THE GENETIC NATURE OF TAXONOMIC DIFFERENCES 259 



T. vulgar e. This grouping of the Tritiaim species was made some time 

 before the chromosome numbers were known, but these completely 

 confirmed the old morphological classification. Crosses between the 

 diploids and either tetraploids or hexaploids are highly sterile, but the 

 hybrids between the Emmer and Vulgare groups have been exten- 



sylvestris 



24 univ. 



Fig. 113. Chromosome Associations in Hybrids between Species of 



Nicotiana. — The numbers of bivalents and univalents formed at meiotic metaphase 

 in the hybrids are given. N. digluta is a synthetic species derived as a tetraploid 

 hybrid between Tabacum and glutinosa. Tabacum itself may be a tetraploid hybrid 

 between sylvestris and tomentosa or Rusbyi. 



(After Sansome and Philp.) 



sively studied. The Fi has 35 chromosomes, 14 from Emmer and 

 21 from Vulgare. The meiosis shows 14 bivalents and 7 univalents. 

 If the Fi and subsequent generations are selfed, the plants which are 

 obtained fall into two groups: a "diminishing" group in which the 

 chromosome number sinks to m = 28, and an "increasing" group in 

 which the number gradually rises to 2n = 42. It seems probable that 

 the 14 chromosomes in the haploid set of Emmer are homologous with 

 14 of the Vulgare chromosomes, the other 7 Vulgare chromosomes 

 being without homologues. The hybrids therefore all form 14 bivalents, 

 which consist of an association between a Vulgare and an Emmer 



