This section is from the "Histology of Medicinal Plants" book, by William Mansfield.
The annular vessels are thickened at intervals in the form of rings (Plate 40, Fig. 1), which extend outward from and around the inner wall of the vessel. In fact, it is the inner wall which is thickened in all the different types of vessels. The ring-like thickening usually separates from the wall when the drug is powdered. Such separated rings occur frequently in powdered digitalis, belladonna, and stramonium leaves. Annular vessels are not, however, of diagnostic importance, because more characteristic cells are found in the plants in which they occur. Not infrequently a vessel will have annular thickenings at one end and spiral thickenings at the other. Such vessels are found in the pumpkin stem (Plate 40, Fig. 1).
Vessels are distinguished from other cells by their arrangement, by their large size when seen in cross-section, and by the thickening of the wall when seen in longitudinal sections of the plant or in powders. The side walls of vessels are thickened in a number of striking yet uniform ways. The chief types of thickening of the wall, beginning with one that is the least thickened, are annular, spiral, sclariform, pitted, and pitted with bordered pores.
Plate 40. Annular and Spiral Vessels.
1. Pumpkin stem (Cucurbita pepo, L.).
2. Two characteristic views of spiral vessels.
3. (A) Upper part of spiral vessel in focus. (B) Under part of spiral vessel in focus.
4. Spiral vessel of the disk petal matricaria (Matricaria chamomilla, L.).