THE BASIS OF THE STEM STRUCTURE. 191 



outline to the central passage, or space. A separation of these 

 three genera then becomes possible, in the first instance, from 

 the fact that in Picea there is a total absence of resin cells, a 

 fact which serves to give it direct connection with Pinus. 

 Pseudotsuga is the most clearly defined of all genera by reason 

 of its spiral tracheids, a feature which serves to differentiate it 

 not only from Larix, but from all other genera without ques- 

 tion. In cases of possible doubt, however, such as might arise 

 through the removal of the spirals by decay, it may be borne in 

 mind that a further differentiation appears in the large size of 

 the resin passages in the fusiform rays of Larix, while in Pseu- 

 dotsuga they are small and often nearly closed. 



The genus Pinus stands by itself as a well-defined group, 

 which it is impossible to confound with any other genus. As 

 already pointed out, it possesses certain characteristics in 

 common with Picea, Larix, and Pseudotsuga, by reason of the 

 presence of resin passages and fusiform rays, as also in the 

 invariable presence of ray tracheids. It is unique, however, not 

 only in the character of the fusiform rays, but in the nature of 

 the pits on the lateral walls of the ray cells and in the structure 

 of the cells themselves. To these features may also be added 

 the fact that the resin passages are large, always with thyloses, 

 and the epithelium cells are thin walled, forming an entire 

 boundary to the central space. Within its own limits, the 

 genus presents certain well-defined differences, which permit of 

 its separation into two subordinate groups, or sub-genera. The 

 first of these divisions is distinguished by the presence of 

 bordered pits on the tangential walls of the summer wood, 

 while the second is distinguished by the entire absence of these 

 elements. 



From the relations thus indicated the various genera have 

 been arranged in such sequence as to exhibit as nearly as pos- 

 sible their true genetic affinities, and the order now adopted 

 may be taken as expressing these relations in their principal 

 aspects. I am fully sensible, however, of the many imperfec- 

 tions which must enter into a work of this nature a work 

 which is announced for the first time and has not yet gained 

 that measure of solidity which can only come from its actual 



