DENNSTAEDTIA-DAVALLIA SERIES 



615 



FlG ' 34 ' 



(After Hooker, from Christ's 



r arrnkrauter.) 



uniformity of structure of the sorus ort 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 Lindsay a, Pteris, Pellaea, and 

 Adiantum, 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 

 Microlepia (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 



. ' 



of its sporangia, while it is protected by an indusium 

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

 .serve 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 * in the Dennstaedtiinae, 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 Dennstaedtiinae is indicated by the position and character 



*L.C. . 1 8. 



FIG. 34 i. 



Cystopteris fragiiis. Pinnule of the form from 

 Tasmania, and its sorus enlarged. (After Hooker, 

 from Christ's Farrnkrciuter.) 



