348 THE ANATOMY OF WOODY PLANTS 



the genus Cephalotaxus. The anatomical evidence points very 

 clearly to the Taxineae as a reduction series, taking its origin from 

 the same general group or plexus as the Podocarpineae. The 

 somewhat meager fossil data at our disposal do not warrant 

 us in assuming a very ancient occurrence for the subtribe Taxineae, 

 since woods of this type are not found earlier than the Tertiary 

 or Cenozoic. It is true that cones and leafy twigs from various 

 levels of the Mesozoic have from time to time been referred to 

 taxineous affinities, but there is no anatomical evidence that they 

 belonged here. So far as any data derived from anatomical 

 structure are concerned, we are not justified in attributing to 

 the Taxineae a great geological age. The external habit as a 

 diagnostic criterion in the Coniferales has been shown in recent 

 years to be almost as misleading as it has proved to be in the 

 Cycadofilicales and other Paleozoic groups. 



Having considered the anatomical organization of the various 

 subtribes of the Coniferales in such detail as comports with an 

 elementary work like the present, before summing up the situation 

 from the evolutionary standpoint we may well make some refer- 

 ence to the anatomical characteristics of coniferous woods which 

 are utilized by paleobotanical investigators. The genera of fossil 

 woods logically increase in number as our knowledge of the ligneous 

 organization of extinct plants becomes fuller and more complete. 

 For the present purpose only a few of the more important genera 

 of fossil woods need be considered. By reason of its relative 

 resistance to decay, wood naturally often becomes isolated from 

 its accompanying tissues and frequently is the sole surviving evi- 

 dence of the former existence of gymnospermous groups. The 

 situation thus presented is a difficult one, and the earlier charac- 

 terizations of fossil wood were naturally largely empirical and 

 made without reference to the facts or principles of comparative 

 anatomy. 



In deference to the prevailing view that the araucarian conifers 

 are the most ancient and form the connecting link between the 

 Coniferales and the Cordaitales, we may consider them first. 

 It has been indicated in a previous paragraph that the mature 

 secondary wood of the two living genera of the Araucariineae is 



