Part 7

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.

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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.

I am with you, father, with eyes and ears all open.

I’ll tell you what it is, Willie, we shall have not only to be inside the tree, but to be able to stay there a few weeks or months to notice all.

That is easy enough. We may as well fancy hundreds of years as millions of miles.

Let us go to the pith. Around here are vessels in the sheath of the pith. The circles of wood are ranged outside. We will pass along one of the Medullary or pith rays, which you saw make the silver lines of the wood, and here we are at the Liber or inner bark.

But what is this soft white wood next to it?

This is the Alburnum, or white wood. When you tap a tree, the sap comes from this part. Now watch what is coming.

I see--the Liber is separating from the soft Alburnum, and there comes a sticking substance of sap oozing between them. What is that?

It is the Cambium, or Albumen; the white of egg is albumen. Look again.

Ah! the cambium is softening the cellular tissue that bound the liber to the alburnum, and while that is growing outward, I find that the other parts are growing upward.

Let us take a peep at the leaf buds. You see they feel the growing influence, and are letting fall bundles of woody matter, which pass into the cambium, and become attached to the liber. The cellular tissue passes in the mass around, along the medullary rays, and a new substance becomes attached to the liber.

But that new substance is another alburnum, father, and our old alburnum is turned a quite hard wood.