154 MEDULLOSBAE [CH. 



vascular network (fig. 439, D, e) which encloses both the main, 

 steles and the meristeles ; it consists of extrafascicular strands- 

 composed of normally orientated bands of secondary xylem and 

 phloem often assuming a fan-hke arrangement and occasionally 

 almost concentric or inversely orientated. These strands are 

 always accompanied by short, usually reticulate, tracheids on the 

 inner margin of the xylem: similar isodiametric tracheids also' 

 occur in the pericycUc region. The extrafascicular strands are 

 beheved to be secondary structures phylogenetically independent 

 of the meristeles and main stele, comparable with the successive 

 cyhnders or arcs of secondary xylem and phloem in some recent 

 Cycads and in some species of Medullosa^. 



The stele is exarch and roughly triangular; except in the 

 broader zone of secondary tissue it agrees with the protostele 

 oi forma a. The secondary xylem (fig. 439, C) possesses numerous 

 medullary rays 3 — 4 cells broad and of considerable depth: the 

 secondary phloem is characterised by the presence of thick-walled 

 elements, presumably sieve-tubes, hke those of Medullosa Leuckarti 

 and M. anglica. 



The ground-tissue is rich in secretory tissue and the stem- 

 surface, from which the leaf-bases have been detached, is hmited 

 by a wide zone of secondary tissue produced by a phellogen. 



Sutcliffia Williamsoni (Seward). 



1876. Myelopteris {pars) Williamson, Phil. Trans. B. Soc. Vol. 166, 

 PI. n. figs. 7, 8; PI. IV. fig. 17. 



1893. BacMopteris Williamsoni Seward, Ann. Bot. Vol. vn. p. i. 



1894. Rachiopteris Williamsoni Seward, ibid. Vol. vttt. p. 207, PI. xin. 

 1906. Sutcliffia Williamsoni Scott, Trans. Linn. Soc. Vol. vu. pt iv. 



p. 62. 



In an account of Myelopteris {= Myeloxylon) pubhshed in 

 1876 WiUiamson included some sections of petioles from the 

 Lower Coal Measures which I afterwards with his concurrence 

 transferred to the genus Rachiopteris as R. Williamsoni. In 

 Rachiopteris Williamsoni the vascular bundles are concentric 

 and not collateral, and are further distinguished from those of 

 Myeloxylon by the association of parenchyma with the tracheids. 



1 Reference should be made to the helpful drawings of models of the vascular 

 system in Miss de Fraine's paper. 



