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.
You see, then, that while the last year’s soft wood becomes a hard ring or layer, another is formed in its place alongside of the liber. The hard wood is dead compared with the softer kind.
I think I understand that. But where does the nourishment come from?
The sap when first rising is watery and nearly colourless. But after exposure to air and light in the leaf, and losing much water in evaporation, it becomes thicker, and, descending by the bark, is prepared, like blood, to renew and to form different parts of the structure.