MIXTAE A HETEROGENEOUS GROUP 623 



these conclusions have been shown to be supported by the facts of anatomy. 

 It must, therefore, be allowed that those Ferns which are associated under 

 the general heading of "Mixtae" are relatively late derivative forms, 

 and that they do not constitute a natural group, any more than do those 

 plants which are heterosporous or those which produce seeds. It would 

 then seem desirable to proceed at once to divide this heterogenous group 

 into true phyletic sequences. But to do this requires much greater com- 

 mand of facts, and especially of those of anatomy, than is at present 

 available. Here it must suffice to recognise the unsatisfactoriness of the 

 present position, and at the same time to give a very few general indications 

 of the form the future system may take. 



It would appear probable that the main bulk of the Mixtae have been 

 derived along a line where the sori were marginal, with the Dennstaedtiinae, 

 the Davalliinae, and Pterideae of Prantl as early representatives of it. This 

 was accompanied by transition through the solenostelic to the dictyostelic 

 structure of the stem. A gradual shifting of the sorus to the under surface 

 of the leaf also occurred, till the condition was reached as seen in the 

 Aspidiinae and Aspleniinae of Prantl. Certain forms allied to these, losing 

 their indusium altogether, constituted one section of the old comprehensive 

 genus Polypodiwn. All through the more advanced members of this 

 sequence the dictyostelic structure of the stem was maintained. Another 

 contingent, with very similar final result, probably arose from forms with 

 superficial sori and basal indusium, allied to the Cyatheae : in this also 

 the dictyostelic structure is seen. A third series, also with superficial sori, 

 is represented by the Gleichenia-Matonia-Dipteris line : it is true that 

 Dipteris is at present the only recognised representative of this sequence 

 which has attained to the rank of the Mixtae, and it has consequently 

 been removed from its old position in Polypodium. It seems, however, 

 not improbable that future investigations may add fresh contingents from 

 the ranks of Polypodium, and possibly from some other genera, and one 

 useful criterion will be found in the stem-structure, for in the recognised 

 forms it is persistently solenostelic. 



The attempt will not be made at present to assort all the remaining 

 forms of Polypodiaceous Ferns into probable phyletic sequences : they are 

 left to be dealt with as knowledge increases. Meanwhile the general view 

 of them will be as of a brush of diverging phyletic lines, which have proved 

 blind. In fact, the ultimate fulness of development of the Homosporous 

 Ferns is that which is before us to-dav. 



