CHAPTER XXVIII.

THE WOOD IN TREES AND SHRUBS.

[Sidenote: How wood is made.]

Perhaps it seemed strange to you when I said in the last chapter that bark makes wood. But so it is. Every year the living inner bark goes to work and makes a layer of wood out of the sap that is in it. This work is done in the warm weather. In the winter there is no wood made. The tree is asleep then.

It is what the bark does that makes the tree larger every year. A new layer of wood is formed by it all up the trunk, and along out to the end of all the branches.

[Sidenote: Its layers.]

The different layers of wood made in the different years are often very distinct from each other. You can see them in a log that has been cut or sawn across. Sometimes they are so distinct that you can count them, and so tell just how many years old the tree is. Here is a representation of the sawn end of the trunk of a tree. You see that the rings of the wood are very plain.

[Sidenote: Pipes in the wood for the sap.]

The wood part of the trunk and branches is full of small pipes. It is through these pipes that the sap goes up from the roots and gets to the leaves. It is in this way that it goes to the very ends of the topmost boughs of the tallest trees. This is very wonderful. How the sap is made to go up such a great distance through these pipes in the wood we do not know. There is only one way that man can make water go so high through pipes. He can do it by a forcing-pump. But we can see nothing like forcing-pumps in the trees. We find nothing but these pipes going from the roots up to the leaves. And the sap is flowing up through them very quietly all the time.

[Sidenote: Sap-pipes numerous.]

In a large tree there is a multitude of these pipes in the wood. And when you look at the huge trunk, think what a quantity of sap there is going up through it all the time to keep all those leaves fresh and green. If you could see it all in one pipe it would be quite a stream.

[Sidenote: Heart-wood.]

If you look at the end of a log you will see that there are two kinds of wood. The wood in the centre is different from that which is around it. It is called the heart-wood. The pipes in it are stopped up, and no sap can go up through it. The pipes for the sap are clear only in the newest part of the wood.

[Sidenote: Pith.]

The use of the pith of trees and plants we do not understand. The pith is very small in trees, but it is quite large in some plants and shrubs. All boys know that it is very large in the elder. It is also large in the stalks of corn, and of the sugar-cane.

_Questions._--How is the wood in a tree made? What is said of the different layers of wood? What is said of the small pipes in the wood? Do we know how the sap is made to go up in them? What is said of the quantity of sap that goes up in the trunk of a large tree? What is said of the two kinds of wood that you see in looking at the end of a log? What do we know about the pith of trees and plants?