174 



BLECHNOID FERNS 



[CH. 



conditions, while the latter were modified in harmony with mesophytic sur- 

 roundings. 



AcROSTicHoiD Derivatives 

 Certain Blechnoid Ferns have adopted an Acrostichoid character of their 

 sori. This seems a natural sequel to the formation of coenosori ; for if the 

 production of sporangia can extend longitudinally so as to link up sori 

 originally distinct,, why not also transversely? Evidence of it comes both 

 from the Lomarioid type, and also from that of Eu-Blechmnn. The former is 

 seen as a slight spread of the sporangia towards the margin in B. atteimatiim 



Fig. 696. Transverse section of a mature fertile 

 pinna of Blechmim attenuatum (Sw.) Mett. The 

 mid-rib is here traversed by two vascular strands, 

 the lateral wings being so curved downwards as 

 to give a quadrangular outline : they end in the 

 recurved indusial flaps, while the sporangia spread 

 over a considerable part of the inner surface. At 

 the adaxial angles are flask-shaped glands, often 

 present in Blechnum. ( x 8.) 



(Sw.) Mett. (Fig. 696), and still more in B. Patersoni (R. Br.) Mett., and in 

 B. pemia-marinai^oxx^ Kuhn (see Studies IV, Pis. XXVI, XXVll). In none of 

 these is the flange a marked feature. An Acrostichoid spread is more fully 

 represented in the genus Stenochlaena, most of the species of which have 

 from time to time been included in the old genus Acrostichiwi i^Syn. Fil. 

 p. 412). It consists, however, of nothing more than climbing Lomarioid 

 Ferns, with the fertile pinna widened out into an Acrostichoid expanse. 

 There is also the monotypic genus Brainea, which is a small Tree-Fern, 

 undoubtedly related to Eii- Blechmim. Thus there is reason to believe that 

 the transition to an Acrostichoid state has occurred repeatedly among the 

 Blechnoid Ferns. 



