132 



ANATOMY OF THE GYMNOSPERMS 



In Larix the same features of contiguity and coalescence may 

 be observed, except that in L. occidentalis the resin passages 

 sometimes form into continuous zones of imperfectly organized 

 structures with the aspect presented in Tsuga Mertensiana. The 

 epithelium is always well defined (fig. 42), and it consists of 

 one, sometimes two, rows of cells. The cells of the first row are 



small, very variable in form and 

 size, thick-walled, and more or 

 less strongly flattened radially. 

 They are also commonly resin- 

 ous and more or less strongly 

 pitted. When there is a sec- 

 ond row of epithelium the cells 

 are essentially like the wood 

 tracheids, and like the paren- 

 chyma tracheids, from which 

 they may be separated with 

 difficulty. The latter, there- 

 fore, which are absent from the 

 summer wood, can be distin- 

 guished from the elements of 

 the spring wood only when 

 the pits on the terminal walls 



FIG. 42. LARIX OCCIDENTALIS. Trans- (fig. 42, pr.t.} are brought into 

 verse section from the inner spring v j ew> or> more ra rely, when the 

 wood showing a pair of resin passages . . 



with the central canals (,-.) ; the thick- P lts on the tangential walls are 

 walled epithelium (<?/.) ; a parenchyma in evidence. Thyloses rarely 



tracheid (fr.t.}, and the summer wood r 



/ w i x 300 occur, and so far they have been 



noted only in L. occidentalis, 



where they are infrequent and thick-walled, and in L. americana, 

 where they are of rare occurrence and thin-walled. In longitu- 

 dinal section the central canal is always continuous, though con- 

 stricted at intervals, a feature in all essential respects the same 

 as in Pseudotsuga. Radially the first row of epithelial cells are 

 short cylindrical, or in L. occidentalis short fusiform, but there is 

 a graduated increase in length outwardly, so that in the second 



s.w. ; 



'.W. 



