THE BASIS OF THE STEM STRUCTURE. iSl 



examples it is clear that satisfactory conclusions can scarcely 

 be drawn from data which are so widely variable in the same 

 species or group under different conditions or at different 

 periods of their growth, and it would seem that evidence from 

 other sources than the external parts of the plant must be 

 obtained before any stable relations can be established. Under 

 these circumstances, it is scarcely to be doubted that data 

 derived from the internal structure of the wood will go far 

 toward satisfying the requirements of the case, and we are led 

 to the belief that such data must form an essential element in 

 any future discussion of the systematic relations of plants. 



The data for a differentiation of the Taxaceae and Coniferae 

 are to be found in the occurrence of resin passages, of isolated 

 resin cells, and of medullary rays containing resin passages, as 

 also in the presence or absence of tracheids with spiral mark- 



ings. 



The Coniferae as a whole are distinguished by their more or 

 less, often strongly resinous wood. This is found in some cases 

 to be due to the presence of numerous large channels, — the 

 resin passages, — which traverse the stem longitudinally for 

 great distances, and such structures are always characteristic 

 of Pseudotsuga, Larix, Picea, and Pinus, more rarely appearing 

 in an imperfectly organized form in Sequoia and Abies. On 

 the other hand, the resinous matter is found to have its orio-in 

 in isolated resin cells, which are variously distributed either 

 through the entire body of the growth ring or localized along 

 the outer face of the summer wood. In either case their pres- 

 ence may be at once determined by the peculiarly dark and 

 resinous color of the contents, or by the structure of the ter- 

 minal walls wherever exposed in transverse section. The walls 

 then show a coarsely pitted structure similar to that of a poorly 

 formed sieve plate. The general law of distribution shows that 

 in those woods which have well defined resin passages the resin 

 cells are wanting. Similarly, those woods which have an abun- 

 dance of resin cells show an absence of resin passages — the 

 one replaces the other. Exceptions to this law naturally occur. 

 Thus in Pseudotsuga and Larix, genera which are distinguished 

 by their prominent resin passages, there are also well-defined 



