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.