XII] PROTECTION OF THE SORUS 237 



Of the Simplices with superficial sori the Marattiaceae and Gleicheniaceae 

 are without any definite indusium. But among the Gradatae there are Ferns 

 with sori in the same position, some without and some with a basal indusium. 

 The Cyatheae and Onocleinae provide examples. The physiological use of 

 the basal indusium is the same as before. The question naturally arises, is 

 the indusium here the strict homologue of that in the Dicksonia-Davallia 

 series, or is it of independent origin ? Sir Wm. Hooker held that it here 

 represents a specialised ramentum, an opinion combated by Mettenius. 

 Von Goebel {Organographies II. Aufl. p. 1 148) holds that it is a derivative of 

 the indusium of the marginal sorus ; but without basing the comparison on 

 grounds wider than the soral characters themselves. For reasons based not 

 only on the sorus but upon a general comparison, which will be detailed in 

 the phyletic section of this work (Vol. II), it is held that the basal indusium 

 of the Cyatheae and Onocleinae is of different origin from that of Dicksoma, 

 and appeared after the sorus had assumed the superficial position. 



Gleichenia, with no indusium and median dehiscence, has as we have seen 

 increased the number of its sporangia in G. dichotonia and pectinata to the 

 point of deadlock as regards dehiscence. Lophosoria, with many points in 

 common, but with slightly oblique dehiscence, is in nearly the same position. 

 But Alsophila and Matteuccia have met the difficulty by adoption of a gra- 

 date sorus and oblique dehiscence, though there is no protective indusium 

 (Fig. 226). There is no reason here to assume abortion to account for its 

 absence, since in Gleichetiia and certain related fossils it appears to have been 

 primitively absent. In Hemitelia, Onocka, and Cystopteris, however, a partial, 

 that is a one sided, indusium is present (Fig. 236, E, O), and in Cyathea 

 there is a very complete basal indusium. Peranenia and Diacalpe appear to 

 be. transitional links between these Ferns and the Aspideae, all having 

 superficial sori. There seems to be no reason to hold any other view than 

 that the indusium in all of them is a new formation, phyletically distinct 

 from that of the Schizaeoid-Dicksonioid Ferns, but serving the same bio- 

 logical purpose. This is the conclusion that naturally follows from a com- 

 parison on a wider basis than the sorus alone affords: in fact these Ferns 

 would appear to have arisen from a non-indusiate Gleichenioid source with 

 superficial sporangia. 



ment, introrse. Z) = strongly modified protective margin of Cassebeera triphylla, Kaulf. , for com- 

 parison with the introrse indusium. E=Cystopteris fragilis, Bernh., indusium attached laterally 

 at a point, extrorse. F= Diella falcaia, Brack., indusium attached laterally along a short line, ex- 

 trorse. G = Nephroditim Filix-mas, Rich. , indusium superior, attached laterally at a point, extrorse. 

 H — Hwnata heterophylla (Desv.), Sm., indusium attached laterally with linear insertion, and joined 

 to the leaf-surface, extrorse. J—Davallia canariensis, Sm., indusium lateral, attached on three 

 sides to the leaf-surface, extrorse. K=Pteridium aquilijium (L.), Kuhn, two indusia, one extrorse 

 the other introrse, fringed. L = Scolopendriuiii brasiliense, Kze, paired indusia over a pair of sori, 

 the one extrorse the other introrse. M= Fadyenia prolifera. Hook., indusium superior, centrally 

 attached, elongated-kidney-shaped. N= Aspidiiim trifoliatuni (L.), Sw., indusium superior, cen- 

 trally attached, circular. = Struthioptei-is germanica, Willd., indusium reduced, since there is 

 also a broad covering margin present. (After Dials, from Engler and Prantl.) 



