Now suppose that laid among the ordinary thin-walled tubes were quite large tubes, so that one could tell the "ring" not only by the thin walls but by the presence of large tubes. That would represent the ring-porous woods, and the large tubes would be called vessels, or _tracheæ_. Suppose again that these large tubes were scattered in disorder thru the layers. This arrangement would represent the diffuse-porous woods. By holding up to the light, thin cross-sections of spruce or pine, Fig. 15, oak or ash, Fig. 16, and bass or maple, Fig. 17, these three quite distinct arrangements in the structure may be distinguished. This fact has led to the classification of woods according to the presence and distribution of "pores," or as they are technically called, "vessels" or "tracheae." By this classification we have: (1) _Non-porous_ woods, which comprise the conifers, as pine and spruce. (2) _Ring-porous_ woods, in which the pores appear (in a cross-section) in concentric rings, as in chestnut, ash and elm. (3) _Diffuse-porous_ woods, in which (in a cross-section) the rings are scattered irregularly thru the wood, as in bass, maple and yellow poplar. In order to fully understand the structure of wood, it is necessary to examine it still more closely thru the microscope, and since the three classes of wood, non-porous, ring-porous and diffuse-porous, differ considerably in their minute structure, it is well to consider them separately, taking the simplest first. [Illustration: Fig. 15. Cross-section of Non-porous Wood, White Pine, Full Size (top toward pith).] _Non-porous woods._ In examining thru the microscope a transverse section of white pine, Fig. 18: (1) The most noticeable characteristic is the regularity of arrangement of the cells. They are roughly rectangular and arranged in ranks and files. (2) Another noticeable feature is that they are arranged in belts, the thickness of their walls gradually increasing as the size of the cells diminishes. Then the large thin-walled cells suddenly begin again, and so on. The width of one of these belts is the amount of a single year's growth, the thin-walled cells being those that formed in spring, and the thick-walled ones those that formed in summer, the darker color of the summer wood as well as its greater strength being caused by there being more material in the same volume. [Illustration: Fig. 16. Cross-section of Ring-porous Wood, White Ash, Full Size (top toward pith).] [Illustration: Fig. 17. Cross-section of Diffuse-porous Wood, Hard Maple, full size (top toward pith).] (3) Running radially (up and down in the picture) directly thru the annual belts or rings are to be seen what looks like fibers. These are the pith or medullary rays. They serve to transfer formative material from one part of the stem to another and to bind the tree together from pith to bark.