122 ANATOMY OF THE GYMNOSPERMS 



Furthermore, from another point of view, the gradual replace- 

 ment of the resin cells appears to be indicated by a correspond- 

 ing reduction in the contained resin. Nowhere is the resin so 

 abundant in the resin cells as in those genera like Podocarpus 

 and Taxodium, which show no development of resin passages, 

 even in their most simple forms ; but with the development of 

 resin sacs, as in Abies or Sequoia, or of resin passages, as in 

 Larix and Pseudotsuga, there is a remarkable diminution of the 

 resin, apparently in direct response to its more ready production 

 by more specialized structures. 



The genus Abies, then, appears to form a transition group, 

 having parallelisms with Dammara and Araucaria through the 

 occurrence of resin tracheids ; with Thuya, Cupressus, etc., 

 through the survival of isolated resin cells approaching oblit- 

 eration ; with Tsuga, Larix, and Pseudotsuga through the devel- 

 opment of rudimentary resin canals leading to the formation of 

 definite resin passages ; and with Sequoia through the survival 

 of isolated resin cells and the development of rudimentary resin 

 canals. Through these parallelisms the connection appears to 

 be most direct with Sequoia, on the one hand, and with Tsuga, 

 on the other. This relation of Sequoia to Abies has been shown 

 by Penhallow on former occasions (59), and has more recently 

 been indicated in other ways by Jeffreys (24) ; but so far as the 

 present evidence is of value it would not permit us to infer that 

 Sequoia, Abies, and Tsuga form a continuous and conterminous 

 series in the order given, but rather that they represent sepa- 

 rate, though short, side lines of development, between which 

 the general sequence is manifested. 



