XII] 



THE MIXED SORUS 



215 



The origin of this state has probably been along not one, but many lines 

 of Descent. Indications of how it came into existence are given at several 

 quite distinct points in the system. For instance among the Dicksonioid- 

 Davallioid series the former have gradate, the latter mixed sori. Demistaedtia 

 apiifolia and piinctilobida have a markedly gradate sorus : but an inter- 

 mediate state is seen in Demistaedtia ritbiginosa (Fig. 208, A), where there 

 are still some signs of the basipetal sequence ; but younger sporangia are 



Fig. 208. A. '^owx's, o{ Dennstaedtia rubiginosa. Cut vertically, and showing a mixed condition 

 in a sorus originally basipetal. B = Davallia Griffithiana, Hk. Young sorus in vertical section 

 showing the first formation of sporangia. C. Old sorus of the same, showing sporangia of 

 different ages intermixed. AUxioo. 



interpolated without order among those pre-existent, while the receptacle, 

 which in gradate sori is convex or elongated, is here relatively flat. These 

 characters approach those of the fully mixed sorus with flattened receptacle 

 seen in Davallia (Fig. 208, C). Another example from a quite distinct affinity 

 is seen in Dipteris conjiigata. But here the progression is directly from the 

 simple sorus as seen in D. Lobbiatta. In the latter all the sporangia appear 

 simultaneously : but in D. conjiigata, which is an advanced species in other 

 respects, the sporangia are produced in succession, so that those of different 



