The cross-section of a buchu stem (Plate C), 1.6 millimeters in diameter, shows a few of the epidermal cells modified into thick-walled, roughish, unicellular trichomes (1). The remaining epidermal cells have a thick, wavy outer wall (2). Beneath the epidermis are several rows of cortical parenchyma cells (3) which extend to the bast bundles and in which are found the secretory cavities with the thin-walled secretory cells (4). The bast fibres (5) occur in continuous bands, varying greatly in size; the walls are whitish and of variable thickness. Inside the bast fibres, the small irregular sieve cells (6) occur in groups, surrounded by the phloem parenchyma (8). The radially elongated cells of the medullary rays (7) extend outward from the xylem, increasing in number in the outer portions of the wood, and extending nearly to the bast fibres. No distinct cambium layer is visible. The conducting cells (9) occur throughout the xylem surrounded by the wood fibres and wood parenchyma (10). The latter is not very abundant in buchu. The medullary rays border on the conducting cells and extend outward to the phloem. The pith parenchyma cells are nearly circular in outline and often show a perforated end wall when a cell happens to be cut just above or below that point.

Plate 100. Cross-Section of Buchu Stems (Barosma betulina [Berg.], Barth. and Wendl.).

1. Hairs. 2. Wavy epidermis. 3. Cortical parenchyma. 4. Secretion cells and cavity. 5. Group of bast fibres. 6. Sieve cells. 7. Medullary rays. 8. Phloem parenchyma. 9. Vessels. 10. Wood fibres, and wood parenchyma. 11. Pith parenchyma.

Mature Buchu Stem

In Plate 101-A is shown the cork formation or secondary growth as seen in the older, larger buchu stems. The wavy epidermis (1), which is the primary epidermis and which has disappeared on many portions of the stem, has thin side walls and dark cell contents (2). Next to the epidermal cells occur several rows of peculiarly arched cork cells with thick, white outer walls (3) and reddish-brown cell contents (4). The cork cambium (5) is typical in form, and it has formed one or two layers of phelloderm cells (6) which have the same form as the cambium cells but with thicker walls. Next to the phelloderm occur the cortical parenchyma cells. The remaining structure of the mature stem is identical with that of Fig. 2.

Plate 101. A. Cross-section of buchu stem (Barosma betulina [Berg.], Barth. and Wendl.).

I, Outer wall of epidermis; 2, Cell cavity of epidermal cell; 3, Wall of cork cell; 4, Cavity of cork cell; 5, Phellogen layer; 6, Divided phellogen cell changing into a cortical parenchyma cell; 7, Cortical parenchyma cell.

B. Cross-section of leptandra rhizome (Leptandra virginica [L.], Nutt.). I, Parenchyma cells undergoing change in the composition of their walls; 2, A break in the epidermal tissue; 3, Parenchyma cells undergoing division.

Powdered Buchu Stem

Powdered buchu stem (Plate 102) has many striking features which make it easy of identification when mixed with buchu leaves. A few unicellular, rough, thick, white-walled trichomes (1) occur distributed throughout the field. They are straight or slightly curved and vary in length from 40 to 100 microns; in thickness at the bast they measure from 10 to 22 microns. The central cavity varies greatly, and in some trichomes seems to have disappeared entirely. The epidermal cells (2) are very characteristic, occurring singly or in groups of two or more. The cells from the older stems often appear reddish brown by transmitted light, while the epidermal cells from the younger stems appear whitish opaque (porcelain-like). They are usually six-sided and angular in outline. The cortical parenchyma cells (3) on transverse view have a rounded cell cavity and intercellular spaces between the walls. The double walls vary in thickness, the greatest thickness being about 9 microns. The parenchyma cells (3) on longitudinal view show square ends and often contain sphaero-crystalline masses of hesperidin. The thin-walled sieve cells and the surrounding cells are scarcely ever seen in the powder. The white-walled pointed stereomes (4) are a characteristic feature of the powder; they vary greatly in length, in diameter and in the thickness of their walls. In a number eighty powder the fibres are mostly broken. The greatest length of the unbroken fibres is 1.25 microns. The thickest wall measured 5 microns and the greatest observed width was 25 microns. The spiral reticulate and scalariform thickened conducting cells occur scattered throughout the powder. The reticulate and scalariform cells usually occur with wood fibres. It is an interesting fact that the spiral thickening in conducting cells is usually separate from the side wall and nearly always appears as indicated at 5. An occasional rosette crystal of calcium oxalate (6) is seen in the field. The wood parenchyma (7), which makes up a very small percentage of the xylem, is not readily found in the powder. The pith parenchyma cells (8) have thick, porous side walls and perforated side walls. The wood fibres (9) usually occur in masses surrounding the conducting cells; when occurring singly, the oblique pores readily distinguish them from the bast fibres.

The diagnostic elements of powdered buchu stems are: First, trichomes; secondly, reddish-brown and white-angled epidermal cells; thirdly, the long, white bast fibres.

Plate 102. Powdered Buchu Stems {Barosma betulina (Berg.], Barth. and Wendl.).

1. Hairs. 2. Epidermal cells, the larger pieces reddish-brown; the smaller aggregations white. 3. Transverse cortical parenchyma. 3'. Longitudinal cortical parenchyma with sphaero crystalline masses of hesperidin. 4. Bast fibres. 5. Spiral, sclariform, and reticulate vessels. 6. Rosette crystals of calcium oxalate. 7. Wood parenchyma. 8. Pith parenchyma with porous side and end walls. 9. Wood fibres.