472 BREAKDOWN OF GENETIC SYSTEMS 



Thus, in all these examples, parthenogenesis is compatible with 

 complete homozygosity. 



There is a second class in which the decisive influence of the 

 mechanical chromosome relationships can be seen : in those 

 aposporous plants which are facultatively sexual but not true 

 breeding. Apospory, in itself, is no more obscure than such tera- 

 tological observations as those of Stow (1930), who induced the 

 formation of a structure in the pollen grain of Hyacinthus resembling 

 the female gametophyte. That such abnormal structures should 

 be able to function is evidently conditioned by sexual failure. 

 Lidforss's experiments show that the lack of viabiUty of the sexual 

 embryo stimulates apospory. In such forms apospory must be 

 conditioned by partial sexual sterihty which itself is sufficiently 

 accounted for by either hybridity or autopolyploidy or cross- 

 fertilisation. 



It is probably of no account whether or not sexual failure is 

 determined by interspecific hybridity. Irregularity of meiosis is as 

 characteristic of non-hybrid tetraploids as of hybrid diploids, and 

 tetraploidy of this kind is probably a determining factor in some 

 species, e.g., Nigritella nigra, which exists in a diploid sexual form 

 (^n = ig) and a tetraploid apomictic form, with nucellar embryony 

 (Afzelius, 1928 ; Chiarugi, 1929). In triploids the conclusion to be 

 drawn is unequivocal : the last sexual act in their history was an 

 act of hybridisation, not necessarily between different species but 

 between gametes with different chromosome numbers. If, in 

 these, hybridisation determined the institution of apomixis, in 

 many others where clear-cut evidence is lacking this kind of 

 origin may be readily presumed from the irregularity of their 

 meiosis. 



In many plants with obligatory parthenogenesis we see that the 

 institution of parthenogenesis was likewise determined by a 

 mechanical defect in the chromosome organisation. Most of them 

 are evidently triploids or autopolyploids of a kind that could not 

 reproduce satisfactorily by way of meiosis. But since they began 

 to be parthenogenetic they have evidently changed. They have 

 acquired a genotypic property of regularly suppressing meiosis. 

 This is shown in a variety of ways, as follows : — 



