Development of Sporangia upon Fern Prothalli. 59 



gonial projections. In the place of the projection nearest to the 

 apex a vegetative bud arose. 



It is possible to draw some general conclusions from this series of 

 cultures. It is a striking fact that in every one of the species, 

 prothalli, which under normal conditions would have produced 

 normal embryos, became, after a longer or shorter period, apo- 

 gamous. Further there was a general similarity in the changes of 

 form and structure of the prothallus, which preceded this result. 

 This form of apogamy, occurring after prolonged cultivation of 

 normal prothalli under special conditions, may be distinguished as 

 induced apogamy, in contradistinction to direct apogamy, by which is 

 meant the immediate production of vegetative buds by prothalli, 

 which are usually incapable of being fertilised. Both forms occur 

 in Nephr odium Filix-mas. 



The causes which appeared to induce apogamy in these prothalli 

 were the prevention of contact with fluid water which rendered 

 fertilisation impossible, and the exposure to direct sunlight. Pos- 

 sibly the temperature also had some effect. The case of Nephrodium 

 Filix-mas shows that the variable condition of the sporophyte, as 

 indicated by cresting, &c., though possibly predisposing to the 

 changes which lead to apogamy, does not stand in any necessary 

 connection with the phenomenon. 



That different decrees of apogamy are distinguishable was also 

 shown by these cultures. The cylindrical process, arising from the 

 apex of the prothallus, or from its under surface, is to be regarded 

 simply as a modification in form and structure of the gametophyte 

 dependent on the altered conditions, and possibly a direct adapta- 

 tion to these. The next stage is seen in cylindrical processes, 

 which, while bearing sexual organs, also produce isolated members 

 of a sporophyte (roots, ramenta, sporangia). It is to be borne in 

 mind, however, that tissue differing from the rest of the process 

 always occurred beneath the last-named structures. The final stage 

 is the production of a vegetative bud capable of further growth as a 

 typical sporophyte. In this a series, leading from the bud arising 

 by transformation of the tip of a cylindrical process, to buds pro- 

 duced on or in the place of archegonial projections, and from this to 

 buds situated on the under surface of the prothallus itself, can be 

 recognised. 



The readiness with which the intermediate form between gameto- 

 phyte and sporophyte and the early stages of vegetative buds 

 reassume the prothalloid form, is worthy of note, as bearing on some 

 cases of apospory. 



These departures from the normal development of the prothallus 

 are not regarded as reversions in the ordinary sense, but as indica- 

 tions of the capability of direct response to altered conditions, 



F 2 



