xxi] 



OSMUNDACEAE 



331 



The striking feature exhibited by these Permian plants is 

 the structure of the protostele, which in Thamnopteris and 

 probably in Zalesskya diploxylon consists of solid xylem sur- 

 rounded by phloem : this may be regarded as the primitive 

 form of the Osmundaceous stele. In Osmunda regalis and in 

 other recent species of the genus the xylem cylinder has the form 

 of a lattice-work; in other words, the departure of each leaf- 

 trace makes a gap in the xylem and the overlapping of the 

 foliar-gaps results in the separation of the xylem into a number 

 of distinct bundles. In Zalesskya gracilis the continuity of 



Fig. 251. Lonchopteris virginiensis. (After Fontaine. ^ nat. size.) 



the xylem is not broken by overlapping gaps ; in this it agrees 

 with Lepidodendron. In Thamnopteris the centre of the stele 

 was occupied by a peculiar form of xylem obviously ill-adapted 

 for conduction, but probably serving for water-storage and 

 comparable with the short and broad tracheae in MegaloxylonK 

 There is clearly a well-marked difference in stelar anatomy 

 between these two Permian genera and Todea and Osviunda : 

 this difference appears less when viewed in the light of the 

 facts revealed by a study of the Jurassic species Osmmidites 

 Dunlopi. 



' Seward (99). 



