英文科學(xué)讀本 第四冊(cè)·Lesson 11 The Oak and the Fir (a Comparison)

Lesson 11 The Oak and the Fir (a Comparison)

In our last lesson on trees we took the oak as the representative of the dicotyledons or exogens, said Mr. Wilson. "There are a large family of timber trees—the firs and pines—which are usually classed with the exogens, but they differ from the oak and other timber trees in many important particulars. Let us find out these points of difference. Here are some acorns. These, you know, are the fruit of the oak tree. Take some of them in your hands and remove the seed from the outer shell. If you open it carefully you will see at once why it is called a dicotyledon. It has the usual double seed-leaves.

Now look at this pinecone. This, you know, is the fruit of the fir and pine family of trees. Notice that it consists of a number of dry hard scales. Each scale is a single fruit. It is the ripened pistil of the flower, but the pistil in this instance, instead of being a hollow receptacle for the seed, is simply a leaf, with the seed attached to its upper surface. As the cone ripens, this pistil-leaf becomes a dry scale, which curls up, and sheds its seed, leaving it to blow away or fall to the ground.

The firs and pines are often called cone-bearing trees, because they are the only trees which bear fruit of this kind. What is the first distinction we have found then between the oak and the fir?"

The fruit of the oak, said Fred, "is a nut, which we call an acorn, and its seed is a dicotyledon; the fruit of the fir is a cone with dry scale-seeds."

We will next examine the stems of the oak and the fir, said Mr. Wilson. "I have a good specimen of each of them here. If you look into them closely, you will not be able to find any difference in the appearance of the two. In each stem there is the same central pith-spot, with the same ringlayers of wood round it, and the same outer covering of bark.

But there is a difference between them, and as it is not one which you can find out for yourselves, I must tell you what it is. The pines and the firs are resinous trees. Their stems yield the inflammable substances known as turpentine and resin. It is easy to detect the resinous smell in these woods. The oak stem is not resinous. It yields no liquid of any sort. Hence, you see, we have a second distinction between these two families of trees. But we must pass on now to the leaves. Here are some oak leaves. Take them in your hands and examine them. Now describe them."

They are like most of the common leaves, said Willie, "and consist of a blade and a foot-stalk. There are ribs crossing the blade, and a network of veins running all over it."

Quite correct, said Mr. Wilson. "They are the usual leaves of the dicotyledon; and, moreover, the microscope would show us the breathing pores, studded all over the blade.

Now look at the pine leaves, he continued. "Here they are. They are more like green spikes than leaves. They have nothing of the shape of other leaves. They are called needle-shaped leaves. They grow in a cluster, and there are from two to five of them to form the cluster. They have small scales at their base. Thus, you see, the difference in the leaves of the two trees makes another distinction."

Are not the firs and pines evergreens, sir? asked Fred.

Yes, they are, said Mr. Wilson, "with the single exception of the larch. This, like most other trees, sheds its leaves in the autumn. All the rest of the pine and fir family are evergreens, and in this again they differ from the oak."