CONDUCTING TISSUE 131 
to bring the different series of bands in focus. The series of 
bands are usually of unequal width and length. 
Sclariform vessels occur in male fern (Plate 42, Fig. 2), 
calamus, tonga root (Plate 42, Fig. 3), and sarsaparilla (Plate 
42, Fig. 1). In each they are characteristic. Sclariform vessels, 
with these few exceptions, do not occur in drug plants. In fact, 
drugs derived from dicotyledones rarely have sclariform vessels. 
They occur chiefly in the ferns and drugs derived from mono- 
cotyledenous plants. Their presence or absence should, there- 
fore, be noted when studying powdered drugs. 
RETICULATE VESSELS 
Reticulate vessels are of common occurrence in medicinal 
plants. In fact, they occur more frequently than any other 
type of vessel. The basic structure of reticulate vessels (Plate 
43, Fig. 1) occurring in different plants is similar, but they vary 
in a recognizable way in different plants (Plate 43, Fig. 2). 
The walls of reticulate vessels are thickened to a greater extent 
than are the walls of spirally thickened vessels. 
PITTED VESSELS 
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. 
PITTED VESSELS WITH BORDERED PORES 
Pitted vessels with bordered pores are of common occur- 
rence in the woody stems and stems of many herbaceous plants 
(Plate 45, Figs. 3 and 4). In such vessels the wall is unthickened 
for a short distance around the pits. This unthickened portion 
