No. 449-] NORTH AMERICAN CONIFERALRS. 345 



nature that the simply dentate tracheid is the most rudimentary, 

 while the strongly reticulated is of the most advanced type of 



The value of the ray tracheid for taxonomic purposes depends 

 upon: (i) its occurrence in certain genera, and (2) its structural 

 peculiarities. The simple wall of the tracheid, in the f;reat 

 majority of cases, affords no basis of specific differentiation, but 

 in the various forms of dentate and reticulated walls ot the 

 second section of Pinus, it is of well defined vahic in this 

 respect. Finns resinosa, P. tJinnbergii and /'. ko mi ens is are 

 all characterized by the occurrence of simi)lc tectli wiiicii are 

 sometimes sparingly developed. This feature is intimately asso- 

 ciated with the occurrence of large, simple and single pits on the 

 lateral walls of the ray cells. From this grouj) /'. doisijlora 

 may be differentiated by the reticulations in the tracheids of the 

 summer wood. Among the hard pines, P. tceda is distinguished 

 by ray tracheids which are typically sparingly reticulated through- 

 out, but on the other hand, P. pabistris and P. cubensis, which 

 probably represent the highest types of the genus, are at once 

 separated from all other species by reason of the extent to which 

 reticulations are developed. 



The relations which the tracheids bear to the parenchyma 

 cells in the general composition of the ray also have an impor- 

 tant bearing upon specific differentiations. In the genus Tsuga 

 the tracheids are sometimes interspersed, affording the first 

 instance of a relation which later becomes most prominent in the 

 higher genera, and the same relation is also expressed in Pseu- 

 dotsuga and Larix. In Picea there is a somewhat stronger 

 tendency to an interspersal which is only expressed fully in 

 Pinus. In the soft pines eleven out of thirteen species show, 

 as a rule, as in the previous genera, that the tracheids are rarely 

 interspersed, P. aristata forming a partial exception, as shown 

 in a sparing interspersal. P. nionophylla and P. monticola, on 

 the other hand, show a strong interspersal of the tracheids, 

 and in this respect they approach the hard pines. In the 

 latter group we again find the first four species character- 

 ized by a rare interspersal. But passing on to the more highly 

 developed species, such types as P. claiisa, P. pabistris and P. 



