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 
Fic. 39. 
Nephrodium 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., \xxvi., 1905, p. 466. 
?Farmer and Digby, Amn. of Bot., 1907, pp. 163-7. 
