When it gets there it finds our old friends the sun-beams hard at work.  If you have ever tried to grow a plant in a cellar, you will know that in the dark its leaves remain white and sickly.  It is only in the sunlight that a beautiful delicate green tint is given to them, and you will remember from Lecture ii. that this green tint shows that the leaf has used all the sun-waves except those which make you see green; but why should it do this only when it has grown up in the sunshine?

The reason is this:  when the sunbeam darts into the leaf and sets all its particles quivering, it divides the protoplasm into two kinds, collected into different cells.  One of these remains white, but the other kind, near the surface, is altered by the sunlight and by the help of the iron brought in by the water.  This particular kind of protoplasm, which is called “chlorophyll,” will have nothing to do with the green waves and throws them back, so that every little grain of this protoplasm looks green and gives the leaf its green colour.

It is these little green cells that by the help of the sun-waves digest the food of the plant and turn the water and gases into useful sap and juices.  We saw in Lecture III. that when we breathe-in air, we use up the oxygen in it and send back out of our mouths carbonic acid, which is a gas made of oxygen and carbon.

Now, every living things wants carbon to feed upon, but plants cannot take it in by itself, because carbon is solid (the blacklead in your pencils is pure carbon), and a plant cannot eat, it can only drink-in fluids and gases.  Here the little green cells help it out of its difficulty.  They take in or absorb out of the air carbonic acid gas which we have given out of our mouths and then by the help of the sun-waves they tear the carbon and oxygen apart.  Most of the oxygen they throw back into the air for us to use, but the carbon they keep.

If you will take some fresh laurel-leaves and put them into a tumbler of water turned upside-down in a saucer of water, and set the tumbler in the sunshine, you will soon see little bright bubbles rising up and clinging to the glass.  These are bubbles of oxygen gas, and they tell you that they have been set free by the green cells which have torn from them the carbon of the carbonic acid in the water.

But what becomes of the carbon?  And what use is made of the water which we have kept waiting all this time in the leaves?  Water, you already know, is made of hydrogen and oxygen, but perhaps you will be surprised when I tell you that starch, sugar, and oil, which we get from plants, are nothing more than hydrogen and oxygen in different quantities joined to carbon.