There is a great variation in the character of the cell contents of medullary rays. In white pine bark (Plate 48, Fig. B1) are deposits of tannin; in quassia wood, starch; in canella alba, rosette crystals of calcium oxalate, etc.
LATEX TUBES
Living latex tubes, like sieve tubes, have a layer of protoplasm lining the walls, and, in addition, have numerous nuclei. In drug plants the nuclei are not distinguishable, but the protoplasm is always clearly discernible.
PLATE 48
A. Cross-section of kava-kava root (Piper methysticum, Forst., f.).
1. Unequal diameter medullary ray cells.
2. Vessels.
3. Wood parenchyma.
4. Wood fibres.
B. Cross-section of white pine bark (Pinus strobus, L.).
1. Wavy medullary rays with tannin.
2. Parenchyma cells.
3. Sieve cells.
Latex tubes function both as storage and as conducting cells. They, like the sieve tubes, contain proteid substances chiefly, yet frequently starch is found. The cells bordering the latex tubes absorb from them, as needed, the soluble food material. While our knowledge concerning the function of latex in some plants is meagre, still in other plants it is practically certain that the latex is composed of nutritive substances which are utilized by the plant as food. In certain other plants the latex appears to be used as a means of resisting insect attacks and as a protection against injury.
There are two types of latex tubes common to plants, namely, latex cells and latex vessels. Latex tubes developing from a single cell do not differ materially from a latex tube originating from the fusion of several cells. In each case the latex tube branches to such an extent that it bears no resemblance to ordinary cells. It would seem that the ultimate branches are formed and develop in much the same manner as root hairs—that is, by a growing tip of the branch. A mature plant may therefore have latex tubes with almost numberless branches (Plate 50, Fig. 1) and be of very great length.
The branches of latex tubes develop in such an irregular manner that it is possible to obtain a cross and a longitudinal section of the latex tubes by making a cross-section of stem. Such a section is shown in the drawing of the cross-section of the rhizome of black Indian hemp (Plate 49, Fig. B).
The color of the latex in medicinal plants varies from a gray white in papaw (carica papaya), aromatic sumac, black Indian hemp, and bitter root, to white in the opium poppy, light orange in celandine, and deep orange in bloodroot (Plate 50, Fig. 2). In each of these cases it is the latex which yields the important medicinal products.