334 



FILICALES 



[CH. 



in recent species of Todea the same kind of reduced tracheae are 

 met with on the inner edge of the xylem^. In both cases the 

 short tracheae are probably vestiges of an axial strand of con- 

 ducting elements which in the course of evolution have been 

 converted into parenchymatous cells. In Lepidodendron vas- 

 culare the mixed parenchyma and short tracheae in the centre 



/ 



-a 



r^ 









W^ 



■P^- 







Fig. 253. Osmundites Kolhei Sew. (^ nat. size.) 



of the stele represent an intermediate stage in xylem reduction, 

 and the arrangement in vertical rows of the medullary paren- 

 chyma in Lepidodendron is precisely similar to that described by 

 Kidston and Gwynne-Vaughan in Thamnopteris. In both cases 

 1 Seward and Ford (03). 



