100 MEDTILLOSEAB [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^ the outer xylem reaches 

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

 form M. stellata var. gigantea^ (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 Hke 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 cyhnders of normally 

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

 successive cambiums either cortical or pericycHc 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*, 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 hke 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 cyHnder 

 {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. ^ jUd. p. 66. 



" Renault (96) A. p. 297 ; (93) A. PI. Lxxi. figs. 1—6. 

 * Brongniart (49) A. p. 77. 



