THE BASIS OF THE STEM STRUCTURE. 185 



belong distinctively to the Coniferae, and they are to be 

 regarded as of subordinate value. Thus in tranverse section 

 the tracheids except in Juniperus are, as a rule, much 

 larger, and there is often a more marked and abrupt contrast 

 between the spring and summer woods. In the radial section 

 the Coniferae commonly show Sanio's bands, which are wholly 

 wanting in the Taxaceae, so far as it is possible to determine 

 from our present investigations. 



Anatomical considerations show that the sequence of genera 

 and also the limitations of those groups, as defined on the basis 

 of general morphology, may require some readjustment. It will 

 therefore be desirable to consider somewhat in detail the various 

 points of affinity which justify the arrangement imbodied in the 

 present treatise. 



The four genera, Libocedrus, Cupressus, Thuya, and Juni- 

 perus, fall into a natural group, of which the common character- 

 istics are the presence of more or less numerous resin cells, the 

 chiefly simple pits on the lateral walls of the ray cells, the thin 

 or sparingly pitted terminal walls of the ray cells, and the 

 absence of resin passages. A more critical examination of the 

 distribution of the resin cells shows that Libocedrus and Juni- 

 perus approach one another somewhat closely in the fact that 

 these elements are disposed in tangential bands, while in both 

 Thuya and Cupressus they are scattering and often appear only 

 in somewhat distant growth rings. The affinity between the 

 first two genera is also greatly strengthened by the great simi- 

 larity of the terminal walls of the ray cells. There is an impor- 

 tant point of divergence, however, in the fact that while in 

 Libocedrus the pits on the lateral walls of the ray cells are 

 simple, in Juniperus they are often more or less conspicuously 

 bordered, a feature which tends strongly to give to this latter 

 genus a decided affinity with Taxodium and Sequoia. The 

 separation of these genera from Thuya and Cupressus thus 

 rests upon well-defined differences in the distribution of the 

 resin cells and the structure of the terminal walls of the ray 

 cells. On the other hand, while Libocedrus approaches them 

 through the character of the pits on the lateral walls of the ray 

 cells, by the same character Juniperus is separable from both 



