14 INTRODUCTION 



(vii) Horizontal resin ducts are formed in some of the 

 medullary rays. According to Willkomm these may be 

 made as early as the second year, but I have not observed 

 any ducts in such young stems. 



Cambium. This is composed of very narrow thin-walled 

 elongated cells. They are mostly about 10-20 /x in tangential 

 direction, but those which form medullary rays are narrower. 



Xylem. This has the characteristically simple form of 

 the Abietineae, and is composed of tracheides, tannin- 

 containing cells, resin ducts, and medullary rays. 



(i) The tracheides compose the greater part of the wood. 

 In transverse section they are 10-20pi square in the spring 

 wood, but in summer wood they may be as little as 5// in 

 radial measurement. They have lignified walls with a single, 

 or occasionally double, row of bordered pits on the radial 

 walls, and the last two or three rows of summer wood have 

 them also on the tangential walls. 



(ii) Tannin-containing cells have the same form in trans- 

 verse section as the tracheides. They have, however, 

 cellulose walls with simple pits, and accurately transverse 

 septa occur at distances of about lOOjx. The simple pits 

 connect with the bordered pits of adjoining tracheides or 

 with the simple pits of medullary ray parenchyma cells. 

 Their contents give reactions for tannin and protein, but not 

 for resin, at any rate in March. They probably correspond 

 to resin cells in allied types. 



(iii) Resin ducts occur scattered irregularly in the summer 

 wood. They are always small, and are formed in the 

 following way. Four cells, which correspond to tracheides, 

 form an intercellular space between them, and, remaining 

 thin walled and alive, secrete resin into this space. The 

 intercellular space then becomes the duct, and the four cells 

 correspond to the epithelium. The walls of the latter may 

 become weakly lignified, though remaining thin, and the 

 cells may divide so that there are ultimately six or eight 

 epithelial cells surrounding a duct. About nine or ten ducts, 

 but generally less, may be formed in the wood of the first 

 year, and corresponding numbers in succeeding years. 



