Sieve Plate

Sieve plates on cross-section (Plate 46, Fig. 2) are polygonal in outline, and the pores are either round or angled. Large sieve tubes and sieve plates occur in pumpkin stem; but, almost without exception, in drug plants the sieve tubes are small and the sieve plate is inconspicuous. When the drug is powdered, the sieve tubes break up into undiagnostic fragments. When studying sections of the plants, the extent, size, and arrangement of the sieve tubes must always be noted.

Medullary Bundles, Rays, And Cells

Function

The medullary ray cells are the lateral conducting cells of the plant. They conduct outwardly the water and inorganic salts brought up from the roots by the vessels and tracheids; and they conduct inwardly toward the centre of the stem the food material manufactured in the leaves and brought down by the sieve cells. The medullary rays thus distribute the in-, organic and organic food to the living cells of the plant, and they conduct the reserve food material to the storage cells, and, lastly, they function in certain plants as storage cells.

Occurrence

The form, size, wall structure, and the distribution of the medullary ray bundles, rays, and cells are best ascertained by studying: first, the cross-section of the plant; secondly, the radial section; and, thirdly, the tangential section.

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.

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.

The Medullary Ray Cell

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.