Wood and Forest. William Noyes
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Название: Wood and Forest
Автор: William Noyes
Издательство: Bookwire
Жанр: Языкознание
isbn: 4064066189327
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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. 208, 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 bass 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 constitutes 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.
Fig. 5. Young Stem, Magnified 18½ 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 mass of cells.
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, constitutes 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 partition wall forming within it. This is the way plant cells increase.
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 constitute 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 passed 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.
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 mass. 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."
Fig. 7. Sap-wood and Heart-wood, Lignum Vitae.
(3) The wood of most СКАЧАТЬ