This section is from the "Histology of Medicinal Plants" book, by William Mansfield.
Pitted vessels are met with most frequently in woods and wood-stemmed herbs. There are two distinct types of pitted vessels - i.e., simple pitted vessels and pitted vessels with bordered pores.
The pitted vessel represents the highest type of cell-wall thickening. The entire wall of the vessel is thickened, with the exception of the places where the pits occur. The number and size of the pits vary greatly in different drugs. In quassia (Plate 44, Fig. 1) the pits are numerous and very small, and the openings are nearly circular in outline. In white sandalwood (Plate 44, Fig. 3). the pits are few in number, but when they do occur they are much larger than are the pits of quassia.
Plate 44. Pitted Vessels.
1. Quassia, low magnification (Purana excelsa, [Swartz] Lindl.).
2. Quassia, high magnification.
3. White sandalwood (Santalum album, L.).