I50 ASPLENIOID FERNS [ch. 



two sides of the sorus, all that is positively required for the origin of the 

 sorus of Athyrium from a Dryopteroid source is present. The steps of the 

 inequality have been sketched by Von Goebel {OrganograpJiie, II, 2, 191 8, 

 pp. II 5 1, 1 195). Abortion of the region of the receptacle nearest to the margin 

 ofthe leaf, and of that part of the indusium, while one or both of the elongated 

 sides of the receptacle retain their fertility, would give respectively the sori 

 of Eu-Asplenium or of Diplaziinn. But the curious point here is that that 

 region nearest to the leaf-margin represents the originally fertile receptacle, 

 according to the comparative history given in the preceding pages. It is the 

 oldest part that becomes non-functional in Asplenmm and Diplaz'uiin. 



The sorus of the Asplenieae may thus, on the basis of comparison, be held 

 to be the last terms of a progressive series of changes that started from a 

 non-indusiate radial sorus, such as is seen in Gkiclienia, Lophosoria, or Also- 

 phila. Hairs are commonly present in the last two genera at the base of the 

 receptacle. Their place is taken in certain Cyatheoids and Woodsioids b\' 

 the indusium inferuin. A zygomorphic or lop-sided development already 

 visible in Peranema becomes more pronounced in Dryoptcris by extension 

 of the lateral pouches of Peranema, and their still greater elongation in 

 DidyniocJdaena, though the primary receptacle still retains its fertility: but 

 as illustrated by the various soral types seen in Diplazium lanceum, that 

 fertility is finally lost in Diplazinm and Asplenium, and it is the product 

 of those secondary pouches of Peranema which remains as the fertile 

 receptacle of these Ferns. Finally, the indusium itself becomes partially 

 or completely abortive in various Asplenioid Ferns, and the penultimate 

 state appears thus as the exposed linear sorus of Ceterach or Plenrosorus. 

 But ultimately there may be a possible Acrostichoid spread of fertility over 

 the leaf-surface, as in RJiipidopteris, if that Fern has its proper place with 

 the Asplenieae. 



The essential point in this phyletic story, as witnessed by the comparisons 

 given above, is that fusion to form coenosori plays no part in it. As the leaf 

 widens it is the individual sorus that lengthens, so that however nearly the 

 result may resemble the coenosori of certain species of Nephrolepis or those 

 typical of Pteris, such similarity can only be held as homoplastic. A closer 

 and more critical question arises, however, in the comparison of the As- 

 plenioid with the Blechnoid Ferns. These have habitually been classed 

 together. That there is reason to believe that those soral similarities which 

 they show are also homoplastic, and that the elongated fertile tracts of the 

 Blechnoids are really coenosori, will be the theme of the next two Chapters. 



Turning now to the natural grouping of the Asplenioid Ferns, they may 

 be arranged in sequence according as their soral characters appear pro- 

 gressively distinct from the Dryopteroid type, as it is seen in the Male 

 Shield Fern. 



