l66 THE MORPHOLOGY OF PTERIDOPHYTES 



to the stipe is different from that in Athyrium, for the two 

 bundles which enter it fuse into a single four-armed strand, 

 instead of into a gutter-shaped strand. The same is true of 

 Phyllitis. That Asplenium and Athyrium are closely related 

 seems fairly certain, since they have the same basic chromo- 

 some number, n = 36, and hybrids between them are known 

 to occur. In Phyllitis the sori occur in pairs, facing each 

 other, along the lateral veins (Fig. 25V), one acroscopic 

 and the other basiscopic. 



Blechnoideae 



Blechnum punctulatum forms a possible intermediate be- 

 tween Phyllitis and the more typical species of Blechnum, 

 for on one and the same frond both types of sorus may 

 occur, some in pairs facing each other and some showing 

 various degrees of fusion along a commissural vein. Wood- 

 wardia has a series of box-Hke sori, on either side of the mid- 

 rib, whose indusia are Uke hinged Hds. The typical Blechnum 

 sorus is a continuous one, as if the adjacent sori of a Wood- 

 wardia had become fused together, with the indusium facing 

 the midrib of the pinna (Fig. 25U). Each has beneath it a 

 commissural vein, which is visible in Fig. 25T, where part of 

 the two sori have been removed to expose it. The British 

 species, here figured, shows a considerable reduction of the 

 fertile lamina, and this reduction process has gone much 

 further in other members of the subgenus Lomaria, where the 

 lamina is almost completely lacking. Such species are 

 markedly dimorphic, for the sterile fronds have a normal 

 unreduced lamina. The genus shows a wide range of habit, 

 for some species are creeping, some are cHmbing, while 

 several have erect trunks, like small tree ferns. 



Adiantaceae 



This is a very diverse family, some members of which show 

 marked similarities with Mohria (Schizaeaceae). Their sori 

 are without indusia and occur along the veins or else form 



