FAILURE OF FERTILISATION 447 



The fifty Datura haploids (cf. Blakeslee, Morrison and Avery, 

 1927) owed their origin to the stimulation of cold (cf. Haberlandt, 

 1927), or to pollination with Datura ferox. 



Special conditions are responsible for the origin of haploids in 

 Limim (Kappert, 1933). Here a single fertilised Qg^ may divide 

 to give twin or multiplet embryos. This polyembryony may take 

 the form of a second haploid cell in the embryo sac growing up 

 beside the normal diploid embryo to give a haploid-diploid twin. 

 A similar process is found in Gossypium and Oryza. 



We can therefore point to five conditions which lead to non- 

 recurrent parthenogenesis in the flowering plants : (i) Exceptional 

 external conditions (temperatures). (ii) Pollination without 

 fertilisation or with fertilisation of neighbouring ovules only, 

 (iii) Accidental fusion of both male nuclei with the fusion nucleus, 

 (iv) The entry of an alien nucleus into the egg-cell without fusion, 

 (v) A genetic propensity for polyembryony. Three of these con- 

 ditions are often called forth by cross-pollination with another 

 species, and it would appear that by this means parthenogenetic 

 seedhngs could be obtained in most groups of the dicotyledons, for 

 they have been found wherever they have been thoroughly searched 

 for. Therefore, although non-recurrent parthenogenesis has, for 

 obvious reasons, been found only under experiment, there is no 

 reason to doubt that it occurs in nature, at least in cross-pollinated 

 plants. 



Nevertheless, two genetical conditions appear to limit the 

 occurrence of non-recurrent parthenogenesis. First, the plant must 

 be sexually fertile and homozygous or nearly so, for the conditions 

 which eliminate the segregates of highly heterozygous organisms 

 (such as interspecific hybrids) and lead to their sterility are much 

 more stringent in haploid parthenogenesis, since the single gametic 

 genotype has to meet all the conditions of life, and not merely those 

 normal to the gamete (c/. Ch. VIII) . It is much more likely to do so 

 if it has the same qualitative constitution as its parent, and this is 

 possible only where the parent is homozygous. Attempts to obtain 

 parthenogenetic seedhngs of heterozygous species of (Enothera have 

 therefore been unsuccessful (Davis, 1931 ; Stomps, 1931)- 



Secondly, a definite hereditary predisposition to produce 



