  A tree has been defined as a woody plant that produces naturally and in its native place one principal erect stem with a definite crown of foliage. A plant thus attaining to the dignity of a tree is said to be arborescent.[1] There are nearly five hundred distinct species of trees growing in the United States,[2] as well as many others peculiar to other countries, yet the great mass of wood everywhere utilized is derived from comparatively few of them.[3] Many woods will be more generally employed as their valuable properties become more familiar or as the supplies of wood now utilized continue to diminish. The same tree is often called by different common names in different places. Nearly thirty names are thus applied to the longleaf pine (Pinus palustris). Such confusion can be avoided only by regarding the recognized botanical nomenclature. The botanical name of a plant consists of two principal terms denoting genus and species. Quercus, for example, is A species is a collection of individuals that might well have sprung from some single root. A genus is a collection of related species. Genera are gathered into families. Families and genera differ with authorities. A variety includes individuals differing slightly from accepted species. Its name when existing is part of the specific name. "Quercus robur var. pedunculata" specifies a variety (pedunculata) of "red" or strong (robur) oak (Quercus). A variety of one botanist is sometimes a distinct species of another. The size and character of the trunk, and the range, locality, or distribution of the tree, have much to do with the utility of the wood, since large or perfect timbers cannot be derived from species characterized by small or crooked trees, and since wood is always more used if it is widely distributed so as to be easily available.[4] Fig. 1.—Some Wood Elements. Wood is made up of cell-structures; as, the true fibre, which originates from several cells; the tracheid (tra-ke-id), which originates from one; the vessel, which is a short, wide tube joined vertically end to end with others of its kind; the pith-ray; the resin-duct, and others,—all of which are often popularly referred to as fibres. The character and the arrangement of cell-structures differ with species. Wood is hard, soft, light, heavy, tough, porous, elastic, or otherwise, because of these differences. Appearance is affected, and woods may be distinguished from one another, because of this fact.[5] Most wood is used in "construction," that is, in mines, railways, houses, and ships, where demand is for size or quantity, and where finish and appearance amount to but little. Much wood is used in decoration and furniture, where appearance, appropriateness, and finish are called for; but these woods, although much in evidence, are infinitely less in quantity than those employed in construction. Some wood is required for implements, turnery, carvings, and small-piece work, where size is secondary and where qualities such as hardness, fine grain, and uniformity, controllable in small pieces, are primary. Some wood is used indirectly, as in the manufacture of paper-pulp, gunpowder, and chemicals. There are also by-products of trees, such as tanbark, turpentine, resin, nuts, and sugar. The weight, strength, and other measurable properties of wood are variable. Weight varies from day to day as water is absorbed and evaporated. Strength differs with grain, age, moisture, specific gravity, and many other things. Two pieces from different portions of the same tree differ from each other. The proportions of sap and heart wood are seldom constant. Results from small specimens may differ from those obtained from larger ones. [6] The botanical accuracy of a specimen is not always certain, therefore figures relating to the physical properties of wood should be employed with greater caution than those relating to the more homogeneous metals. Many of the experiments conducted to establish statements regarding the physical properties of wood have been defective The experiments that have been made to determine the strength of woods may be grouped into the four following divisions: (1) Experiments conducted by the U. S. Division of Forestry (Dr. B. E. Fernow, Chief), under the direction of Professor J. B. Johnson. About forty thousand tests were made, distributed over thirty-one American species, the results, so far as obtained, being undoubtedly the most valuable in existence. The detail considered and methods evolved have in a way reclassed the testing of woods and must influence all future efforts, but results are disappointing in that they have been obtained for so few species, and some of these of commercially secondary importance. These experiments are characterized as follows:
These experiments are originally described in Circular No. 15 and other publications of the U. S. Forestry Division, also in "Materials of Construction," by Professor J. B. Johnson. (2) Experiments conducted for the Tenth U. S. Census by Mr. J. P. Sharpless at the Watertown (Mass.) Arsenal upon specimens botanically selected by Professor Sargent. These experiments are less complete in detail, and averages are based upon infinitely
They are originally described in Vol. IX, Tenth U. S. Census; Executive Document No. 5, Forty-eighth Congress, First Session; in Catalogue of the "Jesup Collection," by Professor C. S. Sargent, and elsewhere. (3) Experiments conducted upon full-sized pieces. The most reliable investigations under this head were either conducted by Professor Lanza, of the Massachusetts Institute of Technology, or else are noted by him in his work, "Applied Mechanics" (ed. 1895, pp. 673-711). They are valuable in that specimens were selected on a commercial rather than on a scientific basis. Professor Lanza claims that such actual pieces are less perfect and show approximately one half the unit strength developed by the more carefully selected smaller specimens. These experiments are characterized as follows:
(4) All other experiments. Many experiments have been made from time to time which, while valuable, are not distinguished by any particular method or principle, such as separate the investigations noted in the preceding articles. Data as to selection of specimens, moisture, and other conditions are either incomplete or else absolutely lacking. Such tests are referred to as are noted in works of Hatfield, Trautwein, Lazlett, Rankine, Thurston, and others. Some of these series are exceedingly valuable comparatively. Professor Rankine and Mr. Lazlett experimented principally upon foreign woods. The figures established by the United States Division of Forestry and alluded to in item 1, page 4, appear, so far as they exist, upon the following pages. Where they do not exist, the leading spaces set apart for them are left vacant for other insertions as preferred. All coefficients are in pounds per square inch. Fractions of pounds in weight and lower figures in coefficients have been omitted as superfluous. It is not always easy to determine the species of living trees, because forms in the forest differ from those in the open, because bark varies with age, and because fruit and leaves of many trees are lacking in the winter. It is easier to tell genus than species—that a tree is an oak, than whether it is a red or a pin oak. Experience is required in this connection. Trees are divided into two general divisions known as Exogens and Endogens.[8] FOOTNOTES PLATE 2. EXOGENOUS STRUCTURE IN WOOD. Yearly Rings or Layers. A section of a Longleaf Pine Tree. A section of Oak showing "porous" structure in yearly layers. (Natural size.) A section of Hard Pine showing "solid" structure in layers. (Natural size.) |
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