158 ONOCLEOID FERNS [ch. 



pinnate, and the pinnae sometimes pinnatifid. The pinnae are broadly 

 lanceolate, with a mid-rib and Pecopterid venation: but there are profuse 

 anastomoses of the veinlets (Fig. 678, G-L). The rhizome contains an 

 extended dictyostele, with some indications of those superficial axillary 

 pockets which are a marked feature in M. struthiopteris (Fig. 679), though 

 absent in M. orientalis and intermedia. This suggests that its creeping habit 

 is secondary. The gradate condition of the sorus has already been alluded 

 to: the indusium, as shown in Fig. 681, consists of only a single layer of 

 cells, in contrast to the massive growth in Cyatliea. The sporangia are as in 

 M. struthiopteris. 



The gametophyte, sexual organs, and embryology of the Onocleoid Ferns 

 appear to follow the usual Leptosporangiate type. Details are given b\- 

 Campbell {Mosses and Ferns, 191 8, pp. 305, etc.). 



Comparison 



It has already been suggested that the relation of the Onocleoid Ferns 

 downwards is with the Cyatheaceae,all of them having superficial and gradate 

 sori. Upwards, as indicated by earlier writers, it is with the Blechnoid Ferns 

 which, as we shall see, have also superficial sori. In fact the Onocleoid Ferns 

 may be regarded as a synthetic group, linking the Cyatheoids with the 

 Blechnoid Ferns. It has also been shown in Chapters XL, XLI, that a deri- 

 vative upward line from the Cyatheaceae has led through the Woodsioid 

 Ferns towards the Dryopteroids; and a firm basis of fact indicates that the 

 Asplenioids are derivative from the latter. The question will then arise as 

 to the relation of these two progressive lines inter se: we shall enquire how 

 far they may be regarded as separate in their history, and therefore distinct 

 in their natural or phyletic classification. As an aid to the structural com- 

 parisons detailed in these Chapters a working hypothesis based on a biological 

 reading of those facts may be introduced here, and tested by its application 

 to a wide area of fact. 



The comparisons already detailed in Chapter XLI indicate that the 

 Asplenioid Ferns were by origin delicate in texture, though certain of their 

 derivative types became in some degree xerophytic. This is seen in many of 

 the smaller species oi Aspleniiim, such as A. ruta-viuraria, as well as in the 

 epiphytic species, such as A. nidus, while xerophytism is specially marked 

 in the structure of ^. Ceterach. But the simpler Dryopteroid types, from which 

 the Asplenioid Ferns are all believed to have been derived, had highly divided 

 Sphenopterid foliage, with that relatively large proportion of surface to bulk 

 which is suitable enough for plants that grow habitually in shade. This 

 condition is perpetuated in Athyrium, which comparison points out as the 

 genus probably nearest allied to Dryopteris: many other Asplenioid Ferns 



