PTEROPSIDA 157 



350 of Trichomanes, of which there is one in the British Isles. 

 Because of their deUcate nature, almost all of them are 

 confined to moist habitats, and most of them are restricted 

 to the tropics, where they commonly grow as epiphytes. 

 The British species H. tunbrigense may be seen growing on 

 rocks constantly wetted by the spray from waterfalls. 



Most filmy ferns have a thin wiry creeping, protostelic, 

 rhizome, from which the fronds arise in two rows. In one 

 species, the stele of the rhizome is reduced to a single 

 tracheid, while in another there is said to be no xylem at 

 all. Some species are completely without roots. The leaf 

 trace is a single strand, which at the base of the stipe 

 shows marked similarity to the stem stele but, higher up the 

 stipe, broadens out into a gutter-shaped strand. The frond 

 is usually much branched, each narrow segment having a 

 single vein, but various degrees of 'webbing' occur and, in 

 one species, Cardiomanes reniforme ( = Trichomanes reni- 

 forme), there is a single expanded lamina. Nevertheless, the 

 venation is open in all species. 



The sori are marginal, and most species are strictly 

 gradate. The vein leading to the sorus continues into a 

 columnar receptacle which, in Trichomanes, can grow by 

 means of an intercalary basal meristem until it forms a 

 slender bristle. The receptacle of Hymenophyllum has more 

 limited powers of growth or may lack them altogether. In 



H, Cyathea Dregii; i, Dennstaedtia cicutaria; J, Micro lepia 

 Speliincae; k, Matteuccia struthiopteris {=Struthiopteris germa- 

 nicd); l, Dryopteris filix-mas; m, Polystichum lobatum; N, 

 Nephrolepis davallioides ; o, Pteris tripartita; p, Pteris cretica; 

 Q, Pteridiiim aqidlinum; R, Athyrium filix-femina; s, Adiantum 

 Parishii; T, Lomaria spicant; u, Blechnum occidentale; v. 

 Phyllitis scolopendrium : w, Asplenium lanceolatum 



(1, outer indusium; 2, inner indusium) 



(a, b, j, u, V, after Diels; c, d, o, Holttum; f, Eames; g, h, s, 

 Hooker; i, Baker; k, l, m, w, Luerssen; n, r, Mettenius; p, q, 

 T, Bower) 



