XXX] MBDULLOSA 107 



secondary conducting tissue about three centres would lead to 

 serious mechanical difficulty: a stem constructed on the plan of 

 Medullosa anglica or the smaller M. picsilla could not increase the 

 thickness of its secondary vascular tissue beyond a certain point 

 without detriment to its efficiency. In some types this difficulty 

 is partially overcome by the production of complete concentric 

 cyhnders of centrifugally developed conducting tissue external 

 to an inner system of concentric steles agreeing individually with 

 those of Heterangium (fig. 415, B). Medullosa anglica, regarded 

 from the point of view of the architectural efficiency of its vascular 

 system, affords a much less promising point d^appui for further 

 evolution than some of the forms described under Medullosa 

 stellata in which the mechanical impasse is avoided by the adoption 

 of the cycadean plan as represented by such genera as Cycas 

 and Macrozamia. The Enghsh species Medullosa centrofilis 

 (fig. 417) affords the first example of a characteristic MeduUosan 

 feature, namely the presence of a small concentric stele in the 

 central region of the stem : this so-called star-ring differs not only 

 in its smaller dimensions but in its more cyhndrical form from 

 the larger peripheral steles. In the later Permian species, e.g. 

 Medullosa porosa and M. Solmsi, the single star-ring of the older 

 M. centrofilis is replaced by a large number of precisely similar 

 conducting strands. These star-rings are structurally comparable 

 with the cortical steles of Cycas and, in position, with the medullary 

 system of bundles in a Macrozamia; they are essentially cauUne 

 and take no part in the emission of leaf-traces. Medullosa 

 Leucharti (fig. 416, H) resembles in its vascular plan M. centrofilis, 

 but in this larger stem there are several star-rings and the en- 

 larged peripheral steles are more or less sinuous. In Medullosa 

 Solmsi (fig. 416, E) the star-rings are still more numerous and 

 the main vascular system consists of a double series of con- 

 centric steles, each agreeing with the larger peripheral steles 

 of M. Leucharti. 



Some of the forms included in Medullosa stellata appear to 

 be very different from M. anglica and M. Leuckarti (ef. fig. 416, 

 F, A, H), but their similarity is apparent if we imagine Medul- 

 losa anglica with only one main stele (with the addition of 

 star-rings) which is stretched tangentially until it becomes a 



