Students should be careful to distinguish between the medullary ray bundle, the medullary ray, and the medullary ray cell. In some plants the bundles are only one cell wide, but in other plants the medullary ray bundle is more than one cell wide, frequently several cells wide. THE MEDULLARY RAY BUNDLE The =medullary ray bundle= is made up of a great many medullary ray cells. These bundles (Plate 106, Fig. 5) are of variable length, height, and width. The bundles are isolated, and they occur among and separate the other cells of the plants in which they occur. Tangential sections show the medullary ray bundle in cross-section. Such sections are lens-shaped, and they show both the width and the height of the medullary ray bundle. The length of the medullary ray bundle is shown in cross-sections. THE MEDULLARY RAY 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. [Illustration: 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.