388 University of California Publications in At/ricultaral Sciences [Vol.2 



2. If one suggests the existence of a factoi- a double dosage of 

 wliicli stimulates parthenogenetic development of a dii)loid egg, it is 

 very easy to explain the observed 2 : 1 ratio. Assuming that two 

 homologous chromosomes of the original triploid plant contain this 

 factor, the gametic ratio among diploid eggs will l)e as follows : 2 eggs 

 bearing a simple dosage of the factor : 1 bearing a double dosage. 

 Since the latter should develop into diploid plants without fertiliza- 

 tion, the proportion of diploids will increase one-third at the expense 

 of the triploids; as a consequence the ratio of diploids and triploids 

 will be equal to 2:1, instead of 1:1. If, however, such an explana- 

 tion is correct, different ratios should be expected among progenies 

 of different trijiloid plants, according to their genetic constitution; 

 l)lants possessing a double dosage of the ''parthenogenetic" factor 

 should give the 2 : 1 ratio, while those possessing a single dosage 

 should produce diploid and triploid offspring in eqmxl numbers. 

 Among the 34 plants investigated 32 have not shown a 1 : 1 ratio but, 

 on the contrary, the progeny of these 32 plants approximated more 

 or less closely a 2:1 ratio; only two plants produced diploid and 

 triploid offspring in equal numbers, the latter circumstance being 

 perhaps merely accidental. 



The reduction division being not j^et completely investigated, it 

 is impossible to say whether or not lagging of chromosomes may 

 account for the observed ratio. On the other hand, some experimental 

 evidence makes it probable that i)arthenogenetic development of 

 diploid eggs does take place. Furthermore, it has been found that 

 the triploid plants set a high number of good seeds if attempts are 

 made to cross them to certain other Crepis species ; instead of hybrids, 

 however, these seeds produce only normal diploid C. capiUaris plants. 

 This phenomenon was especially striking when C. rubra was used as 

 the pollen parent. The production of pure capiUaris plants may be 

 explained, of course, simply as an experimental error due to incom- 

 plete depollination ; on the other hand, however, it seems rather 

 strange that the progeny obtained in this way should consist only of 

 diploid plants, while it is known that under normal conditions about 

 one-third of the progeny always consists of triploids. It seems to be 

 probable, therefore, that parthenogenetic development occurs possibly 

 under the stimulating influence of the foreign pollen, which is, how- 

 ever, incapable of effecting fertilization. Jorgensen's observations 

 (1928) on the parthenogenetic formation of diploid offspring in 

 Solanum give a good illustration of the actual occurrence of the 



