XXX] MEDULLOSA 97 



years later, Weber and Sterzel 1 adopted the same plan as a con- 

 venient method of drawing attention to differences in anatomical 

 characters. As Schenk 2 pointed out, there is a considerable risk 

 in the case of small pieces of stems of attaching excessive import- 

 ance to structural variations, and it is by no means improbable, 

 as he said, that differences which are the expression of states 

 of preservation or stages in development have been incorrectly 

 regarded as distinguishing marks of individual plants. It is, 

 however, convenient to recognise some of the more striking 

 deviations from the type-species by speaking of the different 

 forms as varieties though, as Weber and Sterzel fully admit, 

 such varieties and even some of the species must be looked upon 

 as provisional. Weber and Sterzel give expression to the provi- 

 sional nature of their grouping by classifying the species with 

 their varieties into form-cycles. Under the form-cycle Medullosa 

 stellata five more or less well defined forms are recognised, the 

 type-species being Medullosa stellata var. typica 3 . 



Medullosa stellata var. typica. 



Part of a transverse section of a cylindrical stem is represented 

 diagrammatically in fig. 416, D. Very little of the cortex is pre- 

 served: a parenchymatous axial region with scattered secretory 

 canals contains four oval or cylindrical vascular steles, the stellate 

 columns of Cotta or star-rings of later authors. These are of the 

 same nature as the small central stele in the English Medullosa 

 ccntrofilis. The central region of the stem in this specimen is 

 completely surrounded by a narrow cylinder of inversely orientated 

 secondary xylem and phloem (fig. 416, D), the phloem being on 

 the inner side of the xylem. Beyond the xylem is a parenchy- 

 matous band containing scattered groups of primary xylem 

 tracheids with spiral, scalariform, and reticulate pitting, and 

 this zone, which is usually designated the 'partial pith,' is 

 succeeded by a second and broader, normally orientated, cylinder 

 of secondary xylem and phloem. In this section the two con- 

 centric cylinders separated by the partial pith form a solenostele 

 like that of several recent Ferns except in the presence of secondary 



1 Weber and Sterzel (96) B. 2 Schenk (89). 



3 Weber and Sterzel (96) B. p. 51 ; Schenk (89). 



S. Ill 



