GENERAL CONCLUSIONS l8l 



sporophyte was successfully reared until it produced 

 sporangia; however, as would be expected since it contained 

 only one set of chromosomes, meiosis failed and no spores 

 were produced. ^^ 



Apospory is, in a sense, the reverse process, being the 

 production of gametophytes directly from sporophytes with- 

 out the intermediate formation of spores. Thus, when 

 detached pieces of fern fronds are placed on an agar surface 

 they frequently develop directly into gametophytes of 

 normal shape and form. In such cases, the gametophyte has 

 the same diploid chromosome count as the sporophyte. So 

 numerous are the recorded instances of this phenomenon 

 that BelP^ suggests that it must be general among ferns; 

 yet the exact conditions under which it happens cannot yet 

 be specified. 



As to the causes of apogamy, several theories have been 

 put forward, but the final word has certainly not been said 

 on this fascinating problem. In many cases, ageing of the 

 prothallus seems to be an important factor. Recent work in 

 America^^ on Osmunda, Adiantum and Pteridium has, how- 

 ever, demonstrated that apogamy can be induced by grow- 

 ing the prothaUi on an agar culture medium rich in glucose. 

 Clearly, therefore, under these highly artificial circum- 

 stances, the external environment can be an important 

 factor. That this might be so had been suspected for a long 

 time, since otherwise it was difficult to understand why a 

 diploid zygote developing inside a fertilized archegonium 

 should give rise to a sporophyte, while a diploid cell 

 developing by apospory should give rise to a gametophyte. 

 Confirmation of the view that the internal environment of 

 the archegonium exerts an important formative influence on 

 the nature of the embryo has recently come from experi- 

 ments in which young embryos of Todea were dissected 

 from the archegonium and grown on an artificial medium. ^^^ 

 It was found that those removed before the first division of 

 the zygote developed into flat thalloid structures, whereas 



