XL] INDUSIUM INFERUM 115 



Similarly the problem of the indusium in the superficial series may be held as distinct 

 from that of the marginal series. The two are more nearly parallel than are those of 

 Matonia and Polystichmn : owing to convergent evolution the resemblances are some- 

 times very near. But those who have followed the comparative steps illustrated by 

 Gleichenia, Lophosoria, Alsop/iila, Heniitelia, and Cyathea will have recognised a natural 

 sequence, the gradual steps of which have led, in Ferns with consistently superficial sori, 

 from Mesozoic and perhaps from Palaeozoic times to the present day. The indusium 

 imposed upon it is a relatively late step. On the other hand, the marginal series from the 

 Schizaeaceae to the Dicksoniaceae and Dennstaedtiinae showed an earlier adoption of 

 indusial protection. Both palaeontological and comparative evidence indicates that these 

 two sequences, however similar in certain details, have pursued separate courses since 

 Mesozoic times. Hence the comparatively late appearance of indusial developments in one 

 of them must be held as a separate evolutionary problem from its earlier appearance in 

 the other. However far homoplastic resemblance may extend, arguments will not appear 

 convincing which involve any assumption of strict homogeny. And so the origin of the 

 indusium in the Cyatheaceae and Woodsieae must be considered as an evolutionary 

 problem to be solved according to the facts available from the superficial series only. 



Professor Von Goebel has interpreted the indusium oi Cyathea in terms of a Dicksonioid 

 origin {Organographies II, 2, p. 1149, 1918). He derives it from the lower indusial lip of 

 Dicksonia, closed round the receptacle, which had itself passed to the lower leaf-surface. 

 The condition seen in Hemitelia is held as evidence of this origin. His argument, thus 

 expanded from relatively advanced marginal to relatively advanced superficial types,1 is 

 founded on comparison of the indusia themselves, which are parts of relatively late origin 

 compared with the sporangia they protect, or the leaves and axes that bear them. It 

 leaves oiit of account those intermediate types with naked superficial sori, which would 

 lead, as we shall see, to a different conclusion. Moreover, no reference is made to 

 palaeontological evidence, which demonstrates that the superficial series of Ferns has 

 existed as such, with constantly naked sori, since early Mesozoic times. The whole question 

 must be held to be still open for discussion till a wider sweep of evidence is adduced than 

 the mere form of the indusium as it is seen to-day in the Ferns compared. 



Historically the interpretation of the Cyatheoid indusium stands thus. Robert Brown, 

 in founding the genus Hemitelia in 18 10 {Prod. Fl. Nov. Hall. p. 158), regarded the 

 semilunar basal scale as a "true involucre." Sir Wm. Hooker {Sp. Fit. i, p. ^j) says that 

 to him it appears to be of the nature and texture of the buUate scales common on this and 

 other Cyatheaceae, and embodies this in his specific description of H. capense: "rachis and 

 costa with small bullate deciduous scales, and one lax laciniated one at the inferior base 

 of each sorus." Mettenius, however, resists this idea {Fit. Hart. Lips. p. no, Taf. xxix), 

 on the ground that the base of the scale-like indusium surrounds the inner half of the 

 receptacle, and persists even in the oldest sori, while the neighbouring paleae are inserted 

 in the well-known way. He makes no allowance for specialisation, and demands that the 

 indusium shall be exactly like the paleae in position and endurance, otherwise the com- 

 parison is invalid. It is strange that so thin an argument should have persisted so long, 

 and be quoted as a demonstration in 1918. 



A most valuable contribution to the question was made in recent times by Christ 

 {Farnkrdiiter, 1897, p. 323). In describing the genus Alsophila he remarks on its complete 

 identity with Cyathea ("voile Uebereinstimmung"), with the sole exception of the 

 indusium. He notes the hairiness of the receptacle, and that here and there a scale is 

 found at the base of the sorus, fringed, short, and caducous. He further remarks how this 

 populous genus has exactly the same appearance, structure, and distribution as Cyathea., 



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