RESIN PASSAGES 



133 



prt.- 



row, or in the third if present, they become narrow and very 

 long, and they eventually blend with the parenchyma tracheids 

 through intermediate forms with bordered pits. All of the epithe- 

 lial cells are thick-walled and strongly pitted, and they thus offer 

 a somewhat strong contrast to the rather thin-walled parenchyma 

 tracheids with bordered pits. 



The resin passages of Picea differ from those of Pseudotsuga 

 and Larix in being more 

 strictly segregated, and in 

 consequence there is a con- 

 spicuous absence of contig- 

 uous structures, which may 

 nevertheless sometimes be 

 seen in P. nigra, and espe- 

 cially of coalescent forms. 

 They are usually narrow, 

 but well rounded or oval, 

 and there is far greater uni- 

 formity of structure and 

 form than in any of the pre- 

 ceding types. The epithe- 

 lium consists of one row, 

 one to two rows, or even 

 one to three rows of cells, 

 differences which appar- 

 ently belong to particular 

 species, though no attempt 

 has been made to define 

 the precise limitations of 

 such features. The cells are generally small, round, or radially 

 flattened and thick-walled, though occasionally a cell may be 

 thin-walled, as in P. alba. In cases of thick-walled epithelium 

 the outermost cells merge with similar tracheids, from which 

 they are not readily distinguishable, while the general epithelium 

 becomes extended into a tangentially elongated tract, as in Pseu- 

 dotsuga Douglasii and Pinus. Occasionally thyloses have been 



r 



FIG. 43. PICEA ALBA. Transverse section of 

 a resin passage from the spring wood show- 

 ing the central canal (c.); the thick-walled 

 epithelium (t-'J>.), and the parenchyma tra- 

 cheids (pr.t.). x 300 



