3 22 



THE ANATOMY OF WOODY PLANTS 



It is evident that, in the secondary xylem of the root, parenchyma, 

 conspicuous by its absence in the mature wood of the stem, is 

 abundantly present. Not only is this the case with the root, 

 but the same situation is found in the first annual ring of the 

 vegetative stem and also very strikingly in the woody axis of the 

 ovuliferous cone. The facts here mentioned are of particular 

 significance when correlated with the organization of the Cretace- 

 ous Araucarioxylon 

 shown in Fig. 227. 

 Obviously the 

 parenchyma pres- 

 ent in the older 

 type of araucarian 

 wood is perpet- 

 uated in those re- 

 gions of the living 

 form which we 

 have learned in an 

 earlier chapter to 

 regard as conserv- 

 ative. It may 

 accordingly be 

 logically assumed 

 that woods of the 

 type of the living 

 araucarian conifers formerly possessed longitudinal parenchyma 

 and in this respect are at variance in organization with the ligne- 

 ous structure of the Paleozoic Cordaitales. This conclusion is 

 reinforced by an examination of the effects of injury in the living 

 genera, for the infliction of wounds results frequently in the recall 

 of the lost parenchyma even in the adult axis. 



We may now pass to the consideration of other features which 

 are supposed to indicate a close degree of relationship between 

 the Araucariineae and the Cordaitales. The most important 

 of these are the crowded pitting and the absence of bars of 

 Sanio. In Fig. 232 is shown the organization of the wood in 

 the stem of the seedling of Agathis australis as viewed in longitudinal 



FIG. 231. Longitudinal section of the wood of the 

 root in Agathis australis. 



