RESIN PASSAGES 



125 



structural aspects as the epithelial cells. The interchangeable 

 relation between resin cell and parenchyma tracheid as already 

 shown would lead us to suspect a substitution in the composition 

 of the epithelium, and such substitution does actually occur, since 

 it is often to be noted that the second and third rows may be 

 made up, at least in part, of tracheids. 



In a longitudinal radial section the reservoir is found to have 

 the form of a sac of varying form and size, but generally elongated 



prt. 



r.c 



FIG. 39. SEQUOIA SEMPERVIRENS. Transverse section of a resin cyst showing 

 an associate resin cell (r.c.); the epithelium (ep.) ; the thyloses (///.), one of 

 which contains resin, and one of which is directly derived from an epithelium 

 cell ; the parenchyma (fr.t.). X 300 



parallel with the axis of growth and completely closed at both 

 ends (fig. 40). The epithelium, which immediately defines the 

 limits of the sac, generally consists of short, cylindrical cells, 

 while in the second or outer layer the cells become much elon- 

 gated, being several times longer than broad. Beyond this, the 

 third layer consists of parenchyma tracheids, readily distinguish- 

 able whenever the terminal walls lie near the plane of section, or 

 otherwise recognizable, as already indicated. Certain deviations 

 from this typical structure require examination. The resin sacs 



