THE AM ERIC. 



A LIST. [Vol. XXXVI I L 



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

 are of rare occurrence and thin-walled. In longitudinal section 

 the central canal is always continuous, though constricted 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, or 

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

 and they eventually merge with the parenchyma tracheids through 

 intermediate forms with bordered pits. All of the epithelial 

 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 conspicuous absence of contiguous structures, which 

 may nevertheless sometimes be seen in P. nigra, and especially 

 of coalescent forms. They are usually narrow, but well rounded 

 or oval, and there is far greater uniformity of structure and 

 form than in any of the preceding types. The epithelium con- 

 sists of one row, one to two rows, or even one to three rows of 

 cells, differences which apparently belong to particular species, 

 though no attempt has been made to define the precise limita- 

 tions oL such features. The cells are generally small, round or 

 radially flattened and thick-walled, though occasionally a cell 

 may be thm-walled as in P alba- In cases of thick-walled epi- 

 thelium, the outermost cells meige with similar tricheids fiom 

 which they are not readily distinguishable, while the general 

 epithelium becomes extended into a tangentially elongated tiact 

 as in P auiotmoa dougla n and Pmus. 1 h> lo-es haxe been 

 n<.t..l as of occasional occuiicnce m P mo, a, P pmioin^ '^^^<^ 

 r siklicnus. but the) irc ahNa\s thm u ailed Paiench>ma 

 Irachieds are not obvious in the summer wood, but they are 

 iLu.<,ni/ablt in the spting xxood wheie the> appeal to leplace 

 the lesm tells, though thc\ arc appaienth of'much less ficquent 

 <KcunuKL than in the i^cneia picMouslv discussed In P alba, 

 h<n\.\u (1 1^ 4S p, O, ue sometimes find a radial seiicb of 

 ti uIkuU uluUi lis.) extends lateiall) so as to form an enclosing 



