404 PART III. THE CLASSIFICATION OF PLANTS. 



Propagation by means of gemmae is common in the Hymeno- 

 phyllacese, but it has also been observed in certain Polypodiacese 

 (Vittaria, Monogramme). In Hymenophyllum, the gemmaB are 

 small flat plates of cells ; in species of Trichomanes, Vittaria, and 

 Monogramme, they are short spindle-shaped filaments, consisting 

 of a single row of (6-9) cells, borne on a unicellular stalk or 

 sterigniR ; in Trichomanes, the gemma is attached at its 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 adventitious shoots and gemmae, re- 

 markable and suggestive resemblances to the gametophyte of the 

 Hepaticse. In the general morphology, too, of the gametophyte, 

 there are striking correspondences between the two groups : thus, 

 in both groups (with certain exceptions in both) the first stage in 

 the life of the gametophyte is a filamentous protonema, which is, 

 however, relatively small and short-lived, except in the Fern Tri- 

 chomanes where the gametophyte does not develope beyond the 

 protonematous stage. The protonema in both groups gives rise to 

 a single flattened, expanded shoot, the adult sexual shoot of the 

 Hepaticae, the prothallium of the Ferns ; this bears the sexual 

 organs either directly on a receptacle, or indirectly on a special 

 branch (gametophore). The Fern Trichomanes presents, on the 

 whole, more similarity to the Mosses than to the Liverworts in 

 its more highly developed protonematous stage bearing numerous 

 gametophores, which may be compared to the numerous adult 

 sexual shoots developed on the Moss-pro tonema, though in Tricho- 

 manes there is no such development of vegetative organs as there 

 is in the Moss-shoots. 



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. 257 A) and indusiate, the sporangium sessile or shortly-stalked, 

 and the annulus entire and horizontal. 



Almost all the species are tropical. Trichomanes radicans and Hymenophyllum 

 Tunbridgense and unilateral (or Wilsoni) alone occur in Britain. Some species 

 of Trichomanes have no true roots. 



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

 plete and vertical (Fig. 258 r), that is to say, it is not continuous at the base : 



