Part 3 (2/2)

[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.

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