Читать книгу Trees of the Northern United States - A. C. Apgar - Страница 7

Table of Contents

The stem is the distinguishing characteristic of trees, separating them from all other groups of plants. Although in the region covered by this book the trees include all the very large plants, size alone does not make a tree.

A plant with a single trunk of woody structure that does not branch for some distance above the ground, is called a tree. Woody plants that branch directly above the soil, even though they grow to the height of twenty feet or more, are called shrubs, or, in popular language, bushes. Many plants which have a tendency to grow into the form of shrubs may, by pruning, be forced to grow tree-like; some that are shrubs in the northern States are trees further south.

All the trees that grow wild, or can be cultivated out of doors, in the northern States belong to one class, the stems having a separable bark on the outside, a minute stem of pith in the center, and, between these, wood in annual layers. Such a stem is called exogenous (outside-growing), because a new layer forms on the outside of the wood each year.

Fig. 1.

Another kind of tree-stem is found abundantly in the tropics; one, the Palmetto, grows from South Carolina to Florida. While in our region there are no trees of this character, there are plants having this kind of stem, the best illustration being the corn-stalk. In this case there is no separable bark, and the woody substance is in threads within the pithy material. In the corn-stalk the woody threads are not very numerous, and the pith is very abundant; in most of the tropical trees belonging to this group the threads of wood are so numerous as to make the material very durable and fit for furniture. A stem of this kind is called endogenous (inside-growing). Fig. 1 represents a longitudinal and a cross section of an exogenous stem, and Fig. 2 of an endogenous one.

Fig. 2.

Since all the stems with which we have to deal are exogens, a particular description of that class will here be given. Fig. 1 shows the appearance of a section of an Ash stem six years old. The central portion, which is about as thick as wrapping-twine, is the pith; from this outward toward the bark can be seen the six annual layers of the wood; and then comes the bark, consisting of two portions. First there is an inside layer of greenish material, the fresh-growing portion, and lastly the outer or dead matter. This outer portion must crack open, peel off, or in some way give a chance for the constant growth of the trunk. The different kinds of trees are readily known by the appearance of the bark of the trunk, due to the many varieties of surface caused by the allowance for growth. None of the characteristics of trees afford a better opportunity for careful observation and study than the outer bark.

The Birches have bark that peels off in thin horizontal layers—the color, thinness, and toughness differing in the different species; the Ashes have bark which opens in many irregular, netted cracks moderately near each other; the bark of the Chestnut opens in large longitudinal cracks quite distant from one another. The color of the bark and the character of the scales are quite different in the White and the Black Oaks.

In the woody portion radiating lines may be seen; these are the silver grain; they are called by the botanist medullary rays.

The central portion of the wood of many large stems is darker in color than the rest. This darker portion is dead wood, and is called heart-wood; the outer portion, called sap-wood, is used in carrying the sap during the growing season. The heart-wood of the Walnut-tree is very dark brown; that of the Cherry, light red; and that of the Holly, white and ivory-like. The heart-wood is the valuable part for lumber.

If examined under a magnifying glass, the annual layers will be seen to consist of minute tubes or cells. In most trees these tubes are much larger in the portion that grew early in the season, while the wood seems almost solid near the close of the annual layer; this is especially true in the Ashes and the Chestnut; some trees, however, show but little change in the size of the cells, the Beech being a good example. In a cross-section, the age of such trees as the Chestnut can readily be estimated, while in the Beech it is quite difficult to do this. Boxwood, changing least in the character of its structure, is the one always used for first-grade wood-engravings.

When wood is cut in the direction of the silver grain, or cut "quartering" as it is called by the lumbermen, the surface shows this cellular material spread out in strange blotches characteristic of the different kinds of wood. Fig. 16 shows an Oak where the blotches of medullary rays are large. In the Beech the blotches are smaller; in the Elm quite small. Lumber cut carefully in this way is said to be "quartered," and with most species its beauty is thereby much increased.

Any one who studies the matter carefully can become acquainted with all the useful and ornamental woods used in a region; the differences in the color of the heart-wood, the character of the annual layers, and the size and the distribution of the medullary rays, afford enough peculiarities to distinguish any one from all others.

Branching.—The regular place from which a branch grows is the axil of a leaf, from what is called an axillary bud; but branches cannot grow in the axils of all leaves. A tree with opposite leaves occasionally has opposite branches; while a tree with alternate leaves has all its branches alternate.

Most branches continue their growth year after year by the development of a bud at the end, called a terminal bud. Many trees form this bud for the next year's growth so early in the year that it is seldom or never killed by the winter weather; such trees grow very regularly and are symmetrical in form. Most evergreens are good examples. Fig. 3 represents a good specimen. The age of such trees, if not too great, can be readily ascertained by the regularity of each year's growth. The tree represented is sixteen years old. The branches that started the fifth year, about the age at which regular growth begins, are shown by their scars on the trunk.

Fig. 3.

The terminal buds of many trees are frequently killed by the frosts of winter; such trees continue their growth by the development of axillary buds; but as growth from an axillary bud instead of a terminal one will make a branch crooked, such trees are irregular in their branching and outline. Just which axillary buds are most apt to grow depends upon the kind of tree, but trees of the same variety are nearly uniform in this respect. Most trees are therefore readily recognized by the form of outline and the characteristic branching. A good example of a tree of very irregular growth is the Catalpa (Indian Bean), shown in Fig. 4. The tendency to grow irregularly usually increases with age. The Buttonwood, for example, grows quite regularly until it reaches the age of thirty to forty years; then its new branches grow in peculiarly irregular ways. The twigs of a very old and a young Apple-tree illustrate this change which age produces.

Fig. 4.

There are great differences in the color and surface of the bark of the twigs of different species of trees; some are green (Sassafras), some red (Peach, on the sunny side), some purple (Cherry). Some are smooth and dotless, some marked with dots (Birch), some roughened with corky ridges (Sweet Gum), etc.

The taste and odor of the bark are characteristics worthy of notice: the strong, fragrant odor of the Spice-bush; the fetid odor of the Papaw; the aromatic taste of the Sweet Birch; the bitter taste of the Peach; the mucilaginous Slippery Elm; the strong-scented, resinous, aromatic Walnut, etc.

The branches of trees vary greatly in the thickness of their tips and in their tendency to grow erect, horizontal, or drooping. Thus the delicate spray of the Birches contrasted with the stout twigs of the Ailanthus, or the drooping twigs of the Weeping Willow with the erect growth of the Lombardy Poplar, give contrasts of the strongest character. In the same way, the directions the main branches take in their growth from the trunk form another distinctive feature. Thus the upward sloping branches of the Elm form a striking contrast to the horizontal or downward sloping branches of the Sour Gum, or, better still, to certain varieties of Oaks.

When the main trunk of a tree extends upward through the head to the tip, as in Fig. 3, it is said to be excurrent. When it is soon lost in the division, as in Fig. 4, it is said to be deliquescent.