PTERIDOPHYTA. 705 



ingly improbable. It is more likely that the two groups have had a common 

 origin, and have then developed along entirely different lines 



Alliance XXII. — Filices, Ferns. 



Families: Hymenophyllacece, Polypodiacece, Gyatheacece, Gleicheniacece, Schizceacece, 

 Marattiacece, Osmundacece, Ophioglossacece. 



With us for the most part Ferns have short underground stems bearing a rosette 

 of leaves as in the Male Fern (Asjndium Filix-mas) and Hart's-Tongue Fern (Scolo- 

 pendrium vulgare), or there may be an elongated horizontal underground rhizome, 

 as the Bracken Fern (Pteris aquilina) and Polypody (Polypodium vulgare), the 

 leaves being produced at intervals. In the tropics and sub-tropical regions, on the 

 other hand, Tree Ferns are common. They belong mostly to the genera Cyathea, 

 Alsophila, and Dicksonia. In these a considerable caudex is developed (cf. fig. 347, 

 p. 473, and vol. i. p. 714), which is often enveloped in a perfect plexus of aerial 

 roots. Many Ferns are epiphytic, especially in tropical forests (e.g. PlatyceriuTn 

 alcicorne, fig. 349, p. 475); with us Polypodium vulgare is often met with envelop- 

 ing the trunks and boughs of large trees. The tropical genus Lygodium is charac- 

 terized by its climbing habit, the long rachis of its compound leaf twining like the 

 stem of a twining plant. The view has been held, and is alluded to on p. 12, that 

 the fronds of Ferns are not really leaves but modified stem-structures, the scaly 

 structures that occur on the stem and fronds being regarded as the true leaves. It 

 is sufficient to say that this view is not very widely held amongst Botanists, and 

 that here the term leaf is used as synonymous with frond. Broadly speaking. Ferns 

 love moist and shady habitats ; they grow especially in woods and forests, and on 

 humid rocks beside streams, &c. Generally their leaves are thin and delicate, and 

 ill-adapted to withstand prolbnged desiccation. 



Borne on the leaves of Ferns are the sporangia, tiny capsules in which the spores 

 are developed. The form of the sporangium and the arrangement of these bodies 

 varies in the different families of Ferns — indeed the sporangia afford characters 

 which are used for the grouping and classification of Ferns. In the commonest 

 Ferns (belonging to the predominant family Polypodiaceae) the sporangium resembles 

 two watch-glasses placed together, the rim being occupied by a series of large, thick - 

 walled cells (the annulus), and the whole mounted on a little stalk (cf. fig. 400 "). 

 In other families the stalk may be absent, the annulus incomplete, oblique, trans- 

 verse or altogether wanting, &c., as will be pointed out in treating the several 

 families. The sporangia are aggregated into clusters, the sori, and these are in 

 many cases protected by little outgrowths of the leaf -surface (indusia) or under the 

 infolded margins of the leaf. The form and arrangement of the sori and indusia 

 provide the characters according to which the large family Polypodiacece is sub- 

 divided. 



Hymenophyllacece.— The Filmy and Bristle Ferns. There is generally a 

 rhizome which bears delicate fronds at intervals (cf. fig. 400 2); the lamina of the 



