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.