ANATOMICAL STRUCTURE AND CLIMATIC EVOLUTION 425 



absence of any evidence against its validity. The gymnosperms 

 of the Paleozoic had just begun to develop annual rings toward the 

 end of the epoch, and the phenomenon of tangential pitting had not 

 yet made its appearance. As has been illustrated above, this feature 

 did not become marked until the Mesozoic was well advanced. 



FiG. 294. Details of organization of the wood in Picea canadensis 



The situation in gymnospermous woods did not, however, 

 rest with the appearance of tangential pitting at the end of the 

 annual ring. Not long after this feature had become well estab- 

 lished it was accompanied by another even more important ana- 

 tomical phenomenon. In Fig. 294 is shown a slightly diagrammatic 

 view of the wood of the root of Picea canadensis in three dimen- 

 sions. The illustration includes the transition from one annual 

 ring to the next, and the terminal tracheids manifest the tangential 



