ORIGIN OF HAPLOIDS 443 



possible to " fertilise " an artificially enucleated egg fragment with 

 a normal sperm (cf. Belar, 1933 ; Fankhauser, 1934). 



Crane and j0rgensen have produced haploid plants of Solanum 

 nigrum by inducing parthenogenesis in two ways : (i) a periclinal 

 chimaera of one layer of 5. sisymhrifolium over S. nigrum var. gracile 

 was self-pollinated. Owing, it is believed, to the purely nigrum 

 pollen grains growing less satisfactorily in the chimaera, few ovules 

 were fertilised ; others, apparently stimulated by the pollination, 

 developed parthenogenetically. One haploid plant was raised : it 

 was a dwarf 5. nigrum in character ; (ii) ninety flowers of S. nigrum 

 were pollinated by S. luteum and yielded 43 fruits with 70 seeds, of 

 which 35 germinated ; 7 were haploid, and the remaining 28 diploid. 

 All were maternal in character. Genetical evidence later pointed 

 to somatic doubling of the chromosome number at an early stage in 

 the parthenogenetic embryos, and not to failure of reduction, as the 

 explanation of the origin of the diploid progeny. Similar evidence 

 shows that such haploid parthenogenesis followed by doubling is 

 probably very common when interspecific crosses are made in 

 Fragaria and Nicotiana (East, 1930), and it may reasonably be 

 supposed that these observations are of general significance in the 

 flowering plants. 



When the embryo-sacs of S. nigrum were examined after cross- 

 pollination with 5. luteum, it was found that one or both of the 

 generative nuclei of a pollen grain had often entered an egg-cell and 

 later degenerated, as in artificial parthenogenesis, in the echinoderms. 

 Probably in the same way the egg-cell nucleus degenerates in the 

 occurrence of male parthenogenesis. 



The Triticum compactum haploid arose from an exceptionally 

 large seed — the opposite of what would be expected if the pollen 

 played no part in its development. This suggested to Gaines and 

 Aase that its origin was due to both the generative nuclei of the 

 pollen fusing with the central nucleus to give a tetraploid instead 

 of a triploid endosperm nucleus. 



Triticum monococcum haploids have been produced by pollinating 

 the plants with their own X-rayed pollen which presumably, like 

 that of another species, will stimulate development without con- 

 summating fertilisation. 



