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 » 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 10-20 square in the spring 
wood, but in summer wood they may be as little as 5y 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 100,. 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. 
