The =collenchyma cells= have very small, angled cavities and very thick walls. These cells make up the greater part of the cortex. The =cortical parenchyma= cells are variable in size and shape. The stone cells occur singly or in groups. The walls are thick, white, porous, and striated, and the central cavity is frequently quite large. =Phloem.= The phloem contains sieve cells, phloem parenchyma, and bast fibres. The =sieve cells= have thin, white, angled walls. The =phloem parenchyma= cells are frequently tangentially elongated, otherwise they resemble the sieve cells. The =bast fibres= occur alone or in groups. The walls are thick, white and porous. [Illustration: PLATE 96 CROSS-SECTION OF STEM OF RUELLIA CILIOSA, Pursh. 1. Epidermis. 2. Collenchyma. 3. Parenchyma. 4. Sclerids. 5. Bast fibres. 6. Phloem. 7. Cambium cells. 8. Xylem. 10. Pith parenchyma.] =Cambium.= The cambium cells are rectangular in shape and the walls are thin. =Xylem.= The xylem contains vessels, wood parenchyma, and medullary rays. The =vessels= are large; the walls are thick, white, and angled. The =wood parenchyma= cells are variable in size and shape and the walls are angled. 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): [Illustration: PLATE 97 POWDERED HOREHOUND (_Marrubium vulgare_, L).