1929] Navashin: Triploidy in Crepis 387 



prevent the very formation of gametes bearing extra chromosomes; 

 for the rare instances in which gametes are formed result in i)erfectly 

 viable zygotes; simple or double trisomic individuals. On the other 

 hand, there is a possibility of selective fertilization, although the high 

 percentage of bad pollen in triploids makes it improbable that many 

 pollen grains possessing extra chromosomes could exist. 



Further difficulties are met with in explaining the observed numeric 

 ratio of diploids and triploids which, as may be seen from table 3, 

 is almo.st exactly equal to 2:1, instead of the expected 1:1 ratio. 

 Although the observed ratio is similar to the usual zygotic lethal 

 ratio, there is no reason to suggest any lethal factor of that sort, for 

 under triploid conditions it could not give rise to the expected 1 : 2 

 ratio. A gametic lethal factor of some sort could, of course, give 

 a 1 : 2 ratio, but the first following generation would immediately 

 revert to the normal 1 : 1 ratio, on account of the loss of the lethal. 

 Similarly it is impossible to suggest an}^ stimulating factors which 

 would make the haploid cell win in the struggle for life in the female 

 tetrad in the young ovule. The elimination of trisomies should not, 

 of course, affect the ratio of diploid and triploid offspring. Although 

 it is impossible, from the purely theoretical point of view, to account 

 for this curious ratio which occurs quite regularly in Fj and F2 

 (tables 2 and 3), two reasonable hypotheses may be suggested: 



1. If the odd members of the triploid complex of C. capillaris are 

 regularly distributed to the poles, the extreme variants, i.e., the 

 haploid and diploid germ cells, will be formed in equal numbers. 

 On the other hand, if the odd chromosomes sometimes lag, one of the 

 sister cells of the dyad will receive an incomplete set of chromosomes 

 instead of a foil diploid complex. Cells of that sort, however, are 

 almost incapable of further development as is evident from the 

 absence of trisomies. It is possible that such a female dyad will 

 give rise to a haploid embryo sac, no matter what the position of 

 the haploid component may be, i.e., whether the haploid cell be 

 turned toward the microphile or toward the chalaza. Such a phenom- 

 enon will cause an increase of the number of haploid eggs, and con- 

 sequently of the resulting diploid plants. Similarly, lagging of 

 chromosomes in the intermediate types of distribution may lead to 

 an increase of the relative number of haploid gametes. Despite the 

 reasonableness of such an explanation it still remains quite obscure 

 why such disturbances in the reduction division should cause the 

 observed regular ratio. 



