DENNSTAEDTIA-DAVALLIA SERIES 



615 



FIG. 340. 



uniformity of structure of the sorus on the mixed plan, with flat receptacle, 



and with no definite rule of orientation of the long-stalked sporangia. 

 The connection of the Dennstaedtia-Davallia series 



with such genera as Lindsaya, Pteris, Pellaea, and 



Adiaiifinn, where the sori are marginal, seems beyond 



question, and it is strongly supported by the anatomical 



evidence. But, on the other hand, the sori are liable 



to move from the marginal position : this phyletic 



change is illustrated by very gradual steps. An 



inequality of the lips of the indusium is apparent in 



Mil role pia (Fig. 332 A) : it appears often in greater 



degree in the various forms of Davallia, and is 



specially marked in Leucostegia (Fig. 340), where 



the upper lip appears as the continuation of the leaf- 

 lobe, the lower as a cup-shaped indusium apparently 



some Distance from the margin. Leucostegia has long 



been recognised as closely related to Cystopteris, which 



also has a mixed sorus, without regular orientation j A ^, H ^^) romCh " sts 



of its sporangia, while it is protected by an indusium 



of similar form to that of Leucostegia (Fig. 341). These examples will 



iserve as illustrating a feature which has probably been widely effective 



in the descent of the Leptosporangiate 

 Ferns, viz. the retreat of the sorus from 

 the margin to the under surface of the 

 leaf. From Cystopteris the sequence 

 may with probability be traced on to 

 the Aspideae. It would thus appear 

 that a considerable proportion of the 

 Mixtae are referable in origin to forms 

 with a marginal position of the sorus. 

 A further illustration of probable 

 relationship, in this case to Polypo- 

 dium, is seen in Hypolepis. This 

 genus was included by Kiihn and by 

 Prantl 1 in the I )ennstaedtiinae, though 

 by others it has been placed in the 

 Pterideae. The marginal sori, covered 

 by the reflexed margin of the leaf, 

 consist of a slightly convex receptacle, 

 upon which the sporangia of various 

 ages are inserted in no definite order: 

 the annulus is definitely interrupted 



at the insertion of the .stalk. It is clearly one of the Mixtae, but the 



affinity with the 1 )ennstaedtiinae is indicated by the position and character 



1 /,.(., p. iS. 



Cystopteris fi-n^ilif. Pinnule of the form from 

 Tasmania, am! its .sums enlarged. (After H""l 

 from Christ's Farrnkrauta i 



