The medullary ray cells are radially elongated and rectangular in shape.
Pith Parenchyma. The pith parenchyma cells are large and rounded in shape.
POWDERED HOREHOUND
The structure of powdered horehound is shown in Chart 97. The epidermal cells of the leaf (1) are wavy in outline, the guard cells are elliptical, the stoma lens-shaped, the epidermis often showing hairy outgrowth as in the illustration. The epidermal cells of the petals (2) have irregularly thickened beaded walls. The non-glandular hairs from the calyx (3); the long, thin-walled, multicellular non-glandular twisted hairs (4) from the leaves and stems; long, thin-walled, unicellular hairs (5) from the tube of the corolla; the glandular hairs (6) with a one-celled stalk and with two secreting cells divided by vertical walls; the eight-celled glandular hair (7) as seen in surface and side view; the spiral and reticulated conducting cells (8); the thick, white-walled fibres from the stem (9); the pollen grains (10) with nearly smooth walls.
The diagnostic elements of the U. S. P. horehound are the long, twisted, multicellular hairs (4), the glandular hairs (7), and the pollen grains (10).
POWDERED SPURIOUS HOREHOUND
Marrubium peregrinum, which is a related species of horehound and which is a common adulterant of horehound, has the following structure (Plate 98):
PLATE 97
Powdered Horehound (Marrubium vulgare, L).
1. Epidermis of leaf showing the wavy epidermal cells, stoma, and a clustered hair. 2. Surface view of the petal epidermis. 3. Non-glandular hair from the calyx or corolla. 4. Long, thin-walled, twisted, non-glandular hairs from the leaves and stem. 5. Unicellular non-glandular hair from the tube of the corolla. 6. Glandular hairs with a one-celled stalk and with two secreting cells divided by vertical walls. 7. Surface and side view of the eight-celled glandular hairs. 8. Conducting cells. 9. Fibres from the stem. 10. Pollen grains.