172 BLECHNOID FERNS [CH. 



which here takes no direct part in its formation (Fig. 692, 12, b^ c). Active 

 growth on the side remote from the mid-rib tilts it over, so that it faces 

 centrally (d). It takes an indusioid form, over-arching the receptacle and 

 the sporangia which it bears; the indusial margin, curving strongly, forms 

 a very perfect protection for the receptacle {e). There is at first a gradate 

 sequence of the sporangia {d, e) : but later the 

 sorus becomes pronouncedly "mixed." 



Certain types of Eu-Blechmm are doubly 

 pinnate, such as B. Fraseri (A. Cunn.) Luerss., 

 a species that somewhat resembles B. spicant 

 in the development of its fertile segments: or 

 B. {Salpichlaend) volubile Klf. (Fig. 693, D). 

 The latter is an American species, which has 

 assumed a climbing habit, somewhat like that 

 of Lygodium. It shows variability in its di- 

 morphism, while the vein-endings anastomose 

 closely within the margin of the broad expanse. 



Double pinnation, but with an open venation, Fig- 694- Sadleria cyatheoidesYAi. A 



^ , ., r- 77 • Simple pinnule from the doubly pinnate 



also occurs m the dendroid genus Sadleria leaf, showing the venation, and its lela- 



(Fig. 694). Its coenosori are of the Eu-Blech- tion tothe coenosori. (X3.) 

 noid type, while the venation of the pinnule appears as a quite probable 

 elaboration of the branched veins seen in Matteiiccia (Fig. 678, B), or in 

 Cyathea simiata (Vol. II, Fig. 557). 



The specific details given in these pages will have already outlined a 

 morphological progression within the large genus BlecJinum. This may be 

 held as starting from the Alattenccia-type, with dimorphic leaves, and 

 superficial, non-indusiate sori, disposed in two parallel rows, and covered by 

 a simple curving downwards of the marginal flaps, which thin off almost 

 like an indusium: but no true indusium is present. The sori here are 

 isolated, of radiate form, and the succession of their sporangia is strictly 

 gradate (see Fig. 683, Chapter XLiii). If the rows of sori o{ Matteiiccia were 

 linked together into continuous series the coenosori of Blechnnm would 

 result. In Blechnnm tabulare, lanceolattivi, and others, the fertile pinnae 

 conform very nearly to the type of Matteiiccia. But indications are already 

 seen in these species that the protective flap does not always coincide with 

 the margin as defined by segmentation : the divergence is most marked 

 towards the apex and base of the pinna. Proceeding through the series of 

 species described this divergence increases, both in time and place of origin 

 of the flap, till in Eu-Blechnum the state is reached as seen in B. brasiliense. 

 The series (more fully worked out in Studies IV than as here stated) indicates 

 that the true margin by descent is the indusium-like flap, but that it has 



