5 8 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 Xcphntdiitm 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 l 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 





KM,. 39. 



A ,/,'!>: 'diuin fisL-iuiii-iiias, var. polydiictyluin. 'ris.vju ofpnjthnllus 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 llnmiuli a perforation of the wall, 

 ami fusing immediately with that of the cell it -'inns. ( \lirr Farmer, and Moore, anil 

 Miss l)i;;l>y.) 



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 nu<-U-ur conditions throughout such cycles. The case of 

 .\tli\rintn filix-foemina.) var. clarissima, Jones, may be taken as a first 

 i \ainple where the complete chromosome-cycle is known.'- In this Fern 



1 A'nj'. Soc. /'/-a,-., Ixxvi., 1905, p. 466. 



I; i r and hi^l.y, ./;/;/. ,;/' Jiot., 1907, pp. 165-7. 



