STEMS 257 
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 rectangu- 
lar 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 perigrinum, which is a related species of hore- 
hound and which is a common adulterant of horehound, has 
the following structure (Plate 98): 
The wavy leaf epidermis (1) with stoma; the beaded wall 
petal epidermis (2); the non-glandular, multicellular branched 
hairs (3) from the stem leaves or flowers; the broken pieces and 
branches of the compound hairs (4) scattered throughout the 
