706 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



leaf is often only one cell thick, and stomata occur only in the genus Loxsoma (New 

 Zealand). The other genera, Hymenophyllum (the Filmy Fern) and Trichomanes, 

 are both met with in Great Britain. The former is not uncommon on the rocks 

 beside waterfalls, but the latter {Trichomanes radicans, the Killarney Fern) seems 

 to be almost restricted to the south of Ireland. In this family the sporangia occur 

 at the margins of the fronds on the excurrent veins (see fig. 400^). They are 

 sessile, and the annulus is transverse, i.e. at right angles to the axis of the sporan- 

 gium. The sorus is surrounded by an enclosure formed from the leaf -margin; this 

 investment is cup-shaped in Trichomanes and bivalved in HyTnenophyllum. Often 

 in the former genus the axis on which the sporangia is inserted projects considerably 

 from the cup — hence the name Bristle Fern. In this family the prothallium is 

 unlike that of other ferns, being frequently filamentous and branched; the filaments 

 often bear local expansions, upon which the archegonia are inserted. 



There are about 200 species of Hymenophyllaceee. 



PolypodiacecB. — By far the largest family of Ferns; indeed this family includes 

 more than three times as many species as all the rest of the Pteridophytes together. 

 Almost all our familiar European Ferns belong to it. The character which they all 

 have in common is a stalked sporangium (fig. 400"),. with vertical annulus. The 

 distribution and form of the sori are exceedingly various, The PolypodiaceES have 

 been separated into the following tribes: — Pteridece, Aspidiece, Aspleniece, Daval- 

 liece, PolypodiecB, Grammitidece, Acrostichece. In the Pteridece the sori occur at 

 the margin of the leaf; in the Bracken Fern (Pteris aquilina) the frond is much 

 branched, and the sori are everywhere continuous on the pinnule-margin; they are 

 covered in by an indusium derived from the margin; in the Maiden -hair Fern 

 {Adiantum Oapillis-Veneris) the tip of the pinnule is folded back over the sorus. 

 In the Aspidiece the sori are scattered, circular, and covered in by a circular or 

 kidney-shaped indusium. Aspidium Filix-mas (the Male Fern) belongs to this tribe. 

 The sorus is much elongated and linear in the Aspleniece, and the indusium is 

 inserted on one side of it (e.g. Asplenium Ruta-muraria, figs. 401 « and 401'). 

 The Lady Fern (Athyrium Filix-foemina), Hard Fern (Blechnum), Hart's-tongue 

 (Scolopendrium), &c., are members of this tribe. In the Davallieoe, which include 

 the large tropical genus Davallia, the sorus is near the margin, and inclosed in a 

 pocket-like indusium. In the Polypodiece the sori are circular and scattered over 

 the under surface of the frond. There is no indusium (see fig. 400 =). The Oram- 

 mitidece resemble the last-named in the absence of an indusium. The sori usually 

 follow the veins, frequently forming very elegant reticulations on the under surface 

 of the leaf, as in the tropical genus Hemionitis. The Gold and Silver Ferns (Gym- 

 nogramm^) belong to this tribe. In the AcrostichecB the whole under surface is 

 covered with sporangia, and there is no indusium. Examples are, Rhipidopteris 

 (fig. 400*), Platycerivm (fig. 349, p. 475), and Acrostichum. 



Nearly 3000 species of Polypodiaceae are known. 



Cyatheacece.— This family includes the' Tree-ferns (fig. 347, p. 473). The 

 annulus of the sporangium is slightly oblique; it is only indifferently represented in 



