The medullary ray cell (Plate 48, Fig. 1) is one of the individual cells making up the medullary ray bundle and the medullary ray.
The cross-sections of the cells which are seen in tangential sections show the cells to be mostly circular in outline when they occur in the central portion of medullary ray bundles of more than two cells in width; but they are more irregular in outline when the medullary ray bundle is only one cell wide. Even the cells of the three or more cell-wide bundles have irregular, outlined cells at the ends of the bundle and on the sides in contact with the other tissues.
The length and height of the medullary ray cell are shown in radial sections; while the width and length of the medullary ray cells are shown in cross-sections.
Structure of Cells
The structure of the individual cells forming the medullary rays differs greatly in different plants, but is more or less constant in structure in a given species.
The medullary rays of the wood usually have strongly pitted side and end walls, while the medullary rays of most barks are not at all, or only slightly, pitted. In most plants the cells are of nearly uniform size. Frequently, however, the cells vary in size in a given ray, as shown in the cross-section of kava-kava.
Arrangement of the Cells in a Ray
The union of any two cells in a ray is also of importance. In quassia the medullary ray cells have oblique end walls, so that on cross-section the line of union between two cells is an oblique wall. In most plants the medullary ray cells have blunt or square or oblique end walls, so that the line of union is a straight line.
In most plants the cells are much longer than broad, but the cells of sassafras bark are nearly as broad as long.
The walls of the cortical medullary ray cells and the medullary rays of most roots and stems of herbs are composed of cellulose; while the walls of medullary ray cells occurring in woods are frequently lignified.