58 ALTERNATING GENERATIONS 



in which the doubling of the chromosomes is initiated by the nuclear 

 fusion. 



The first case of apospory to be cytologically investigated was that 

 of Nephrodium pseudo-mas, var. cristata apospora, where the prothallus 

 grows directly out from the margin or surface of the leaf. It was shown 

 in this case by Miss Digby * that there is no nuclear change involved, 

 but that both sporophyte and gametophyte have a number of chromosomes 

 about 50. This result would at first sight appear to show that the 



FIG. 39. 



Neph.rodnt.ni pseudo-mas, var. polydactylum. Tissue of prothallus where an apogamous 

 growth is to be formed, showing to the left a cell with two nuclei, while an adjoining cell 

 has none. At the centre a nucleus is seen passing through a perforation of the wall, 

 and fusing immediately with that of the cell it enters. (After Farmer, and Moore, and 

 Miss Digby.) 



chromosome-criterion had hopelessly broken down. But a better under- 

 standing of such cases is obtained when the whole nuclear cycle is 

 considered, than by contemplation of a single phase of it. It has been 

 above noted that there is a frequent relation between apogamy and 

 apospory in the life-cycle of the same individual : it is important to 

 know the nuclear conditions throughout such cycles. The case of 

 Athyrium filix-foemina, var. clarissima, Jones, may be taken as a first 

 example where the complete chromosome-cycle is known. 2 In this Fern 



1 Roy. Soc. Proc., Ixxvi., 1905, p. 466. 



2 Farmer and Digby, Ann. of Bot., 1907, pp. 163-7. 



