[Footnote 5: Roth, U. S. Forestry Bul. No. 10, pp. 64-71. Also von Schrenk, U. S. Dept. Agriculture, Bureau Plant Industry Bul. No. 14, pp. 12-16.] [Footnote 6: Only because imperfections are more likely in larger pieces. Large and small pieces of equally perfect wood are equally strong. (See Publications U. S. Forestry Div. and J. B. Johnson's "Materials of Construction," p. 462.)] [Footnote 7: It should be noted that the selection and preparation of specimens require the exercise of more judgment than the simple testing of specimens, if the conclusions are to be such that they can be generalized from.] [Footnote 8: This division coincides with that by which they are separated into Dicotyledons and Monocotyledons.] [Illustration: 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.)] EXOGENOUS TREES. (_Dicotyledons_.) Exogenous trees are those the trunks of which are built up by rings or layers, each deposited consecutively upon the outside of the others. A section exhibits first a central point or canal known as a pith-cavity, next and consecutively the annual layers, and finally the bark. The woods of this series are familiar to all. The oaks, pines, and practically all of the merchantable lumbers are among them. The forests are widely distributed, and the species are so numerous as to present an almost infinite range of possibilities. [Illustration: FIG. 2.--SECTION BOXELDER, SHOWING PITH-CAVITY AT CENTRE.] The young wood of exogenous trees is porous. It permits the passage of sap and is known as sapwood (Alburnum). As a layer is enclosed by others and retreats from the surface of the tree, it becomes denser, its canals are filled with gums or tannin, color changes, and the result is heartwood (Duramen). This change goes forward rapidly in some trees, such as locusts, so that their sections appear to be almost wholly heartwood; other species require longer time, and sapwood then predominates. Heartwood gives stability to the tree, but is not needed in its physiological processes. It is tougher, heavier, stronger, and more valued in construction. Sapwood is vitally essential to the life of the tree, but is lighter, weaker, less durable, and less valued in construction. Sapwood is pliable, and the sapwoods of several trees are valued for this reason.