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.
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.