WOODS. 147 



toxylon and Santalum), bitter products (Quassia) — 

 in general, products of destructive metabolism. 



Although from the medical standpoint the woods are of 

 small interest, they are of more importance to the phar- 

 macognocist, for they are frequently used to adulter- 

 ate ground drugs, and they form, though in small quan- 

 tities, an important part of the stems and roots of official 

 drugs. 



In the narrow sense of the word the "woods" include 

 the xylem elements of the dicotyledonous stem; the 

 phloem elements of the bundle generally adhere to the 

 bark. 



These elements, it may be recalled, are: 



(i) Ducts or vessels. 



(2) Tracheids. 



(3) Woody fibres (libriform). 



(4) Parenchyma. 



(a) Wood parenchyma. 

 (6) Simple parenchyma. 



(i) The ducts or vessels are characteristic of woody 

 tissues. They are usually lignified, have thick walls, 

 and the end cell membranes have become absorbed. 

 The walls are provided with variously shaped and sized 

 pores and may be sculptured in a variety of ways, giving 

 rise to. the various types of vessels. 



(a) Spiral. — Cell wall thickening spiral. 



(6) Annular. — Cell wall thickening annular or ring-like. 



(c) Reticulated. — Cell wall thickening net-form. 



(d) Pitted. — Pits or pores which may be simple or 

 bordered. Various transitional forms are to be encoun- 

 tered and they all spring from the cambium. 



(2) The tracheids are to be distinguished from the ducts 

 by the presence of cross walls that have not been ab- 

 sorbed. In other respects they resemble the ducts quite 

 closely. 



The wood of the Conifers is distinguished by having 



