Page:The Oak.djvu/81

 about six to ten fibers end in a single parenchyma cell, and thus are formed short, tangential rows of wood-parenchyma cells, intercalated, as it were, between the radial rows of other elements (Fig. 12, p). It often happens, moreover, that reticulated and pitted vessels are closely surrounded by wood-parenchyma.

The secondary medullary rays exist as single radial rows of cells, agreeing in form, etc., with the cells of the primary medullary rays. In contact with one another or with wood-parenchyma their walls have simple pits, but they have bordered pits where they abut on tracheids or vessels. In winter these cells are filled with starch. On tangential sections (Fig. 15) it is easy to see how the vertical groups of cells have the same origin as the groups of wood-parenchyma cells—the difference being that the cambial cells which are going to be transformed by horizontal divisions, etc., into vertical rows of ray parenchyma, undergo repeated tangential longitudinal divisions, and so continued radial rows are formed. The cells of these rays are often much shorter than those of the wood-parenchyma, yet all gradations occur. The mother-cells may be very long, evidently corresponding to two, and they may also divide in the radial longitudinal plane, and the ray become biseriate.

These secondary rays start (on the transverse section) from the first large vessels, or from younger ones, or they may start from other points. The ray may sometimes cease within the first year's bundle: but the difficulty