6 Weiss, A Tylodcndron-like Fossil. 



tissue surrounding the primary wood of Pitys antiqiia.^ 

 But these isolated patches are never as large as those of 

 Pitys antiqiia, and are sometimes almost continuous with 

 the main woody cylinder. In these small groups it has 

 not been possible to note much difference in the size of 

 the tracheids, and we cannot therefore determine whether 

 they had a mesarch or endarch structure, though the 

 particular group figured may suggest a mesarch arrange- 

 ment. From longitudinal sections it would appear that 

 the primary wood is somewhat sinuous in its course — 

 sometimes bending away from the secondary wood 

 towards the pith, sometimes joining close up to the 

 secondary tracheids. This would to some extent explain 

 the irregularity of these isolated groups of tracheids. If 

 these groups represent the primary wood, we may, I 

 think, conclude that their feeble development represents 

 the transition to a different type of vascular development, 

 and nearer to that found in recent Gymnosperms. The 

 occurrence would, indeed, be an analogous case to that 

 described by Zalessky^ in Mesopitys Tchihatcheffi, though 

 occurring probably in a different series of forms. Similar 

 isolated groups of tracheids are figured by Wieland in 

 Cycadoidea Wielandi, where sometimes two groups may 

 be found on the same radius. The longitudinal section 

 represented in PL IL, Fig. 7, may possibly represent two 

 such radial groups, or both isolated tracheids may belong 

 to a larger group, cut tangentially. 



The main xylem masses commence with small 

 tracheids, but only in one case could a distinct spiral 

 vessel be observed, otherwise both the isolated and the 

 innermost tracheids show scalariform markings. The 



* Scott, D. II. " Studies in Fossil Botany," 1909. Vol. ii., p. 516. 



° Zalessky, M.D. "Iltude sur Tanatomie du Dado.xylon Tchihatcheffi." 

 Memoircs du comity geologiquc, St. retersbourg, 191 1. 



