100 MEDULLOSEAE [CH. 



M. stellata belongs there may be flatter and tangentially elongated 

 vascular strands in addition to the cylindrical star-rings ; these 

 are termed plate-rings. 



In Medullosa stellata var. lignosa 1 the outer xylem reaches 

 a breadth of 4 cm. and the star-rings are reduced to one. The 

 form M. stellata var. gigantea 2 (fig. 416, K) is of special interest 

 as an example of a stem reaching a diameter of nearly 50 cm. 

 and having as many as 43 large and small star-rings in the axial 

 region. A large tubular stele like that of the type-species (fig* 

 416, D) surrounds the central region, but in this form the cylin- 

 drical stele a is succeeded by concentric cylinders of normally 

 orientated xylem and phloem (fig. 416, K, 66) produced by 

 successive cambiums either cortical or pericyclic in origin. This 

 type of stem presents a striking resemblance to stems of Cycas 

 and Macrozamia except in the possession of a double cylindrical 

 stele consisting of both centripetal and centrifugal secondary 

 xylem and phloem separated by a zone of primary xylem (partial 

 pith). 



Medullosa gigas Renault. 



This species was founded on a piece of stem from the Permian 

 of Autun 3 , consisting almost entirely of secondary xylem, which 

 Brongniart had previously placed in his genus Palaeoxylon*. 

 The secondary xylem reaches a diameter of 45 50 cm. and in 

 the portion of the central region preserved there are a few vascular 

 strands like the star-rings of other species. The considerable 

 development of secondary xylem indicates a form of stem similar 

 to some forms of M. stellata (e.g. fig. 416, F), but as the available 

 data are insufficient for accurate determination Renault's specific 

 name is retained. Renault describes the internal xylem cylinder 

 (i.e. the centripetal xylem) as very slightly developed or as hardly 

 visible, a feature in which the French specimen shows a nearer 

 approach to the structure of a recent Cycad. 



1 Weber and Sterzel (96) B. p. 63. 2 Ibid. p. 66. 



3 Renault (96) A. p. 297 ; (93) A. PI. LXXI. figs. 16. 



4 Brongniart (49) A. p. 77. 



