XXl] OSMUNDACEAE 333 



petioles and the petioles of scale-leaves with an aborted lamina. 

 A similar association of two forms of leaf is seen in the existing 

 American species Osmunda Claytoniana and O.cinnamomea. The 

 cortex and armour of leaf-bases are penetrated by numerous 

 diarch roots. The xylem cylinder, six to seven tracheae broad, 

 is characterised by the narrower diameter of its innermost ele- 

 ments and — an important point — by the fact that the detachment 

 of a leaf-trace does not break the continuity of the xylem 

 cylinder (fig. 252). Each leaf-trace is at first elliptical in 

 section; it then becomes curved inwards and gradually assumes 

 the horse-shoe form as in Zalesskya and in the recent species. 

 The single endarch protoxylem becomes subdivided until in 

 the petiole it is represented by 20 or more strands. 







FiQ. 252. Osmundites Dimlopi Kidst. and G.-V. Portion of xylem showing the 

 departure of a leaf-trace. (After Kidston and Gwynne-Vaughan; x 36.) 



In the continuity of the xylem cylinder this species of 

 Osmundites shows a closer approach to Todea harbara or 

 T. superha (fig. 221, B) than to species of Osmunda; it differs 

 from Zalesskya in having reached a further stage in the re- 

 duction of a solid protostele to one composed of a xylem 

 cylinder enclosing a pith. This difference is of the same kind 

 as that which distinguishes the stele of Lepidodendron rhodum- 

 nense from L. Harcourtii. In Lepidodendron short tracheae 

 occasionally occur on the inner edge of the xylem cylinder, and 



