XLi] INDUSIUM SUPERUM 127 



been traced through the Gleicheniaceae (Chapter XXIV), the Protocyathe- 

 aceae (Chapter xxxil), to the Cyatheaceae (Chapter xxxill), these last 

 supplying the innovation of a basal indusium. Taking up the sequence, 

 probably from some type of the Cyatheaceae with semi-lunar basal indusium 

 as it is seen in Heiuitclia, the innovation of lop-sidedness or zygomorphy 

 makes its first appearance in the indusium covering the previously radial 

 receptacle. This is itself tilted towards the leaf-margin in the stalked sorus 

 of Peranenia: it becomes markedly lop-sided and even horse-shoe shaped in 

 the Male Shield Fern, which however still retains traces of the basipetal 

 sequence of its radiate and gradate ancestry, though this is soon merged 

 in mixed sporangia borne on the zygomorphic receptacle. 



The type of sorus of Diyopteris filix-nias thus constructed, and interpreted 

 in phyletic terms, may be held as a centre for further advance in at least 

 three directions — the Aspidioid,the Polypodioid,and the Acrostichoid. These 

 advances are registered most clearly in the sorus, but they may accompany 

 other changes in the sporophyte. Of these the most obvious is the adoption 

 of a reticulate venation, in place of the open venation of the Male Shield 

 Fern and other relatively primitive types, a feature shared by typical forms 

 of PolysticJiuin. In varied detail vein-fusion has been a feature of the old 

 genera Cyrtomiuni Presl, Cyclodiiun Presl, and oi Eu-Aspidiuvi Hook. i^Syn. 

 Fil. p. 258): particularly of the genus Meniscmin Schreb., now merged in 

 Dryopteris under Section II, Goniopteris Presl (Diels, E. and P. I, 4, p. 177). 

 Vein-fusion is also characteristically seen in Luerssenia Kuhn, and Fadyenia 

 Hook. In all of these the reticulate venation may be held as an indication 

 of advance from the open venation, and it is biologically intelligible where 

 the leaf-area is extended. 



The ''indusium SUPERUM'' as seen in ASPIDIUM K^Y) Polystichum 



There can hardly be any two opinions as to the evolutionary relation of 

 the sorus oi Dryopteris to that seen in Aspidium (Swartz, 1801., 138 species), 

 Polystidmm (Roth, 1799, 112 species), and Cyclodiiun (Presl, 1836, 2 species). 

 Nor yet can their affinity one with another be doubted; indeed they were 

 all included under Aspidium by Sir Wm Hooker {Sp. Fil. Vol. IV, p. 6). He 

 describes them as Ferns very various in form, size, and composition, with 

 veins either free or variously anastomosing. But he remarks that in their 

 systematic arrangement entire dependence cannot be placed either on the 

 exact uniformity of their venation, or on the shape of their "involucres." 

 These occasionally vary, being sometimes orbicular, sometimes cordate on 

 the same species, and sometimes the form and the point of insertion appear 

 to be intermediate between the two. The venation typically free \n Polystichum 

 may in the broader-leaved types of Aspidium rise to a high degree of 



