CONDUCTING TISSUE 131 



to bring the different series of bands in focus. The series of 

 bands are usually of unequal width and length. 



Sclarifomi vessels occur in male fern (Plate 42, Fig. 2), 

 calamus, tonga root (Plate 42, Fig. 3), and sarsaparilla (Plate 



42, Fig. i). 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. i) 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. i) the pits are numerous and very small, and the 

 openings are nearly circular in outHne. 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 



