ORIGIN OF APOMIXIS 467 



of the sperm was necessary to stimulate development without 

 fertilisation, as in the related parthenogenetic species. In this case 

 parthenogenesis must be regarded as a genetic character arising by 

 mutation. 



(iv) Hybridisation. Ostenfeld (1910), by crossing facultatively 

 sexual species of the Pilosella section of Hieracium, with purely 

 sexual species, e.g., H. mirantiactim x H. auricula, produced a 

 heterogeneous progeny of three types : (i) sterile ; (ii) purely 

 asexual, probably aposporous ; and (iii) asexual when self -pollinated, 

 sexual when crossed with good pollen. When we consider this in 

 relation to Lidforss's observations on Rubus we see that with 

 plants having the genetic faculty of producing aposporous offspring, 

 the greater their hybridity the less the chance of their successfully 

 producing sexual offspring — owing to the sterility inherited as a 

 concomitant of high segregation — and therefore the greater the 

 proportion of asexual seedlings produced on self-fertilisation. The 

 extreme type, being sexually sterile, will be obligatorily apomictic. 

 But in the same group we find plants not having the faculty of 

 apomixis and sexually sterile as well. Thus the apomixis is con- 

 ditioned both by the sterility of the hybrid and by its particular 

 genotype, which is not inherent in its hybridit}^ 



Harrison (1920) crossed two sexual species, Tephrosia creptiscularia 

 and T. bistortata (Lepidoptera) and obtained four parthenogenetic 

 females amongst normally sexual progeny. When this happened 

 on a second occasion the offspring of the parthenogenetic female 

 showed segregation of recessive characters (Peacock, 1925). This 

 should happen with the kind of parthenogenesis found by Seller in 

 the Lepidoptera. Seller (1927) crossed parthenogenetic and sexual 

 races of Solenobia triquetrella, and the progeny showed variation in 

 behaviour between the normal and that obtaining in the 

 parthenogenetic race, where pairs of nuclei fused after the second 

 segmentation division in the embryo. There were various other 

 irregularities, with the result that different individuals and parts of 

 individuals had different numbers of chromosomes. 



(v) The evidence of Chromosome Numbers. It was early noticed 

 that apomictic species of flowering plants had, as a rule, higher 

 chromosome numbers than their sexual relatives. The following 



