164 MEDULLOSEAB [CH. 



cylindrical and band-like vascular strands forming an irregular 

 anastomosing system (fig. 441, B) and by crowded spiral leaf- 

 scars on the exposed face. The appearance presented by the 

 transverse section figured by these authors, while suggesting 

 comparison with Medullosa, reveals a distinctive character, namely 

 the absence of a definite peripheral system of vascular rings such 

 as forms a striking feature of the continental MeduUosae. A more 

 complete description was afterwards pubhshed by Schenk^ who 

 recognised more fully the peculiar features and hinted at the 

 possibihty that the species might more appropriately be regarded 

 as a member of a distinct group. Solms-Laubach^ went a step 

 further and instituted the generic name Steloxylon, and in a later 

 pubhcation gave a fuller account of the anatomical characters. 

 The complete stem must have reached a diameter of approximately 

 13 cm. The homogeneous ground-tissue forms a matrix enclosing 

 an anastomosing vascular system of cyhndrical or oval steles 

 (fig. 441, C). Each strand consists of a band of secondary xylem 

 tracheids with one or several rows of circular or oval bordered 

 pits on the radial walls and narrow medullary rays usually 1 — 2 

 cells broad and 1 — 4 cells deep, though occasionally deeper. No 

 phloem is preserved. The tissue in the centre of each stele is 

 very imperfectly preserved, but it is clear that the secondary 

 xylem enclosed a central region ('partial pith') hke that in the 

 steles of a Medullosa, doubtless consisting of primary xylem and 

 conjunctive parenchyma. 



The stem is covered with leaf-bases of oval or circular section 

 and between them are small organs, probably multicellular hairs 

 (fig. 441, A). A leaf -base consists of an outer zone of strengthening 

 tissue and a parenchymatous ground-tissue traversed by two or 

 more small vascular strands which assume various forms. These 

 petiolar strands are simply portions of the main vascular system 

 which bend outwards at the periphery of the anastomosing network. 

 The more noteworthy features in which Steloxylon differs from 

 Medullosa, particularly such species as M. stellata and M. Leucharti, 

 are (i) the crowded and comparatively small leaf-bases in place 

 of the massive decurrent petioles of Medullosa; (ii) the supply 



> Schenk (89) p. 525, PI. I. figs. 1—16. 



= Solms-Laubach (96) B. p. 62; (97) p. 197; (10) p. 542, PI. ni. fig. 9. 



