[Illustration: Fig. 417. _Medullosa centrofilis._ Transverse section showing the deep-seated ‘periderm’ (broken line) surrounding four steles. (After de Fraine.)] _Medullosa stellata_ Cotta. Cotta[254] described _Medullosa stellata_ as a stem characterised by the occurrence of several many-rayed stellate columns (‘vielstrahlige Sternsäule’) in a pith enclosed by a double cylinder of secondary xylem. The so-called pith is the central ground-tissue of the stem and the double ‘striated ring’ of Cotta is a cylindrical stele identical in structure with each of the steles of _Medullosa anglica_ but having a tubular form instead of forming a relatively broad and short band (_cf._ fig. 416, D and A). Goeppert[255] in his _Permian Flora_ gave a detailed account of the species, some of his sections being cut from Cotta’s material, and by the employment of varietal epithets emphasised the range of variation within the limits of the type. Goeppert and Stenzel[256] and, several years later, Weber and Sterzel[257] adopted the same plan as a convenient method of drawing attention to differences in anatomical characters. As Schenk[258] pointed out, there is a considerable risk in the case of small pieces of stems of attaching excessive importance 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 provisional 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_[259]. _Medullosa stellata_ var. _typica_.