XXXV 



PHYLETIC GROUPING 



333 



becomes a leading feature. The Gymnogrammoid Ferns centre round Gynino- 

 gramme, and though this phylum is less definite than any of the others, it 

 may perhaps be traced as derivative, in part or in whole, from forms such as 

 the living Plagiogyria, and ultimately from an Osmundaceous source. The 

 Blechnoid Ferns centre round Blechmnn, and have in common the fusion of 

 superficial sori of a primitively Cyatheoid type into linear sequence, but 

 along evolutionary lines quite distinct from the fusion seen in Pteris. The 

 Dryopteroid Ferns centre round Dryopteris, and differ from the Blechnoids in 



>3o tyyoptt-yi J a. tea. 



Phyletic Scheme for the more Primitive Fihcales. 



maintaining the individuality of their superficial sori, which are linked with 

 those of the Cyatheoids by unmistakeable intermediate types. The Dipteroid 

 Ferns may be held as derivatives from the type of Dipteris, a genus which 

 already shows in D. conjiigata the advance to a broad leaf-surface, and a 

 mixed type of sorus, advancing towards the acrostichoid state. The com- 

 parative working out of the natural affinities of the Leptosporangiate Ferns 

 along such lines, and the discussion of their grouping in relation to these 

 suggested central types, must be held over as the subject for Volume ill. 



nOfEXTY LIBRARY 



M. C SMe CqIUs^ 



