xxi] 



OSMUNDACEAE 



327 



occur just within the edge, (ii) an inner zone of broader 

 and shorter tracheae (fig. 248, x i). The protoxylem elements 

 {px, fig. 248) are characterised by a single series of scalariform 

 pits, while the metaxylem elements have multiseriate pits like 



^x 



Fig. 248. Zalesskya gracilis (Eich.). Transverse section of part of the stele: 

 ph, phloem; xi, xii, xylem ; px, protoxylem. (After Kidston and 

 Gwynne-Vaughan. x20.) 



those on the water-conducting elements of recent Osmundaceae. 

 The tracheae show an interesting histological character in the 

 absence of the middle substance of their walls, a feature recog- 

 nised by Gwynne-Vaughan' in many recent ferns. External to 

 ' Gwynne-Vaughan (08). 



