284 



THE GAMETOPHYTE, AND SEXUAL ORGANS 



[CH. 



they do not reach the surface of the soil. The vegetative region is occupied 

 by a symbiotic fungus (Fig. 274, parts shaded in c and d), but it dies out about 

 the time that the sexual region begins to elongate, the growth being com- 

 pleted at the expense of the starch accumulated internally as a consequence 

 of its activity. There is a partial sexual differentiation; for instance, Fig. 274, 

 a and c, are exclusively male. But both antheridia and archegonia may be 

 present on the same prothallus, the former appearing first, as in Fig. 274, b, d. 

 The antheridia are very large and deeply sunken, but the archegonia have 

 projecting necks. 



All the Ophioglossaceae have saprophytic underground prothalli, but it 

 is recorded by Mettenius that when those of Ophioglossiim pediinculosuni 

 reached the light at the surface of the soil their cylindrical form became 

 flattened and lobed, and they assumed a green colour, but did not develop 

 further in the light. In Botrychiiim Lunaria and virginianum. there is a 

 tendency to a horizontal, thick, flattened form with localisation of the sexual 

 organs upon the upper surface. This may have an adaptive significance, so 

 that the surface arrests rain-water percolating downwards, which, bathing the 

 sexual organs, would facilitate fertilisation. Otherwise the approximately 

 cylindrical form is maintained. 



There are certain important differences between these prothalli and those 

 of Lycopods which suggest that their similarity of form depends more upon 

 similarity of mode of life than upon real affinity (Lang, I.e. p. 50). Comparison 

 within the Filicales is more fruitful, however different this type of Fern-pro- 

 thallus may appear to be from the rest. It is supported by the resemblance of 

 the sexual organs to those of the Marattiaceae. But in these the prothallus is 

 of the usual dorsiventral type. 

 It is found, however, that under 

 special conditions of cultiva- 

 tion the flattened form may be 

 lost even in Leptosporangiate 

 Ferns, the prothallus continu- 

 ing its growth as a cylindrical 

 process, with structure, apical 

 meristem, and sexual organs 

 like those of the sexual region 

 of the Ophioglossaceae (Fig. 

 275). This suggests that if it 

 were possible to cultivate ordi- 

 nary Fern-prothalli, of a fleshy 

 type like those of the Marat- 

 tiaceae, below the surface of 

 the soil the cylindrical form might be attained, as in the Ophioglossaceae. 



Fig. 275. Prothalli of Scolopendritim, after Lang. They 

 have grown out unfertilised into " cylindrical processes," 

 arising (A) from the apex, {B) from the under surface of 

 the prothallus, and bearing sexual organs all round. ( x 6.) 



