The structure of powdered insect flower stems (Chart 99) is as follows:
The epidermal cells of the stems are prominently marked with stoma and angled, striated wall cells (Fig. 1). On cross-section (Fig. 2) the stem is seen to be made up of epidermal cells with thick outer and thin side walls (Fig. 2). The T-shaped hairs (Fig. 3) are longer than those found on any other part of the plant. The fibres (Fig. 4) are the most characteristic part of the powder. They are elongated, and the walls are white and slightly porous and of nearly uniform thickness. They occur free in the field or in groups of two or more. The cross-section view of these fibres is shown in Fig. 5. The pith parenchyma (Fig. 6) is abundant and is composed of thick, porous-walled cells. On cross-section the cells are rounded and are separated by intercellular spaces. The conducting cells (Fig. 8) vary from spiral to reticulate.
CHAPTER III
WOODY STEMS
BUCHU STEM
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.
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 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.