526 STUDIES IN FOSSIL BOTANY 



and Pityeae, was wholly centrifugal in its development, 

 the first-formed spiral elements lying on the interior 

 margin, next the pith (Fig. I go, B, px) ; they are 

 localised in groups, often projecting somewhat into the 

 pith, and marking the position of the primary bundles. 

 In stems from Shore, however, which I have recently 

 examined, showing the discoid pith and other characters 

 indistinguishable from those of a typical Cordaites, arcs 

 of centripetal xylem are present at definite points, no 

 doubt corresponding to the position of the leaf-traces. 

 The structure appears to be essentially the same as in 

 Poroxylon, though the number of elements in the 

 centripetal part of the xylem-strands may be smaller. 

 In the Cordaiteae the old " Cryptogamic " or centripetal 

 wood appears to have been on the verge of extinction, 

 and its presence or absence may here be of little 

 taxonomic significance. 



In Cordaiteae of the type described by Renault, the 

 elements of the wood are radially arranged throughout, 

 so that in transverse sections there is no distinction 

 between primary and secondary xylem (see Fig. 189, x). 

 In radial sections, however, such as that from which 

 Fig. 1 90, B, was drawn, we find a marked change, from 

 the pith outwards, in the structure of the walls of the 

 tracheides. The narrow spiral elements of the proto- 

 xylem are succeeded by wider spiral tracheides, and 

 these again by scalariform elements. It is not until 

 many rows have been passed, that we come to the 

 pitted tracheides (bt) which form the bulk of the wood. 

 The transitional region between primary and secondary 

 xylem was thus more extensive in these plants than in 

 most of the recent Conifers. 



