The medullary ray (Plate 47) is a term used to indicate that part of a medullary ray bundle which is seen in cross-sections and in radial sections. In cross-sections the length of the ray will be as great as the length of the bundle, and the width of the ray will be as great as the width of the medullary ray bundle at the point cut across. In longitudinal sections the medullary ray will differ in height according to the thickness of the bundle at the point cut.
When the medullary rays extend from the centre of the stem to the middle bark, they are termed primary medullary rays; when they extend from the cambium circle to the middle bark, they are termed secondary medullary rays. As the plant grows, the diameter of the organ becomes greater and the number of medullary rays are increased. In each of these cases the medullary rays may be one or more than one cell wide, according to whether the medullary ray bundle is one or more than one cell wide. Even in the same plant the width of the medullary rays will vary if the bundle is more than one cell wide, according to width of the medullary ray bundle at the point cut across.
PLATE 47
Radial Longitudinal Section of White Sandalwood
(Santalum album, L.)
1. Medullary ray.
2. Wood fibres and wood parenchyma.
On cross-section the medullary rays are seen to vary greatly. In many plants they are more or less straight radial lines, as in quassia (Plate 105, Fig. 2); while in other plants they form wavy lines where they bend or curve around the conducting cells, as in piper methysticum, kava-kava (Plate 48, Fig. A).
In the study of powdered drugs the radial view of the medullary rays is most frequently seen.
In a perfect radial section (Plate 107, Fig. 2) the medullary rays are seen as tiers of cells in contact throughout their long diameter, and they run at right angles to the long diameter of the other cells. This view of the rays shows the length and height of the medullary ray. In logwood the rays are often forty cells high. In powdered barks, woods (Plate 47), and woody roots the radial view of the medullary rays is frequently diagnostic.
In guaiacum officianale wood the medullary rays are one cell wide on cross-section, and up to six cells high on the tangential section. In santalum album the rays are from one to three cells wide on cross-section, and up to six cells high on tangential section. In the greater number of plants the rays are more than one cell wide.