234 MORPHOLOGY OF GYMNOSPERMS 



of resin canals from the secondary wood. The same method of rea- 

 soning apphed to the resin canals and their appearance as a traumatic 

 response in Taxodineae and Cupressineae results in the conclusion 

 that they have been derived from the Abietineae. This has been done 

 by Jeffrey notably for Sequoia (78). This genus was long supposed 

 to be without resin ducts, but they were found in S. gigantea in the 

 ovulate strobilus, in the first annual ring of vigorous branches of adult 

 trees, and in leaf traces of very vigorous leaves of adult trees. In 

 S. sempervirens resin ducts are absent from all these regions; but in 

 both species resin ducts appear in the wood of both shoot and root 

 as a result of injury. It was inferred that these resin ducts, appear- 

 ing as a traumatic response, were an ancestral feature of Sequoia; 

 and if so, the suggestion of connection with the Abietineae is inevitable. 



Penhallow (96) has reached a very different conclusion from 

 a study of the resin canals. According to his series, resin cells occur 

 in the more primitive conifers. These cells are successively scattered, 

 zoned, and segregated. Segregated resin cells may give rise to resin 

 cysts, such as occur in Abies, Tsuga, and Sequoia. In Larix, Pseudo- 

 tsuga, and Picea the continuous system of resin canals shows its 

 derivation from resin cysts by the constricted walls; while in Pinus 

 the walls are no longer constricted and are lined by an epithelium. 

 This series leads to the inference that the Abietineae are the most 

 modern of conifers, and Pinus the most specialized genus. 



The discussion of the interrelationship of tribes is deferred to 

 a later section (p. 308), but the two views outlined above serve not only 

 to emphasize the use that is being made of resin canals, but also the 

 kind of facts in reference to them that are thought to be of service. 

 It is evident that a series may be read in either direction, and that a 

 traumatic response may not necessarily reveal an ancestral character. 



The cotyledons have been found to be the last region of the body 

 to lose the primitive features of the vascular bundle. If the mesarch 

 structure has disappeared from all other regions of the body, it may 

 have been retained by the cotyledons, which have been appropriately 

 called the most "conservative" organs. For this reason, it is of inter- 

 est to note that the resin canals are not found in the cotyledons of 

 the Cupressineae, as witnessed by Actinostrobus, Callitris, Libocedrus, 

 Thuja, Cu press us, and Juniperus; but they occur in the cotyledons 



