[PKNHALLOW] NOTES ON TERTIARY PLANTS B5 



to point out that the chiefly 3-seriate form of the larger rays as seen 

 in the tangential section, has been selected as the basis of specific differ- 

 entiation. 



Biological Considerations. 



The present studies have served to give emphasis to the fact that 

 in the Coniferse, certain species show more or less well defined devia- 

 tions from the structural characteristics which may be regarded as 

 constituting normal specific or generic features. It was shown some 

 years since that in those coniferous woods which have resin passages 

 traversing the stem longitudinally, there are also similar resin passages 

 extending radially through certain of the medullary rays which thereby 

 become specially altered,^ and the law of association thus indicated, 

 has been found to be so constant for the Xorth America Coniferge, 

 that where one of these structures is found the other may be inferred. 

 ^Nevertheless, it was pointed out at that time that certain well defined 

 exceptions to this law are to be recognized in species of Abies and 

 Sequoia, where imperfectly organized resin passages traverse the wood 

 longitudinally, but without a trace of corresponding canals in the 

 medullary rays. At that time the possible significance of these facts 

 did not appear, and they were looked upon as of a sporadic nature and 

 therefore of litth significanice, but the more recent discovery that 

 they have persisted in some cases since Cretaceous time, has invested 

 them with a new interest and makes it important that their relation 

 to the general course of development should be ascertained. 



Although the material at hand is not very extensive, it neverthe- 

 less seems desirable to examine the data it presents with a view to 

 determining the bearing of the facts upon the evolution of the species, 

 and at least to establish a basis which may serve as the starting point 

 for further treatment as additional material may appear in the future. 



In order to gain a clear conception of the nature of the facts in 

 question, and their full significance from the standpoint of develop- 

 ment, it will be necessary to briefly review certain structural characters 

 of the Coniferae and establish their bearing upon the relations of the 

 various species and genera, and for our present purpose it will be 

 useful to consider the Taxacege and Coniferœ as a whole. While the 

 present discussion relates exclusively to the anatomical characters 

 developed within the xylem region, and therefore takes no cognizance 

 of those characters of foliage and inflorescence upon which relations 

 are more commonly established, it will nevertheless come within the 

 scope of our present purpose to eventually compare the relations deter- 



^ Trans. R, Soc. Can., IL, iv., 39. 



