_The medullary rays._ While most of the elements in wood run longitudinally in the log, it is also to be noted that running at right angles to these and radially to the log, are other groups of cells called pith rays or medullary rays (Latin, _medulla_, which means pith). These are the large "silver flakes" to be seen in quartered oak, which give it its beautiful and distinctive grain, Fig. 32, p. 38. They appear as long, grayish lines on a cross-section, as broad, shining bands on the radial section, and as short, thick lines tapering at each end on the tangential section. In other words, they are like flat, rectangular plates standing on edge and radiating lengthwise from the center of the tree. They vary greatly in size in different woods. In sycamore they are very prominent, Fig. 13. In oak they are often several hundred cells wide (_i.e._, up and down in the tree). This may amount to an inch or two. They are often twenty cells thick, tapering to one cell at the edge. In oak very many are also small, even microscopic. But in the conifers and also in some of the broad-leaved trees, altho they can be discerned with the naked eye on a split radial surface, still they are all very small. In pine there are some 15,000 of them to a square inch of a tangential section. They are to be found in all exogens. In a cross-section, say of oak, Fig. 14, it can readily be seen that some pith rays begin at the center of the tree and some farther out. Those that start from the pith are formed the first year and are called primary pith rays, while those that begin in a subsequent year, starting at the cambium of that year, are called secondary rays. [Illustration: Fig. 13. Tangential Section of Sycamore, Magnified 37 Diameters. Note the large size of the pith rays, A, A (end view).] The function of the pith rays is twofold. (1) They transfer formative material from one part of a stem to another, communicating with both wood and bark by means of the simple and bordered pits in them, and (2) they bind the trunk together from pith to bark. On the other hand their presence makes it easier for the wood to split radially. The substance of which they are composed is "parenchyma" (Greek, _beside_, to _pour_), which also constitutes the pith, the rays forming a sort of connecting link between the first and last growth of the tree, as the cambium cells form new wood each year. [Illustration: Fig. 14 Cross-section of White Oak. The Radiating White Lines are the Pith Rays.] If a cambium cell is opposite to a pith ray, it divides crosswise (transversely) into eight or ten cells one above another, which stretch out radially, retaining their protoplasm, and so continue the pith ray. As the tree grows larger, new, or secondary medullary rays start from the cambium then active, so that every year new rays are formed both thinner and shorter than the primary rays, Fig. 14.