XLix] SUPERFICIAL DERIVATIVES 277 



dermal appendages, has been advanced in Vol. II, p. 307, and it is more fully 

 stated in Chapter XL, on the Woodsieae. Its extreme variability in Alsophila 

 and Cvathea, noted by Christ, supports this view {Farnkrduter, p. 323). The 

 smaller size, lower spore-output, and greater precision of structure of the 

 sporangia in the Cyatheoids are in accord with the introduction of the gradate 

 sequence, and find their homoplastic parallel in the Hymenophyllaceae. 



The Woodsieae (Chapter XL), which may for the most part be regarded 

 as arctic and mountain congeners of the Cyatheoids, form an important 

 bridge to the very large series of the Dryopteroid Ferns. They introduce 

 two fresh features among the Superficiales, viz. a mixed condition of the 

 sorus, and its lop-sidedness or zygomorphy. The radial sorus of Woodsia is 

 gradate, with a basal indusium, the various forms of which readily accord 

 with an upgrade origin of a coherent cup, derived from hairs such as are 

 seen in a like position in Gleichenioid Ferns, in Lophosoria, and in Alsophila 

 (see p. 114). The gradate state characteristic of Woodsia is hardly more 

 than indicated in Diacalpe and Pcraiiema, for profuse interpolation of 

 sporangia soon intervenes upon the widened receptacle, giving the state 

 typically present in Dryoptcris. Finally Hypoderris is a specialised shade- 

 form, with widened reticulate blade : but the mixed sorus with basal indusium 

 is still radial as in Woodsia, from which it is probably a derivative. 



In Peranema the receptacle, at first radial, as it is in all earlier Super- 

 ficiales, soon becomes lop-sided and stalked (p. 113). Its indusium, which covers 

 the receptacle completely, is unequally developed, being absent on the side 

 next the leaf-margin : thus a narrow slit is left facing outwards from the mid- 

 rib, while the receptacle itself is tilted over in the same direction, and covered 

 by the stronger growing side of the indusium. This zygomorphy of the 

 sorus corresponds to that general in Dryopteroid Ferns, while the habit, 

 anatomy, and chaffy scales of Peranema all point to its synthetic position 

 between Woodsia and Dryopteris. Probably Cystopteris and Acrophorus find 

 their natural affinity here (Chapter XLViIl). 



The view at which we arrive by following the phyletic line thus traced is 

 that the reniform Dryopteroid sorus is a zygomorphic derivative of a super- 

 ficial radial sorus, protected by an indusium originally basal, and derived by 

 modification of dermal appendages. A comparatively slight modification of 

 it, by extension of the receptacle right and left so as to encircle the stalk 

 of the indusium, and fusion of its edges, gives the '■'indusium superum " as 

 seen in PolysticJmm (Von Goebel, I.e. p. 1 1 50). In other Ferns of this affinity 

 various degrees of abortion of the indusium are seen leading to its complete 

 absence: as in Meniscium^ ox Phegopteris. The latter, long included as a 

 section oi Polypodium, consists of Ferns that are ex-indusiate Dr)'opteroids. 

 Lastly, the genera Polyboirya, Stenoseviia, together with the pinnate species 

 o{ Leptoehiliis, are Dryopteroids in which the individuality of the ex-indusiate 



