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to be at least 3000 years old. Some of them are 35 feet in diameter and nearly 300 feet high (Fig. 246). Think of a tree that was at least 250 years old when Rome was founded. What is this date?

But when you

think of a tree, you may not have in mind a tree in a great forest, nor one of the Big Trees of California, but a cool shade tree in a city park, or the trim rows of little trees of a peach orchard in Michigan, or a cherry orchard in Wisconsin or Utah, or an orange grove in Florida or California; or you may think only of a comfortable apple tree in your own yard, which gives you both shade and refreshment on a hot day in summer. The trees are indeed our friends, and the more we understand them the more we shall appreciate them.

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325. How Does a Tree Get Its Shape? - Look at the branches of a spreading elm or hard maple or birch as they stand out against the sunset sky in winter, and see

how graceful and beautiful they are.

Branches are the arms of the tree, spread open to receive the gifts of the sky.

If you think of it, you will realize that the shape of a tree is due to its branches, and that trees usually branch in one of two ways. What is the shape of a Christmas

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FIG. 247. An elm and a poplar, showing the two types of branching.

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tree? It is that of a cone, is it not? Why does it have this shape? Is it not because its branches come out with regular spaces between them, and each set of branches is a little shorter than the set just below it? So we have trees that are cone-shaped and taper toward the top. Name some other cone-shaped trees (Fig. 247).

Suppose that the trunk of the tree, instead of going straight up to the top, divides into branches, and these divide in their turn into smaller branches. What shape will the tree have? It will be spreading, will it not, like

an elm tree or a willow tree? Name some other trees that are spreading. Which class of trees makes the better shade, the cone-shaped, or the spreading?

You can see how much the stem has to do with the shape of a plant, shrub, or tree. If the stem is strong and stiff, it will be able to bear a great deal of weight; if it is weak, it will need a support, as in climbing vines, such as the Virginia creeper. Notice the different kinds of trees in a high wind. Which are most stiff, and which yield most to the wind?

326. What is the Inside of a Tree Trunk Like? Interesting as the outside of a tree is, the inside shows us still more wonderful things. If you cut across a young stem, such as that of a lilac, birch, maple, box elder, or pine, you will be surprised at the ringlike structure, beginning with a circular center and extending to the bark on the outside. Look at the cutting across a large tree, as seen in the end of a log, and you will see a similar structure. Perhaps you have heard people say that these are the "rings of growth" of the tree, and that one is added every year. Probably it would be better to say that one is added for every growing season. What is the difference?

Let us see what we can make out of this ringlike structure. The botanist has shown us that there are four distinct regions in such a stem (Fig. 248) :

(1) The outer covering, or skin. In the leaf (see § 300) we called this the epidermis; it has the same name here.

(2) The next layer inside the epidermis is soft tissue, which is generally green. It is called the cortex.

(3) The third layer from the outside is the layer of wood,

(4) The very central part of the stem is the pith. When a tree decays at the center, which of these layers has decayed most?

The rays, like spokes of a wheel, that pass out from the pith through the wood, until they reach the green cortex, are called the pith

Heart Wood

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Sap Wood

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FIG. 248. How a cutting across a woody stem looks, when smoothed so as to show the markings.

rays. They divide the wood into bundles. Do all plants have this arrangement? If you look at the stem of Indian corn, or of a lily, or a rhubarb, you will see that the woody bundles in these plants are not arranged in circles around the pith, but are scattered through the pith. Woody plants with two cotyledons in the embryo,

like our common trees, have this ringlike structure, while plants with only one cotyledon, like Indian corn and the lily, do not. Is it not strange that there should be such a difference in the way in which wood is arranged in a stem? See Fig. 249.

What is the bark of a tree? Is it not the old layers of the epidermis, dead, but not cast off from the tree? Why does the bark of a tree split? Can you not see that since the bark cannot stretch very much, the time will come when the growing tree will be too large for its bark,

and the bark will crack? In which direction does the bark crack? What does this show as to the way in which the bark is being stretched? Usually the bark adheres to the tree for a long time, and protects the growing stem. How?

Examine the bark of several trees, and see how they differ from one another. Describe the bark of the birch, the shagbark hickory, the pine, and the sycamore, if you can find these trees. Do you think you ought to strip the bark from a growing birch tree?

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FIG. 249. - A cut

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ting across the stem lengthwise cutting at the side goes through some woody bundles.

of Indian corn. The

327. What are Sapwood and Heartwood? Have you ever seen a hollow tree, in which the pith and a great deal of wood next to it have disappeared, while the tree still has branches and leaves and carries on its life? Sometimes a tree trunk is split into several strips, and yet each one lives. How can this be? The explanation is that in a large tree the really living part of a tree trunk is near the outside, and that in some diseased trees the inner part has disappeared. We know that the water and dissolved food (the sap) rise from the roots to the leaves, and so must pass up through the stem. The sap passes only through the woody part of the stem. But as the tree grows older, the sap stops flowing through the inner wood, which is near the pith, and passes up only through the outer, younger rings of wood, near the cortex. This does not mean that the inner wood necessarily rots. The inner, older wood is called the heartwood; the younger wood is called sapwood. Which do you think makes better wood for lumber? The heartwood is the

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