But this broth does not lose all its water. There is still enough left to carry it back through the leaf into the branches and stem, and even down into the root once more. In fact, the prepared food is now sent to just those parts of the plant which most need it. Perhaps it is laid up beneath the bark, to help make new buds which will burst into leaf and flower another year. Or perhaps it goes down to help the roots put out new branches and fresh root hairs. Or possibly it is stowed away in such an underground stem as that of the lily, or the crocus bulb, and is saved for next year’s food. Once in a while some of this prepared food is stored in the leaf itself. When a leaf is thick and juicy (“fleshy,” the books call it), we can guess that it is full of plant food. Do you recall the _Bryophyllum_,—the plant we talked about a few days ago? Its wonderful leaves, you remember, gave birth to a whole colony of new plants. You may be sure that these leaves had refused to give up all the food sent to them for cooking in the sun. You can guess this from their thick, fleshy look, and you can be sure of this when you see the baby plants spring from their edges; for without plenty of nourishment stored away, these leaves could never manage to support such a quantity of young ones. LEAF GREEN AND SUNBEAM But the earth broth which the roots supply is not the only article of importance in the plant’s bill of fare. The air about us holds one thing that every plant needs as food. This air is a mixture of several things. Just as the tea we drink is a mixture of tea and water, and milk and sugar, so the air is a mixture of oxygen and nitrogen, and water and carbonic-acid gas. Oxygen, nitrogen, and carbonic-acid gas,—each one of these three things that help to make the air is what we call a gas, and one of these gases is made of two things. Carbonic-acid gas is made of oxygen and carbon. Now, carbon is the food which is needed by every plant. But the carbon in the air is held tightly in the grasp of the oxygen, with which it makes the gas called carbonic-acid gas. To get possession of this carbon, the plant must contrive to break up the gas, and then to seize and keep by force the carbon. This seems like a rather difficult performance, does it not? For when a gas is made of two different things, you can be pretty sure that these keep a firm hold on each other, and that it is not altogether easy to tear them apart. Now, how does the plant meet this difficulty? You cannot guess by yourselves how this is done, so I must tell you the whole story.