CONIFERALES 351 



Pinus. Further, both megasporophylls and microsporophylls 

 in the two cases produce paired sporangia. The microsporangia 

 of Ginkgo present a common difference with those of the Abie- 

 tineae from lower forms in owing their dehiscence to a mechanical 

 layer derived, not from the epidermal, but from the fibrovascular, 

 tissues. An additional feature of affinity is supplied by the 

 winged character and the internal organization of the microspores 

 in the two groups, for they are practically identical. The structure 

 of the tracheids of the wood in the Abietineae and Ginkgoales is 

 significantly similar in the presence of opposite pitting and bars 

 of Sanio, a common feature which distinguishes them from the 

 Cordaitales and other older gyirmospermous groups. Further, 

 although the pitting and other details of organizatipn of the 

 tracheids in the two groups is of the modern gymnospermous 

 type, the structure of the xylem in primitive regions clearly shows 

 a filiation with the conditions characteristically presented by the 

 Paleozoic gymnosperms. 



If a strong argument for the primitive position of the Abie- 

 tineae is supplied by a comparison with the structural features of 

 the Ginkgoales, an even more cogent one is furnished by their 

 resemblance in important anatomical characteristics to the Cor- 

 daitales. It is the leaf of the extinct genus Prepinus which mani- 

 fests, as has been indicated in earlier pages, the most categorical 

 and distinct similarity to the foliar organization of certain Cor- 

 daitales. Not only is Prepinus the only known representative 

 of the Coniferales to show distinct and unmistakable centripetal 

 wood as distinguished from transfusion tissue, but it manifests 

 its affinity to cordaitean forms by the presence of a double trans- 

 fusion sheath in relation to the centripetal wood of the foliar bundle. 

 It is further impossible to deny for Prepinus a close degree of 

 relationship with Cretaceous species of Pinus which, like Prepinus, 

 possess a well-marked double transfusion sheath and differ from 

 the more primitive genus only in the absence of true centripetal 

 wood. The agreement in foliar organization between Prepinus 

 and cretaceous species of Pinus on the one hand and the Cordaitales 

 on the other cannot be overlooked in any discussion of the evo- 

 lution and affinities of coniferous subtribes. Although the wood 



