starch you would not be alive to-day, and I would
not,—in short nobody would.
All our lives depend upon starch. So when we come right down to the
truth, our lives depend upon chlorophyll, because that makes all the
starch there is in the world.
You do not think our lives depend upon starch? Wait and see.
Chlorophyll makes starch. Never forget that as long as you live. Forget
your own name if you want to, but do not forget that chlorophyll makes
starch.
You see starch is the raw material of which plants are made.
After the chlorophyll has made starch, the starch is dissolved, or
_melted_ you would likely say, and so is carried all over the plant in
the sap. Some parts of the plant change the starch into sugar; for sugar
is made of the same things as starch, only in it the carbon, hydrogen,
and oxygen are put together a little differently, just as you can make
several kinds of cake from flour, butter, sugar, milk, and eggs by
stirring them together differently and mixing them in different
proportions.
You cannot make cake without flour, sugar, eggs, and milk, and usually
butter. But if you have these ingredients you can make a great many
kinds of cake.
Starch is the material of which the plant makes a large part of its
substance.
Some parts of the plant that need sugar make it from the starch, and we
find more or less sugar in all plants. There is, as you know, a great
deal in the nectar of flowers, but other parts of the plant need it too,
so sugar is a matter of importance to plants as well as to people. But
sugar, remember, is made generally from starch, no matter in what part
of the plant we find it.
[Illustration: [Leaves]]
The sweet sap in the sugar maple is made from starch; so is the sweet
juice of the sugar beet and of the sugar cane. All the sugar we use,
excepting that in homeopathic pills, is made from starch. The sweet
juice of fruits, berries, apples, peaches, oranges, contains sugar,
which the plant has made from starch. In green fruit the starch has not
yet been changed into sugar, so it is not pleasant to the taste.
Some parts of the plant need thick walls, like wood or bark, and these
are made by the protoplasm from starch; they are not sugar, however, but
a very tough, firm substance so unlike sugar that you wonder how it can
be made of the same materials. But it is, for starch is the substance
from which both are made.
There are other things in the plant besides starch, and there are things
which are not made from starch; for instance, there are acids and
minerals of different kinds and there is protoplasm, but the greater
part of every green plant is formed from starch.
Some plants make more starch than they need at once, so they store it
away for future use, just as people raise extra supplies of wheat and
corn, and store them away until they want them.
The potato plant, for instance, stores a large quantity of starch in the
potatoes underground. A potato is nearly all starch, and the sweet
potato stores up sugar as well as starch in its underground parts.
The potatoes have a reason for this, and, if let alone, would use up the
starch and sugar another season; but we do not let them alone, as you
know. We too need starch, and so we dig up the potatoes and eat them
instead of leaving them for the plant.
A great many plants store up starch in their seeds that the young plant
may have food enough to start growing. All our grains do this. Wheat,
rye, oats, barley, rice, corn, and all other grains are only the seeds
of plants which have been stored full of starch. Peas and beans are also
starch-filled seeds. Cabbages store food made from starch in their big
thick leaves. Beets store sugar and other starch-food materials in their
thick roots; so do carrots and parsnips and turnips. Onions store it in
their bulb leaves underground.
[Illustration: [Leaves]]
You begin to see now how important starch is to our lives. Nearly all
the vegetables and grains and fruits we eat are composed almost entirely
of starch or the materials of starch. Even meat is made from starch, for
what do the animals we kill for meat live on?
Why, plants of course, and chiefly the starch they find in plants.
So now we are just where we started,—we see we really do owe our lives
to starch, and we owe starch to chlorophyll, so of course, we owe our
lives to chlorophyll. I wonder if we shall think of this next time we
look at the green leaves everywhere in the fields and woods.
I wonder if these green leaves will not look more beautiful than ever
when we think of the work they are doing.