XL] INDUSIUM INFERUM 117 



AlsopJiila itself, and more constantly in Hemiielia and Cyathea^ by lateral extension or 

 possibly by webbing, or both, produced that flattened organ which has been styled in 

 those Ferns the ^Hndiisijitn inferiiiii." The whole question requires to be examined afresh, 

 not in one species or only in a few, but very widely throughout the Cyatheaceae, before 

 any final decision can be taken. But meanwhile the facts stated above appear to justify 

 the working hypothesis that the '■'■ indiisiinn infenun^' is in origin of the nature of a dermal 

 appendage : and that it arose by transformation of such dermal appendages as are borne 

 by the vegetative parts. 



This is in fact a slight modification of the view first suggested by Sir William Hooker 

 for the Cyatheaceae. It was met with opposition on somewhat slender grounds by 

 Mettenius ; this opposition is, however, supported by a general unwillingness that exists 

 to entertain the idea of polyphyletic origins. There is a natural tendency in evolutionary 

 morphology to read all apparently similar organs in terms of that which is the most 

 familiar, and so to reduce the categories of parts to their simplest terms. Instead of 

 yielding to this we may in the present instance contemplate how a protective indusium 

 has arisen not only from a Schizaeoid-Dicksonioid source (which has apparently supplied 

 the type general for the Marginales), but also from a Gleichenioid-Cyatheoid source 

 (which apparently supplied the type general for most of the Superficiales) : another source 

 has been the Matonioid, but it has had only a restricted application: another again gave 

 those distal lobes of the sporangiophore of Hel)]iinthostachys, which are functionally pro- 

 tective as a rudimentary indusium (Vol. 11, Fig. 364, G). A fifth type of protection is that by 

 hairs with a stellate or discoid head interspersed among the sporangia, as in Platycerium 

 (Hooker, Gen. Fil. Plate LXXX, B\ or Polypodium lineare Thbg. {,Gen. Fil. Plate xvill). 

 The variety of adaptive methods such as these last points a warning against too rigid a 

 search for uniformity of origin of indusia, however similar in form or structure. 



Turning now to the Woodsieae, the development of the indusium in them has interesting 

 features bearing on its probable dermal origin. It has been suggested for the Cyatheaceae 

 that lateral extension and possible webbing may have been involved in the formation of a 

 membranous indusium from the more primitive hairs seen in the sori of non-indusiate 

 related Ferns. Schlumberger's excellent analysis of the details in the developing sorus of 

 Woodsia ilvcnsis and obtusa is reproduced as Fig. 647 (p. 102). In his discussion of the 

 facts he refers the indusium of the Woodsieae to the type seen in Cyathea, of which he holds 

 it to be a reduced form {I.e. p. 406). He contemplates the dissolution ("auflosung") of the 

 continuous cup into individual hairs. But in view of the general argument of the preceding 

 paragraphs, for us his thesis must be inverted. We shall see in these drawings from 

 Woodsia early stages in an upward construction of a cup-like indusium from the primitive 

 constituent hairs, such as were already present in the non-indusial sori of Lophosoria, or 

 Alsophila. In Fig. 647, i, 2 of W. ilvensis the simple hairs (/, i) appear isolated : in 

 Fig. 647, 6 of W. obtusa some of them are extended laterally, and segmented so as to 

 form flattened scales : in Fig. 647, 5 the receptacle is surrounded by an upgrowth forming 

 a ring-like wall, lateral fusion of the constituent hairs being prevalent ; the ring is open on 

 the side towards the leaf-margin, but later it may be completed (Fig. 647, 7, s). The final 

 consequence of such steps would be a more or less perfectly cup-like indusium, with a 

 fringed lip, the laciniae indicating the constituent hairs or scales. The fringed indusium 

 of Hypoderris will bear a like interpretation (Hooker, Gen. Fil. Plate l). When it is remem- 

 bered that many species of Cyathea are lofty Tree-Ferns, while Woodsia and Hypoderris 

 are small low-growing Ferns, the more complete indusial protection of the sori in the 

 former finds its biological rationale : while the latter, subjected to less drastic demands, 

 may be held to have retained a more rudimentary state of their indusia. On the other hand, 



