XL] ZYGOMORPHY OF THE SORUS 113 



is flattened and wide, and there is a mixed succession of the sporangia. The 

 latter are inconstant in the structure of the stalk and in segmentation, 

 suggesting steps in reduction towards an advanced Leptosporangiate type. 

 Incidentally the frequent presence of a hair, glandular or not, upon the 

 sporangial stalk is distinctly reminiscent of Dryopteris filix-vias. In fact 

 Hypodcrris appears to be synthetic between the Cyatheoid and Dryopteroid 

 Ferns, while it affords another instance of the widely homoplastic assumption 

 of a mixed sorus, associated with flattening of the receptacle, and diminution 

 in size of the long-stalked sporangia in which the annulus is definitely 

 interrupted at the insertion, and the stalk may be reduced to a single row 

 of cells. 



Other advances not less interesting are seen in Diacalpe and Peranema, 

 and particularly in relation to their sori. In both of these monotypic genera 

 the stock is traversed by a dictyostele of very similar character to that seen 

 in Dryopteris filix-inas, and the leaf-trace is highly divided: but there are 

 no perforations. The sorus of Diacalpe conforms nearly to the Woodsia type, 

 but with mixed ages of the more numerous sporangia, while these resemble 

 in structure and in spore-output what is seen in Dryopteris. On the other 

 hand, by its glandular hairs and the segmented lid-cell in the antheridia 

 the gametophyte points rather to Woodsia z.\\di Cyathea. Peraiieiiia, with the 

 very Dryopteroid habit of its sporophyte, is particularly suggestive in its 

 soral features. At first sight its stalked sorus recalls that of Marattia 

 {Eiipodiiini) Kaidfussii ]. Sm. But such comparisons are only superficial: 

 the stalk of the sorus may be attributed in either case to an intercalary 

 extension below the receptacle: and this being an isolated phenomenon, 

 occurring in two very distinct families of Ferns, must needs be homoplastic. 

 The real interest in the sorus of Peranevia lies in its one-sidedness combined 

 with the mixed condition. 



{a) ZYGOMORPHY OF THE SORUS 



For the first time in the upward sequence of Ferns with superficial sori the sorus in 

 Peranenia departs definitely from that radial construction which characterises all the earlier 

 types where sporangia are associated sorally. This remark refers not only to the families and 

 genera above named, but also to the Marattiaceae and Matoniaceae. Indications of zygo- 

 morphy of the sorus appear, it is true, in the half-moon-shaped indusium of Hemitelia, and 

 in the incomplete ring of the indusial cup of Woodsia obtiisa: in both of these the indusium 

 is incomplete on the side next the margin of the pinnule. The same holds also ior Peranema: 

 but there the gap is combined with a lop-sided development of the whole receptacle, which 

 becomes still more pronounced in Dryopteris and related Ferns, as will be seen later. 

 This progression appears to be analogous to the secondary adoption of zygomorphy in the 

 shoots and flowers of Angiosperms. It is held to be a derivative state acquired in relation 

 to circumstances not uniform the zygomorphy appearing in relation to the form of the 

 part that bears the sori. 



Kill 8 



