8 AMERICAN FOREST TREES 



figure or "grain" of southern yellow pine is largely due to the contrast 

 between the dark summerwood and light springwood of the rings. The 

 same is true of ash, chestnut, and of many other woods. 



Pores Wood is not the solid substance it seems to be when seen 

 in the mass. If magnified it appears filled with cavities, not unlike a 

 piece of coral or honeycomb; but to the unaided eye only a few of the 

 largest openings are visible, and in some woods like maple, none can be 

 seen. The large openings are known as pores. They are so prominent 

 in some of the oaks that in a clean cut end or cross section they look like 

 pin holes. Very little magnifying is required to bring them out distinct- 

 ly. A good reading glass is sufficient. 



Pores belong to hardwoods only. The resin ducts in some soft- 

 woods present a similar appearance, but are far less numerous. All 

 pores are, of course, situated in the annual rings, but in different species 

 they are differently located as to spring and summerwood. In some 

 woods the largest pores are in the springwood only and therefore run in 

 rings. Such woods are called "ring porous," and the oaks are best 

 examples. In other species the pores are scattered through all parts of 

 the ring in about the same proportion, and such woods are called 

 "diffuse porous," as the birches. Softwoods have no pores proper, and 

 are classed "non-porous." 



Medullary Rays A smoothly-cut cross section of almost any oak, 

 but particularly white oak and red oak, exhibits to the unaided eye 

 narrow, light-colored lines radiating from the center of the tree toward 

 the bark like spokes of a wheel. They are about the breadth of a fine 

 pencil mark, and are generally a sixth of an inch or less apart. They 

 are among the most conspicuous and characteristic features of oak wood, 

 and are known as medullary or pith rays. 



Oak is cited as an example because the rays are large and promi- 

 nent, but they are present in all wood, and constitute a large part of its 

 body. They vary greatly in size. In some woods a few are visible 

 unmagnified ; but even in oak a hundred are invisible to the naked eye 

 to one that can be seen. Some species show none until a glass is used. 

 Some pines have fifteen thousand to a square inch of cross section, all of 

 which are so small as to elude successfully the closest search of the 

 unaided eye. 



The medullary rays influence the appearance of most wood. 

 They determine its character. Oak is quarter-sawed for the purpose of 

 bringing out the bright, flat surfaces % of these rays. The prominent 

 flecks, streaks, and patches of silvery wood are the flat sides of medul- 

 lary rays. In cross section, only the line-like ends are seen, but 

 quarter-sawing exposes their sides to view. 



