英文科学读本 第四册·Lesson 07 The Oak Stem

Lesson 07 The Oak Stem

I had often wondered, sir, at the beautiful markings of the various ornamental woods used by the cabinet-maker, said Fred, as they were putting the things away after the lesson. "I think I understand it clearly now."

I want you to be quite clear as to those medullary rays, said Mr. Wilson. "In all these stems there are two kinds of matter. There is the hard, fibrous, woody tissue and the softer tissue of the pith. In the young tree you have seen that the stem is almost all pith, and that the pith is the channel for the upward passage of the sap. As the new layers of wood are formed round it year by year, the pith becomes more and more compressed, and it sends out thin walls of its own substance, radiating to the outside of the stem, so as to separate the woody tissue into wedge-like blocks. These radiating walls of soft matter are the medullary rays. They carry on the work of the pith, and form channels for the flow of the sap."

But the sap does not always rise through the medullary rays, does it, sir? asked Willie.

As the tree grows from the outside, said Mr. Wilson, "the older part of the stem is subject to more and more pressure. The walls of soft matter between the wedges of wood become thinner and thinner, till they are the finest sheets, and the medullary tissue itself has ceased to be living matter. After this it no longer carries up the sap. When the stem is cut across, only the thin edges of these partition walls are seen, and they appear to be mere lines."

Thank you, sir, said Fred. "These are the lines then that the cabinet-maker calls the silver-grain of the wood."

Yes, Fred, they are, said Mr. Wilson. "The beauty of the wood depends upon the closeness of its texture, and the skill of the workman in cutting it so as to expose this silver grain. Which part of the woody stem did you say was first formed?"

The central part of the stem, sir, is the oldest wood in the tree.

Tell me again what happens to this part as new layers are formed one by one around it?

The outer layers press upon it more and more, sir, and it becomes denser and denser.

Quite true, said Mr. Wilson, "and it becomes the hardest wood in the tree. It is called heartwood. Heartwood is always selected where strength and durability are required. The heartwood becomes darker and more deeply coloured than the layers round it, and it is the graduation of coloring that makes our cabinet woods so beautiful.

Where do we find the newly-formed wood in the stem?

The last-formed layer of wood is always found on the outside, just under the bark, said Willie. "It is called the sap-wood. It is formed from the sap which has passed upwards through the stem and leaves. The sap, after it has flowed back from the leaves, is deposited in this part of the stem, where it forms the new layer of sap-wood."

A very good answer, my boy, said Mr. Wilson. "This sap-wood is at first soft, pulpy matter, and while in that state it affords a convenient channel for the upward passage of the sap, which can no longer rise through the dead flattened cells of the pith and the medullary rays."