278 THE GAMETOPHYTE, AND SEXUAL ORGANS [CH. 



flattened scales, are present : the prothalli of Hemitelia share the feature 

 common for the Cyatheoids, viz. the presence of flattened scales. These 

 facts again illustrate the parallelism between the two generations in respect 

 of their appendages already noted for Notholaena. Nor is it surprising that 

 the similarity should exist, for both are mere stages in the completed life- 

 cycle. 



The arrangement of the gametangia in Ferns appears to be in high 

 degree fortuitous, and dependent on circumstances. In the case of green 

 prothalli light of sufficient intensity is necessary for the formation of both 

 antheridia and archegonia, but the former require less intensity than the 

 latter (Nagai, Flora, 19 14, p. 326). The position of the antheridia on the 

 Nepkrodmm-\.yp& of prothallus may be marginal or superficial : they are 

 usually on the surface, and directed downwards. They appear as a rule 

 before the archegonia, and consequently are most numerous in the basal 

 region of the prothallus, and later their formation may cease altogether on 

 the protlfalli where archegonia are being formed. The consequence is a 

 partial separation of the sexes, which is accentuated by the fact that starved 

 prothalli usually bear antheridia only. 



The archegonia being borne as a rule on the lower surface of the massive 

 cushion, and in acropetal succession, are more regularly arranged than the 

 antheridia. Their position appears, however, to be determined by the inci- 

 dence of light : for if both sides of a prothallus are about equally lighted 

 archegonia may be formed on both surfaces. This result has its interest for 

 comparison with what is seen in Polystichuni anomalum, in which the sori 

 may arise from the upper instead of the lower surface of the leaf. In both 

 a transfer of the stimulus of formation of the part is held to have taken 

 place : in respect of the archegonia the transfer can be correlated with ex- 

 ternal conditions. But it does not seem probable that the same holds with 

 the sori, for Sir Wm. Hooker states that the Fern retained its anomaly under 

 cultivation at Kew, where the conditions must have been very different from 

 those of the uplands of Ceylon {Species Filictim,\\, p. 27). In this instance, 

 as in its characters at large, the sporophyte seems less directly susceptible to 

 the impress of circumstances than the gametophyte (see p. 217). 



The Filamentous Type 

 There is another type of prothallus characterised by a more persistent 

 filamentous structure than that seen as a consequence of growth under 

 diminished light in the instances above described. It is found in the 

 Hymenophyllaceae, and in Schizaea. The prothallus of ^. pusilla is com- 

 posed entirely of branched filaments : the same appears to hold also for 

 5. bifida : flattened expansions have not been recorded in them (Britton 

 and Taylor, Torrey Bot. Club, Vol. xxviii, 1901), nor in 5. nipestris (von 



