You have been told that it absorbs oxygen from the air, and also carbonic acid gas--that is, carbon and oxygen. This carbonic acid floating in the air is absorbed in the day-time, though the leaf exhales, or breathes out, oxygen gas while the sun shines. _Does it not do so at night?_ No; then the leaf throws out some carbonic acid gas undigested. There is this use of the oxygen being taken in by the leaf, that it acts beneficially upon the sap to thicken it, &c. What is the true shape of the leaf? The course of veins or branching vessels. The fleshy part of the leaf is of cellular tissue. I have heard that our leaves differ from English ones: how are they different? I know they do not fall off in autumn. English leaves are made to fall, as from the first they are nearly divided from the _petiole_ or leaf stalk, the mark of division being to be seen in the budding leaf. Have our Australian leaves that mark? No, they have not. The petiole expands or becomes leafy, and the leaves, therefore, do not readily fall. There is yet another difference. While the glands or tumours for imbibing moisture are on the underside of the leaf in England, they are on both sides of it here. Instead, too, of being horizontal, our leaves are set in edgewise on their stems. I often notice the great difference there is in leaves, father. Yes, they differ in size, structure, direction of veins, situation, form, duration, kind of surface, &c. But what have you to say of those plants which have no leaves? I could never see any in our sheoak nor cherry. Quite true. Our _Casuarina_ or oak family, and the _Exocarpus_, or cherry, have no leaves. They have, however, long hanging knotted twigs at the end of branches. They have been called the Pines of Australia. Is light necessary to leaves as well as air? It is. Sunlight acts chemically to produce changes in substances. It is believed necessary to enable the leaf to separate the carbon from the oxygen in carbonic acid gas. Leaves love light, and will turn toward it. Some even follow the sun in its daily motion. In a dark place, plants soon fade, and the leaf turns white. * * * * * Well, father, we have got rid of the root of our gum tree, as well as its bark and leaves--what shall we take next? I seem to know a good deal, but I cannot yet make out how the tree grows. But you are prepared now to understand that part of my lesson to come. You see the rings of wood, and the rings of bark, and you want to know how they come there. Yes, that I do. Of course I could say that the sap makes wood, as I know that bread and butter make flesh; but I want to understand how the change takes place. Let us then imagine ourselves, Willie, to be spirits able to be inside of the tree, without taking up any room, and with our eyes about us to watch all that is done. I wish I could do that; it would be better than flying. Don’t talk about it, but jump inside along with me.