CONJUGATION IN FERNS 105 



of the neck separate so as to form an opening (Fig. 52, B). The 

 spermatozoa appear to be attracted to the opening by an acid 

 secretion discharged therefrom. One of them makes its way 

 down the neck to the ovum and fertilizes it by the usual process 

 of conjugation. 



The zygote begins to develop, by cell-division, within the 

 venter of the archegonium,and forms a young sporophyte, which 

 for some time remains attached to the prothallus as shown in 

 Fig. 50, B, drawing nutriment therefrom by means of a special 

 temporary organ known as the foot. Presently, root, stem and 

 leaf are developed and the sporophyte becomes self-supporting. 



One very remarkable fact in connection with the sexual 

 process in the fern remains to be noticed. The gametes, as we 

 have seen, are normally produced in special sexual organs and 

 are themselves perhaps as highly differentiated in relation to 

 the function of conjugation as gametes ever are. It has been 

 found, however, that if the normal sexual union between ova 

 and spermatozoa be prevented a conjugation may take place 

 between nuclei from adjacent vegetative cells of the prothallus, 

 resulting in the formation of an embryo sporophyte by so-called 

 apogamy. In these cases it is obvious that the sexual process 

 is not really suppressed, but simply transferred to ordinary 

 prothallial cells, which, though they do not conjugate under 

 normal circumstances, have retained the power of so doing when 

 occasion arises. It seems probable, however, that in some cases 

 true apogamy, or suppression of the sexual process, occurs, the 

 embryo sporophyte arising from the prothallus without any 

 conjugation of gametes. 



The gametophyte or prothallus of an ordinary fern is, as we 

 have already seen, produced by the development of a unicellular 

 spore (Fig. 49), and in most cases is monoecious or hermaphro- 

 dite, bearing both male and female sexual organs and male and 

 female gametes. In the less commonly known and not very 

 fern-like " heterosporous " forms, however (Isoetes, Salvinia and 

 Marsilea), the gametophyte is dioecious or unisexual, there being 

 distinct male and female prothalli, and this sexual differentia- 

 tion affects not only the prothalli themselves but the spores from 

 which these are developed. Hence in these forms we find small 

 microspores which produce male prothalli, and large megaspores 

 which produce female prothalli. The spores themselves are set 

 free from the parent sporophyte, but the prothalli are very much 



