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.
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.