Part 21

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.

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.

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.