WOOD-ELEMENTS. These vary in details, but are similar in this regard that all partake of the nature of minute tubes. The cavities within the tubes are the "lumina." A cell-cavity, or lumen, may be empty, or it may contain water or other com pounds. Wood-elements are of several kinds, as wood-fibers, tracheids, vessels, wood-paren chyma fibers, and pith-ray cells. Each of these classes of wood-ele ments includes several varieties.
The walls of all wood-elements are thickened and appear under the microscope as double lines.
The young primary wal1, is a very thin, practically imperforate and continuous membrane, which constitutes the first outline of the cell. This membrane originally surrounded the protoplasm and other materials that were contained within the living cell.
The secondary thickening is laid on later and gives strength to the wood , element. It is seldom, if ever, but contains pits of char acteristic shapes. The layer is times disposed in ridges on the inside of the cell, much as a spiral stair case is placed within a tower. This structure is never present within wood-fibers, but is occasionally found within tracheids and vessels.
Holes or thin spots in the walls of wood-elements are known as "pits." Some pits are round, while others are elliptical or slit-shaped. They are further divided into what are known as "simple pits" and "bordered pith." Pits are "simple" when the walls that extend out from the middle lamella are nearly parallel, and they are "bordered" when the walls that extend out from the middle lamella diverge.
The bordered pits that are present in the walls of tracheids, vessels, and some wood-fibers, are invaria bly paired exactly in position with similar pits in the walls of ad joining elements. They do not open through, however, but are closed by partitions which exist in the primary walls or "middle lamellae." There is usually a thickened disc in the middle of the partition that is known as the "torus." Wood Fibers.
Ordinary wood-fibers are long, slender, com paratively smooth-surfaced, and sharp-pointed wood-elements.
The walls are thick and lignified, and the pits are usually simple; that is, they are without borders. Wood-fibers are not found in conif erous woods, but are nearly always present in, and are regarded as char acteristic of the so-called broadleafed woods. to which they contribute much strength and hardness. The wood fibers also give mechanical strength to the living tree and probably con tribute in some way to the transporta tion of water through the tree, from the roots to the foliage.
(Tra-ke-ids). Tra cheids are elongated, taper-pointed cells, with peculiar markings, which appear, either in the form of bordered pits, located, for the most part, on the radial surfaces of the tracheids, or else in the form of ridges, variously disposed upon the inner walls. Tra cheids are the wood-elements upon which coniferous woods largely de pend for strength. They are charac teristic of coniferous woods and although they do exist in many of the broadleafed woods are then in variably subordinate to wood-fibers and vessels.
Tracheids serve in the living tree because they contribute to its chanical support, and also because the bordered pits are so designed as to assist very materially in the tion of water through the stem from the roots to the leaves.
The means by which water is thus raised has been credited to root pressure, transpiration, and osmotic pressure.' Vessels.
These compound structures are formed by the breaking down of partitions that exist between the abutting ends of simpler or shorter structures, known as "vessel-segments." Tubes of very considerable length are formed in this manner and, as is the case with oak, are often so large in diameter that they can be seen with the unaided eye. These large cavities are commonly referred to as "pores," and the vessels themselves have been variously named by plant anatomists as pores, canals, ducts, tubes, vasa, tracheae, tracheal-tubes, and fistulae.
Vessels Differ with Spe cies. The central cavities of lumina of the vessels of some species are open, while those of other species are obstructed by parenchymatous growths known as "tyloses." Air can readily be blown through several feet of red oak, even before it has been seasoned, because tyloses are absent in the vessels of this species. On the contrary, a pressure of one hundred pounds per square inch is sometimes insufficient to force air through a single inch of unseasoned white oak, because the vessels of that species contain quantities of tyloses.
A vessel increases. in thickness by means of layers that are gradually deposited on its walls. Several layers of unequal thickness can often be distinguished with the aid of a powerful microscope. The thick ened portions of the walls give strength, while the unthickened por tions permit water and materials in solution to pass in and out. The differences in thickness are evidenced by markings such as are shown in the picture.
Thesecompound structures are made up of short, cubical or oblong cells, arranged in rows that pass radially from the center of a tree to its circumference. Pith-rays differ strikingly from other wood-elements in that they are arranged horizontally. They cross the tree, bind the vertical wood-ele ments together, and also serve as a vital link between the living elements of the tree. The cells of which pith-rays are composed resemble those making up wood-parenchyma fibers in form and structure, and because of the fact that they, too, contain various foods and products connected with the life-processes of the tree. The terms pith-ray, medullary ray, and ray mean the same.
Pith-rays are plainly visible in some woods, as oaks, but are not easily visible in other woods, as poplars, even when a hand magni fying glass is employed. Pith-rays contribute to the appearance of "quartered oak," which with other "quartered woods," are obtained by cutting logs radially (see Fig. 25). When cut in this way the pith-rays are split and their larger surfaces are exposed. Otherwise, in the tangential cut, the pith-rays are cut through vertically and appear as short lines. Pith-rays are not visible in some woods except when very thin pieces are placed under a compound microscope.
The small, cubical or oblong cells, of which are composed, are indented with minute, simple pits. The pith-rays of some conifers also contain, in addition to the small parenchyma cells, one or more rows of peculiar flattened tracheids, known as "ray-tracheids." Resin ducts are also present in some of the pith-rays that exist in the pines. Pith-rays may be divided into primary and secondary pith-rays. The first are those that extend completely through from the pith-cavity at the center of the tree to the bark, while the second are those that do not extend through thus completely.
The function of the has been described as follows:' "The have, for their primary function, the radial transmission and storage of food. Their intimate relation with the cells of the phloem at their outer and with the xylem parenchyma along the inner course, and the fact that we usually find them gorged with food, points to this conclusion. The short, vertical extent of the rays, and their isolation from each other renders them unsuited for the vertical or longitudinal transmission of foods. If they were of value in this respect girdling would not prevent the downward flow of foods." fir, is interrupted by constrictions. In some woods resin-canals are simple cavities known as "cysts." Resin-canals and resin ducts are the same.
The resin-passages that exist in the trees that produce commercial resins have received most attention. Tschirch divides these passages, as they exist in the pines, into "primary resin-ducts" and "secondary resin-ducts." The former, scattered through the heartwood and the sapwood, produce comparatively small quantities of resins, while the latter, formed in the outer sapwood of trees that have been wounded, pour crude turpentine over the wounded surfaces in order to protect them. The turpentine of commerce is obtained from these "secondary resin-canals." It will be seen that the resins produced by the "primary canals" are physiological products, whereas those produced by the "sec ondary canals" are pathological products. Tschirch has demonstrated that the seat of resin-production is in a mucilaginous layer (epithelium cells, Fig. 15) that lines the inside of the resin canal.' Arrangement of Wood Elements.
The character of wood depends not only upon its wood-elements but also upon the way in which these wood-elements are arranged. Most wood elements are arranged up and down, a fact that explains the comparative ease with which most woods are split. But besides this, there is a horizontal arrangement. The pith-rays pass radially, that is horizontally, from the center of the tree to its circumference, and bind the vertical wood-elements together. The arrangement of wood-elements is much more regular in some woods, as pines, than in others, as eucalyptus and lignumvit; and woods are easy or difficult to work, in proportion as their elements are thus arranged in a simple or a complicated manner.
Certain materials are associated with, although they do not form part of, the wood-elements, and such compounds are notable because they exert a material influence upon the character of the wood-elements, and, therefore, upon the character of the wood. Of these associated materials, the most important is water, which acts by distending the wood elements and thus making them weaker and more pliable. The influence of moisture is so great as to require further notice (see "Moisture in Wood ").
Influence of Cellular Structure upon Chemical Composition and Physical Properties of Wood.
Chemical and physical prop erties of woods are influenced by the character and arrangement of the wood-elements, and by the qualities and quantities of the materials associated with these wood-elements. Chemical com position, strength, weight, appearance, and other properties re garded by those who use woods depend much upon these details.
The cellular characteristics of woods are most evident in their cross-sections. The cross-sections of different species differ from one another, but each one exhibits certain traits that remain constant for that species, and in many cases these traits are suffi cient to serve as a means of practical identification. For example, it will be noted that the section of white oak (Plate I) contains large vessels but is without resin-duct s, whereas the section of long leafed pine (Plate I) contains resin-ducts but is without vessels.