APOGAMOUS FERNS. THE GENERAL PHENOMENON 



and de Bary at the end of the last century, and the available literature on the subject 

 is in a rather unsatisfactory state. From the cytological point of view there is, in my 

 experience, only one existing account which can be accepted as adequate, namely, that 

 by Dopp (1932) on Dryopteris remota. Other well-known and much-quoted papers, e.g, 

 Farmer and Digby (1907) on ' Nephrodium pseudomas wslts. polydactyla'' Wills and Dadds, 

 Allen (19 1 4) on ' Aspidium falcatum\ Steil (19 19) on ' Nephrodium hirtipes\ while con- 

 taining some correct observations are so seriously incomplete as to be actively mis- 

 leading. 



The central cytological problem posed by all these plants is that of reconcihng the 

 absence of a sexual nuclear fusion with the presence of an apparently normal meiotic 

 process in the development of the spores. It is obvious that some compensating process 

 must exist at some point in the life cycle to stabilize chromosome numbers and to 

 prevent the progressive diminution which a repeated succession of meioses would other- 

 wise very rapidly bring about. 



Some authors have looked for this compensating process in the gametophyte, and 

 Farmer and Digby (1907) in particular believed that they had found it when they 

 detected nuclear migrations in the vegetative cells of some of the prothalli o{ ' Mphrodium 

 pseudomas ( = Dryopteris Borreri) var. polydactyla', and interpreted this as 'pseudo-fertiliza- 

 tion'. The observational evidence for the existence, under certain circumstances, of 

 nuclear migrations may be accepted as valid, but no demonstration was offered of 

 either nuclear fusion or the origin of sporophytic tissue from cells with a higher chromo- 

 sorfle number than the rest of the prothallus. At that date, indeed, it is doubtful 

 whether cytological technique was sufficiently advanced to permit of a numerical 

 demonstration of this kind, and certainly these authors, having examined meiosis, 

 were unable to detect any numerical or other peculiarity about the process. Since in 

 fact the vars. 'polydactyla'' differ in no way from the other cases, in all of which, as will 

 shortly be seen, the compensating process looked for is in the sporangium, it is to be 

 hoped that 'pseudo-fertilization' after passing as a fact for a quarter of a century will 

 shortly cease to be quoted. 



All subsequent authors have correctly concluded that the process which compensates 

 for the apparently normal meiosis is to be looked for in the sporangium. That the 

 process itself was not at once discovered may perhaps be explained in part by the un- 

 expected complexity of the sporangial development, and also probably in part by the 

 fact that all the earlier workers appear to have been consciously looking for a pseudo- 

 sexual process and were therefore predisposed to find it, in spite of incompleteness, 

 now obvious, in the evidence before them. 



The first serious investigation of sporangial development in an apogamous fern was 

 by Allen in 19 14 on ' Aspidium falcatum\ now more correctly known as Cyrtomium 

 falcatum. She observed, correctly, that sporangia could be found with eight spore mother 

 cells and with sixteen spore mother cells, and from this she concluded prematurely that 

 one type turned into the other by fusion of the mother cells in pairs, a view which was 

 apparently supported by the detection of occasional intermediate stages. Allen's 

 evidence on the chromosome numbers was very seriously defective, as will be shown 

 below, but she deduced, correctly, that the meiotic process, though normal in 



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