DENNSTAEDTIA-DAVALLIA SERIES 613 



cases from the type of the Gradatae, but might also be produced directly 

 from the sorus of the type of the Simplices. It must suffice here to trace 

 some probable lines of phyletic origin which have so far emerged, though 

 others may eventually be recognised. 



DENNSTAEDTIA-DAVALLIA SERIES. 



The best accredited case is seen in Ferns with marginal sori, and it 

 has been found that among them there are forms which lead from the 

 Gradatae towards those genera with mixed sori which have been grouped 

 by Prantl as the Pterideae. 1 It has already been seen that though the 

 sorus of Dennstaedtia is typically basipetal (see Fig. 332 bis, A), occasional 

 departures from the strict sequence exist in D. apiifolia, while in D. rubi- 

 ginosa the sorus retains some signs of the basipetal sequence, but younger 

 sporangia occur interpolated without order among those pre-existent, while 

 the receptacle is more flattened (Fig. 339 A). It will be seen that these 

 characters approach those seen in Davallia. 



The genus Davallia, as it stands in the Synopsis Filicum, is a compre- 

 hensive one. Sir William Hooker remarks 2 of the Davalliae : " No two 

 authors are agreed as to the limits of this group, nor of the genera which 

 compose it ; and no wonder, seeing how gradually the genera seem to 

 run one into another. To me the genera seem to have been need- 

 lessly multiplied, upon very insufficient grounds, so that in many cases 

 I cannot even adopt them as sub-genera." The result of Sir William 

 Hooker's view, as thus expressed, has been that he grouped several genera 

 of other authors under the comprehensive genus Davallia. But the 

 tendency has since been to reinstate some of his sub-genera as substantive 

 genera, the most prominent case being that of Microlepia : the characters 

 derived from the sorus have been described above, and justify the removal 

 of Microlepia from the genus Davallia. 



From the Eu-Davallia of the Synopsis Filicum, observations have been 

 made on D. griffithiana, Hook, pyxidata, Car., canariensis, Smith, solida, 

 Swartz, and divaricata, Blume. In all of these the sorus shows various 

 ages of sporangia intermixed, while they are inserted upon a wide, flat 

 receptacle. The sporangia themselves have long stalks when mature, so 

 that the ripe sporangial head is raised far above those of the younger 

 sporangia, and thus scattering of the spores is ensured without an elongated 

 receptacle. 



The development has been specially studied in D. Gi'iffithiana. Fig. 339 B 

 shows a young sorus with the first sporangia appearing. It may be noted 

 that on the flat receptacle the first sporangium is in a median position, 

 and this may be taken as a slight trace of basipetal succession ; but even 

 this is not constant, and as the development proceeds any superficial cell 



1 Arb. K. Bot. Gart. Breslau, vol. i. (1892), p. 17. 

 2 Species Filicitni, i., p. 150. 



