154 MEDULLOSEAE [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-like 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 pericyclic region. The extrafascicular strands are 

 believed to be secondary structures phylogenetically independent 

 of the meristeles and main stele, comparable with the successive 

 cylinders or arcs of secondary xylem and phloem in some recent 

 Cycads and in some species of Medullosa 1 . 



The stele is exarch and roughly triangular; except in the 

 broader zone of secondary tissue it agrees with the protostele 

 of 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, like 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 limited 

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



Sutcliffia Williamsoni (Seward). 



1876. Myelopteris (pars] Williamson, Phil. Trans. R. Soc. Vol. 166, 

 PI. n. figs. 7, 8; PL iv. fig. 17. 



1893. Rachiopteris Williamsoni Seward, Ann. Bot. Vol. vn. p. I. 



1894. Rachiopteris Williamsoni Seward, ibid. Vol. vni. p. 207, PI. XIIT. 

 1906. Sutdiffia Williamsoni Scott, Trans. Linn. Soc. Vol. vn. pt iv. 



p. 62. 



In an account of Myelopteris (= Myeloxylori) published in 

 1876 Williamson 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 Frame's paper. 



