Wood and Forest Part 3

(1) Transverse, a plane at right angles to the organic axis.

(2) Radial, a longitudinal plane, including the organic axis.

[Ill.u.s.tration: Fig. 3.

A.

A, B, C, D, Transverse Section.

B, D, E, F, Radial Section.

G, H, I, J, Tangential Section.

B.

A, B, C, Transverse Section.

A, B, D, E, Radial Section.

B, C, E, F, Tangential Section.

(3) Tangential, a longitudinal plane not including the organic axis.

If a transverse section of the trunk of a conifer or of a broad-leaved tree is made, it is to be noted that it consists of several distinct parts. See Fig. 4. These, beginning at the outside, are:

(1) Rind or bark (a) Cortex (b) Bast

(2) Cambium

(3) Wood (a) Sap-wood (b) Heart-wood

(4) Pith.

[Ill.u.s.tration: Fig. 4. Diagram of Cross-section of Three Year Old Stem of Ba.s.swood.]

(1) The rind or _bark_ is made up of two layers, the outer of which, the "cortex," is corky and usually scales or pulls off easily; while the inner one is a fibrous coat called "bast" or "phloem." Together they form a cone, widest, thickest, and roughest at the base and becoming narrower toward the top of the tree. The cortex or outer bark serves to protect the stem of the tree from extremes of heat and cold, from atmospheric changes, and from the browsing of animals. It is made up of a tough water-proof layer of cork which has taken the place of the tender skin or "epidermis" of the twig. Because it is water-proof the outside tissue is cut off from the water supply of the tree, and so dries up and peels off, a ma.s.s of dead matter. The cork and the dead stuff together are called the bark. As we shall see later, the cork grows from the inside, being formed in the inner layers of the cortex, the outer layers of dry bark being thus successively cut off.

The characteristics of the tree bark are due to the positions and kinds of tissue of these new layers of cork. Each tree has its own kind of bark, and the bark of some is so characteristic as to make the tree easily recognizable.

Bark may be cla.s.sified according to formation and method of separation, as scale bark, which detaches from the tree in plates, as in the willows; membraneous bark, which comes off in ribbons and films, as in the birches; fibrous bark, which is in the form of stiff threads, as in the grape vine; and fissured bark, which breaks up in longitudinal fissures, showing ridges, grooves and broad, angular patches, as in oak, chestnut and locust. The last is the commonest form of bark.

The bark of certain kinds of trees, as cherry and birch, has peculiar markings which consist of oblong raised spots or marks, especially on the young branches. These are called lenticels (Latin _lenticula_, freckle), and have two purposes: they admit air to the internal tissues, as it were for breathing, and they also emit water vapor.

These lenticels are to be found on all trees, even where the bark is very thick, as old oaks and chestnuts, but in these the lenticels are in the bottoms of the deep cracks. There is a great difference in the inflammability of bark, some, like that of the big trees of California, Fig. 54, p. 209, which is often two feet thick, being practically incombustible, and hence serving to protect the tree; while some bark, as canoe birch, is laden with an oil which burns furiously. It therefore makes admirable kindling for camp fires, even in wet weather.

Inside the cork is the "phloem" or "bast," which, by the way, gives its name to the ba.s.s tree, the inner bark of which is very tough and fibrous and therefore used for mat and rope making. In a living tree, the bast fibers serve to conduct the nourishment which has been made in the leaves down thru the stem to the growing parts.

(2) The _cambium_. Inside of the rind and between it and the wood, there is, on living trees, a slimy coat called cambium (Med. Latin, exchange). This is the living, growing part of the stem, familiar to all who have peeled it as the sticky, slimy coat between the bark and the wood of a twig. This is what const.i.tutes the fragrant, mucilaginous inner part of the bark of slippery elm. Cambium is a tissue of young and growing cells, in which the new cells are formed, the inner ones forming the wood and the outer ones the bark.

In order to understand the cambium and its function, consider its appearance in a bud, Fig. 5. A cross-section of the bud of a growing stem examined under the microscope, looks like a delicate mesh of thin membrane, filled in with a viscid semi-fluid substance which is called "protoplasm" (Greek, _protos_, first; _plasma_, form). These meshes were first called "cells" by Robert Hooke, in 1667, because of their resemblance to the chambers of a honeycomb. The walls of these "cells" are their most prominent feature and, when first studied, were supposed to be the essential part; but later the slimy, colorless substance which filled the cells was found to be the essential part.

This slimy substance, called protoplasm, const.i.tutes the primal stuff of all living things. The cell walls themselves are formed from it.

These young cells, at the apex of a stem, are all alike, very small, filled with protoplasm, and as yet, unaltered. They form embryonic tissue, _i.e._ one which will change. One change to which an cell filled with protoplasm is liable is division into two, a new part.i.tion wall forming within it. This is the way plant cells increase.

[Ill.u.s.tration: Fig. 5. Young Stem, Magnified 18-1/2 Diameters, Showing Primary and Secondary Bundles. _By Courtesy of Mrs. Katharine Golden Bitting._

E, epidermis, the single outside layer of cells.

C, cortex, the region outside of the bundles.

HB, hard bast, the black, irregular ring protecting the soft bast.

SB, soft bast, the light, crescent-shaped parts.

Ca, cambium, the line between the soft bast and the wood.

W, wood, segments showing pores.

MR, medullary rays, lines between the bundles connecting the pith and the cortex.

MS, medullary sheath, the dark, irregular ring just inside the bundles.

P, pith, the central ma.s.s of cells.]

In young plant cells, the whole cavity of the chamber is filled with protoplasm, but as the cells grow older and larger, the protoplasm develops into different parts, one part forming the cell wall and in many cases leaving cavities within the cell, which become filled with sap. The substance of the cell wall is called cellulose (cotton and flax fibers consist of almost pure cellulose). At first it has no definite structure, but as growth goes on, it may become thickened in layers, or gummy, or hardened into lignin (wood), according to the function to be performed. Where there are a group of similar cells performing the same functions, the group is called a tissue or, if large enough, a tissue system.

When cells are changed into new forms, or "differentiated," as it is called, they become permanent tissues. These permanent tissues of the tree trunk const.i.tute the various parts which we have noticed, viz., the rind, the pith and the wood.

The essentially living part of the tree, it should be remembered, is the protoplasm: where there is protoplasm, there is life and growth.

In the stems of the conifers and broad-leaved trees--sometimes together called exogens--this protoplasm is to be found in the buds and in the cambium sheath, and these are the growing parts of the tree. If we followed up the sheath of cambium which envelopes a stem, into a terminal bud, we should find that it pa.s.sed without break into the protoplasm of the bud.

In the cross-section of a young shoot, we might see around the central pith or medulla, a ring of wedge-shaped patches. These are really bundles of cells running longitudinally from the rudiments of leaves thru the stem to the roots. They are made of protoplasm and are called the "procambium strands," Fig. 6.

[Ill.u.s.tration: Fig. 6. Three Stages in the Development of an Exogenous Stem. P, pith; PB, primary bast; SB, secondary bast; C, cambium; PR, pith ray; PW, primary wood; SW, secondary wood; PS, procambium strands. _After Boulger._]

In the monocotyledons (endogens) these procambium strands change completely into wood and bast, and so losing all their protoplasmic cambium, become incapable of further growth. This is why palms can grow only lengthwise, or else by forming new fibers more densely in the central ma.s.s. But in the conifers and broad-leaved trees, the inner part of each strand becomes wood and the outer part bast (bark).

Between these bundles, connecting the pith in the center with the cortex on the outside of the ring of bundles, are parts of the original pith tissue of the stem. They are the primary pith or medullary rays (Latin, _medulla_, pith). The number of medullary rays depends upon the number of the bundles; and their form, on the width of the bundles, so that they are often large and conspicuous, as in oak, or small and indeed invisible, as in some of the conifers. But they are present in all exogenous woods, and can readily be seen with the microscope. Stretching across these pith rays from the cambium layer in one procambium strand to that in the others, the cambium formation extends, making a complete cylindrical sheath from the bud downward over the whole stem. This is the cambium sheath and is the living, growing part of the stem from which is formed the wood on the inside and the rind (bark) on the outside.

In the first year the wood and the bast are formed directly by the growth and change of the inner and outer cells respectively of the procambium strand, and all such material is called "primary;" but in subsequent years all wood, pith rays, and bast, originate in the cambium, and these growths are called "secondary."

[Ill.u.s.tration: Fig. 7. Sap-wood and Heart-wood, Lignum Vitae.]

(3) The _wood_ of most exogens is made up of two parts, a lighter part called the sap-wood or splint-wood or alburnum, and a darker part called the heart-wood or duramen, Fig. 7. Sap-wood is really immature heartwood. The difference in color between them is very marked in some woods, as in lignum vitae and black walnut, and very slight in others, as spruce and ba.s.s. Indeed, some species never form a distinct heart-wood, birch (_Betula alba_) being an example.

In a living tree, sap-wood and heart-wood perform primarily quite different functions. The sap-wood carries the water from the roots to the leaves, stores away starch at least in winter, and in other ways a.s.sists the life of the tree. The proportional amount of sapwood varies greatly, often, as in long-leaf pine, const.i.tuting 40 per cent.

of the stem.

As the sap-wood grows older, its cells become choked so that the sap can no longer flow thru them. It loses its protoplasm and starch and becomes heartwood, in which all cells are dead and serve only the mechanical function of holding up the great weight of the tree and in resisting wind pressures. This is the reason why a tree may become decayed and hollow and yet be alive and bear fruit. In a tree that is actually dead the sap-wood rots first.

Chemical substances infiltrate into the cell walls of heart-wood and hence it has a darker color than the sap-wood. Persimmon turns black, walnut purplish brown, sumac yellow, oak light brown, tulip and poplar yellowish, redwood and cedar brownish red. Many woods, as mahogany and oak, darken under exposure, which shows that the substances producing the color are oxidizable and unstable. Wood dyes are obtained by boiling and distilling such woods as sumach, logwood, red sanders, and fustic. Many woods also acquire distinct odors, as camphor, sandalwood, cedar, cypress, pine and mahogany, indicating the presence of oil.

As a rule heart-wood is more valuable for timber, being harder, heavier, and drier than sap-wood. In woods like hickory and ash, however, which are used for purposes that require pliability, as in baskets, or elasticity as in handles of rakes and hoes, sap-wood is more valuable than heart-wood.