XXX] MEDULLOSA 101 



Medullosa porosa Cotta. 



The second of Cotta's species^, which has been fully investigated 

 by Weber and Sterzel, is constructed on the same plan as that of 

 M. stellata, but the stem is distinguished by the greater number 

 ■of star-rings and, more especially, by the presence of an outer 

 system of vascular strands in the axial region (fig. 416, M) : these 

 form a frequently interrupted cylinder of anastomosing strands 

 characterised by the feeble development of secondary xylem and 

 phloem or by the absence of this tissue on the outer face of the 

 strands. The component parts of this outer series occasionally 

 fuse with the internal star-rings. 



Medullosa Solmsi Schenk^ var. typica Web. and Ster.* 



This type has a large axial region containing several very 

 small star-rings enclosed by two concentric zones of separate 

 plate-rings (fig. 416, E) each consisting of a complete flattened 

 cylinder of secondary xylem and phloem enclosing primary xylem. 

 As the complete cylindrical stele of the stem of Medullosa stellata 

 shown in fig. 416, D, was compared with the solenostele of a 

 Fern, so in this stem (fig. 416, E) the vascular cylinder may be 

 compared at least superficially with a dictyostele. From the 

 inner circle of plate-rings strands are given off in the form of 

 star-rings and these pass through the gaps in the outer system, 

 eventually breaking up in the cortex into numerous collateral 

 bundles. In another form of this species, var. lignosa (fig. 416, L), 

 the axial region is enclosed by a circle of plate-rings like those 

 in the type-form, but these are succeeded by a circle of very 

 asymmetrically developed and large steles with the outer xylem 

 and phloem much broader than the inner. Moreover in this 

 form additional cylinders of normally orientated vascular tissue 

 are added as in M. stellata var. gigantea and in some recent Cycads. 

 It is noteworthy that the secondary wood of Medullosa Solmsi 

 is rather more compact than in other species, a feature in which 

 it to some extent agrees with the South African genus Ehexoxylon. 



1 Cotta (32) B. p. 63, PI. xii. figs. 6, 7. 



2 Schenk (89) p. 339. 



» Weber and Sterzel (96) B. p. 541, Pis ii. iii. 



