STEMS 



237 



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 



