6o ALTERNATING GENERATIONS 



budding. But an exact parallel is found in Athyriwn filix-foemina, var. 

 darissima, Bolton, and in Scolopendrium vulgare, var. crispum Drummondae, 

 in which the embryo arises from the unfertilised ovum. 1 It may be 

 remarked that the phenomena thus seen in the last-named Ferns and in 

 Marsilia correspond essentially to what has been described for certain 

 Phanerogams. 2 It thus appears that in a number of cases, systematically 

 apart from one another, a diploid condition of the gametophyte is associated 

 with apogamous development from a diploid unfertilised ovum : the 

 abnormality is initiated by omission of the reduction in the spore-mother- 

 cell, and consequently the diploid state is continued in the gametophyte, 

 which is normally haploid. It is important to note in such cases that 

 a double number of chromosomes may be present without producing 

 fundamental change of form or of external character in the gametophyte. 



The further question will then present itself, whether under any 

 circumstances the converse is possible, or has been observed, viz. that 

 the phase normally diploid, that is, the sporophyte, may be haploid? 

 Strasburger states (I.e., p. 166) that no case has come under his 

 observation in which the generation normally diploid has only the reduced 

 number of chromosomes. No case of a haploid sporophyte has yet been 

 proved beyond doubt : but a reasonable probability has been established 

 by Farmer and Miss Digby in the case of Lastraea pseudo-mas, var. 

 cristata, Druery (I.e., p. 180). The detached leaf of this plant produces 

 prothalli from its margin or surface, which bear occasional antheridia, 

 but the sporophyte is apogamous. The chromosome-number in the 

 prothallus is about 60 : in the embryo the number varies considerably, 

 one mean being 60, another mean number being about 78. No migration 

 of nuclei was observed, nor is there any reduction in the whole cycle. 

 The relatively small number of chromosomes in the nuclei of the sporophyte 

 is striking, and suggested to Farmer and Miss Digby that the gametophyte 

 character had been impressed on the sporophyte the converse, in fact, of 

 what was seen in the varieties of Athyriwn and in Marsilia. A comparison 

 of the chromosome-number (60, 78) with that in normal Lastraea 

 psetido-mas (144) certainly indicates that this is the probable condition 

 of the apogamous sporophyte of Druery's variety : that the sporophyte is 

 irregularly haploid, and that the whole cycle is essentially haploid 

 throughout. 3 



It still remains to refer briefly to two other modifications of the normal 

 cycle of alternation in Archegoniate Plants, so as to complete the tale of 

 those which have been observed : I mean sporophytic and gametophytic 

 budding. The former has already been mentioned in the case of 



farmer and Digby, I.e., p. 171. 



2 Eii-alcheniilla, Thalictrum, Antennaria, Taraxacum. See Strasburger, I.e., p. 139. 



3 The examples quoted illustrate the more important modifications of the chromosome- 

 cycle hitherto described. For further details reference must be made to the papers from 

 which these have been derived. 



