GROUP III. PTERIDOPHYTA : FILICIN.E. 371 



centre to the stalk, so that its long axis is at right angles to the 

 stalk ; in Vittaria and Monogramme, the gemma is attached to the 

 stalk by one end. The sterigmata are developed either singly or 

 several together, from a single cell of the prothallium ; and the 

 gemmae may be borne singly or several together on one sterigma. 



It will have been observed that the gametophyte of the homo- 

 sporous leptosporangiate Ferns presents, in its development, its 

 root-hairs, its propagation by gemmae, remarkable and suggestive 

 resemblances to the gametophyte of the Hepaticse. In the general 

 morphology, too, of the gametophyte, there are striking corres- 

 pondences between the two groups : thus, in both groups (with 

 certain exceptions in both) the first stage in the life of the game- 

 tophyte is a filamentous protonema, which is, however, relatively 

 small and short-lived, except in the Fern Trichomanes where the 

 gametophyte does not develope beyond the protonematous stage. 

 The protonema in both groups gives rise to a single flattened, ex- 

 panded shoot, the adult sexual shoot of the Hepaticae, the prothallium 

 of the Ferns, bearing the sexual organs. 



Order 1. Hymenophyllaceae ; this order contains the simplest forms 

 The leaf-blade almost always consists of a single layer of cells ; the sorus 

 is always marginal (Fig. 214 A) and indusiate, the sporangium sessile or 

 shortly-stalked, and the annulus entire and horizontal. 



Almost all the species are tropical. Trichomanes radicans and Hymeno- 

 phyllum Tanbridgense and unilaterale (or Wilsoni) alone occur in Britain. 

 Some species of Trichomanes have no true roots. 



Order 2. Polypodiaceae. The annulus of the stalked sporangium is 

 incomplete and vertical (Fig. 215 r), that is to say, it is not continuous at 

 the base : indusium present or absent. Almost all our native Ferns belong 

 to this order, which is exceptionally rich in genera. 



The following are the chief families : 



(a) Pteridece. Sori coalescent along the margin of the leaf (Fig. 214 6')," 

 with a spurious indusium. Pteris (Pteridium) nquilina, the Bracken, has 

 a stem which grows at some depth below the surface of the soil, and throws 

 up every year a single large, much-segmented leaf (frond) : it has also a 

 true lateral indusium. Adiantum, the Maiden-hair Fern, belongs to this 

 group, as also Cheilanthes. 



(6) Aspleniece. The sorus, which is situated on the under surfaca of the 

 leaf, is elongated or linear, and the lateral indusium springs from the vein 

 to which it is attached (Fig. 214 E). Aaplenium Ruta muraria, the Wall- 

 B,ue, is nob uncommon on walls and rocks ; A. Trichomanes is also abund- 

 ant, with simple pinnate leaves and a shining black rachis. Athurt:im 

 F'dix foemina, the Lady Fern, is common in damp woods. Scolopendritim 

 vulgare, the Hart's-tongue, with entire leaves, is common in damp hedge- 

 raws and woods (Fig. 217). Blechnum (Lornaria), the Hard Fern, as also 



