352 MORPHOLOGY OF GYMNOSPERMS 



advanced characters is more perplexing. This is indicated by 

 Scott's statement (i6o) that if any one of the tribes of conifers could 

 be regarded as distinct in origin, it would be the taxads. It seems 

 impossible to interpret the resemblances to Ginkgo as meaning any- 

 thing more than the retention of common characters derived from a 

 common ancestor, presumably in this case the Cordaitales. At the 

 same time, it would seem impossible to connect the Taxineae, which 

 have no history earlier than the Cretaceous, directly with the paleozoic 

 Cordaitales. The only mesozoic stock they suggest is that from 

 which the modern Abietineae, Taxodineae, and Cupressineae have 

 come; and they differ from Abietineae in some of the important 

 morphological features shown by Taxodineae and Cupressineae. 

 The geographical association also enforces this suggestion, and the 

 conclusion seems reasonable that the living Taxineae represent a 

 phylum which was derived early from the abietineous stock, and 

 which has retained cordaitean features that have disappeared among 

 existing Abietineae, Taxodineae, and Cupressineae. The greatest 

 objection to this connection lies in the structure of the ovulate stro- 

 bilus, but the condition in Taxineae, as contrasted with that of the 

 three tribes of Pinaceae, is one of those differences in degree rather 

 than in kind (see p. 322) which mark ancient separation at a time 

 when both groups bore more resemblance to their common ancestral 

 stock. 



The position of PhyUodadus among Taxaceae has given rise to 

 such diversity of opinion that a summary of the evidence may be 

 useful. The three alternatives are (i) to include it with Taxineae, 

 (2) to include it with Podocarpineae, or (3) to constitute for it an 

 intermediate tribe. The taxad resemblances are (i) structure of 

 ovule, (2) symmetrical arillus, (3) ovulate strobilus, (4) mesarch 

 bundles, (5) markings of tracheids. The podocarp resemblances 

 are (i) male gametophyte, (2) structure of stamen, (3) winged pollen 

 grains, (4) megaspore membrane, (5) "spongy tissue." The taxad 

 features are, on the whole, more superficial and variable; and the 

 podocarp features are more fundamental, so fundamental, in fact, 

 that an intermediate tribe seems unjustifiable. It seems evident 

 that the testimony is strongly in favor of including PhyUodadus in 

 Podocarpineae (174). 



