XXXVIII] PTERIDIUM 45 



commissure, is seen in Fig. 609, C. It shows the vein-endings, their positions 

 being indicated by the indentations of the upper surface; the tracheides 

 elongated in the plane of the section link them together, forming the 

 receptacular commissure. The sporangia of various ages are seen intermixed, 

 and covering the whole length of the commissure. The condition is that seen 

 as a transparency in Lindsaya in Fig. 602. The conclusion from such facts 

 is inevitable, that in Ptefiduun as in Lindsaya the coenosorus is the result 

 of a lateral linkage of marginal two-lipped sori, originally of gradate type, 

 such as are seen in the Dicksonioid Ferns. It does not necessarily follow 

 that these genera are of common descent, but at least their similarity of soral 



Fig. 610. Sorus of/", aqiiilinti/n var. c-aiidatii/n cnt vertically, showing a regular 

 basipetal succession of the four sporangia. ( x 150.) 



construction does suggest a common origin, from some Dicksonioid-Denn- 

 staedtioid source. The relation of Pteridiiim to Deimstaedtia is specially 

 indicated by the external habit of the creeping rhizome, the elongated rachis, 

 and much branched deltoid blade, with open venation, and dermal hairs, not 

 scales. The polycyclic stele is, however, advanced beyond that of the 

 Dennstaedtiinae in respect of its prevalent perforation, though this is 

 occasionally seen in Dcnnstaedtia itself The gradate, two-lipped sorus of 

 Dicksonioid type is still traceable in the development of Pteridiuiii, but it 

 is disguised by the lateral linkage into coenosori, with their vascular com- 

 missures connecting the otherwise free veins. On this view Pteridium stands 

 as a synthetic type between the Dicksonioids and the Pteroid Ferns. It will 

 now be shown comparatively within the latter group how, by gradual steps, 

 the condition seen in Ptcris itself may probably have been attained, and 

 finally even that oi AcrosticJiuni. 



