436 BREAKDOWN OF GENETIC SYSTEMS 



In others, parthenogenesis is facultative : parthenogenetic females 

 may give rise to sexual females and sexual males. In some of these 

 (e.g., Daphnia, Banta et alii, 1926, and Rotifera) the succession of 

 sexuality and parthenogenesis is irregular, in others they come in 

 a definite cycle (e.g., in Aphides). The conditions immediately 

 determining this cycle will be seen later. 



(h) Haploid Parthenogenesis. With the haplo-diploid system 

 of sex-differentiation found in many animals, haploid partheno- 

 genesis gives male offspring (Ch. IX). 



(ii) Ferns (Pteridophyta) . In the ferns the two generations have 

 an independent existence, and it is therefore possible to distinguish 

 the different abnormal sexual processes more readily than in the 

 flowering plants. Nevertheless a great deal of the older work in 

 this field is, as Manton (1932) says, ambiguous. All types naturally 

 occurring involve the suppression of meiosis and fertilisation and 

 may be classified as follows : — 



(a) Diploid Parthenogenesis, i.e., without loss or substitution 

 of the special organs — spores and gametes (e.g., Marsilia Drummondii, 

 Strasburger, 1907). In this type two diploid spores are formed as 

 a result of the failure of reduction, instead of four haploid ones, and 

 the egg-cells developed on the diploid prothallia (gametophytes) 

 grow into sporophytes without requiring fertilisation (cf. Gustafsson, 



1935). 



(b) Apospory, i.e., with substitution of unspecialised cells for the 



spores (e.g., Athyrium Filix fcemina var. clarissima Bolton, and 

 Scolopendriiim vulgare var. crispum DrummondcB , Farmer and 

 Digby, 1907). In this type the gametophyte does not develop from 

 a spore but as a purely vegetative outgrowth of the sporangium or 

 the tip of the frond. It is diploid, meiosis having been completely 

 omitted, and it produces diploid egg-cells. These develop without 

 fertilisation, but perhaps require the stimulation of the sperm as in 

 pseudogamy (cf. Andersson-Kotto, 1931). 



(c) Apogamy, i.e., with substitution of unspecialised cells for the 

 gametes (Nephrodium hirtipes, Steil, 1919). In this type the 

 sporophyte arises as a purely vegetative outgrowth from the 

 gametophyte (prothallium), which is diploid, having arisen from 

 unreduced spores. In the example, antheridia were produced 



