Inner Bark. The cells forming the inner bark are medullary rays, parenchyma, sieve cells, and storage cavities.
PLATE 103
Cross-Section of Unrossed White Pine Bark (Pinus strobus, L.)
1. Cork cells of the epidermis. 2. Parenchyma cells filled with chlorophyl. 3. Intercellular space. 4. Secretion cavity with resin. 5. Secretion cells. 6, One or more circles of parenchyma filled with chlorophyl. 7. Parenchyma. 8. Medullary rays. 9. Sieve cells. 10. Storage cavities.
The medullary rays form wavy lines. The medullary ray cells are radially elongated, rectangular in shape, and they contain granular cell contents. The sieve cells are either square or rectangular in shape. The walls are thin and white. The storage cavities are either filled with starch or with prisms and tannin.
POWDERED WHITE PINE BARK
White pine bark (Plate 104) when powdered shows the following characteristic elements:
The microscopic structure of a powdered white pine is as follows: The epidermis (1) consists of reddish-brown masses, irregular in outline. The outer parenchyma cells are of a bright-green color, owing to the presence of chlorophyll. (The above elements are not usually found in the rossed bark.) The parenchyma (3) with starch usually occurs in longitudinal sections accompanied with sieve cells. Often the tissue separates transversely, showing the medullary rays (4) with their granular cell contents (9) and the inner parenchyma cells filled with starch and the surrounding sieve cells.
The crystals are nearly perfect cubes and occur singly (5) or in groups (6). On the longitudinal section of the bark the crystals occur in parenchyma cells surrounded by a reddish cell content and form parallel rows which are very characteristic. The resin occurs either as white, angled fragments (7) in a water mount, or as globular mass (8) or as reddish-brown pieces (10). The starch is very abundant and is distributed through the field. The diagnostic grain is lens-shaped, with a cleft hilum, which is nearly straight, or slightly curved, and runs parallel to the long diameter of the grain. The addition of ferric chlorid T. S. will show the presence of tannin by forming a dark coloration. The identification of the starch is facilitated by the addition of a weak Lugol’s solution, which imparts a blue coloration to the starch grain.
The form, amount, and distribution of the cells composing the bark differ greatly in different plants.