The Aneuploids 339 



ceived a (hromosomal pair from .V. ghttijiosa, \\hi(h contributed the 

 necrotic factor tor resistance. Alien additional races included a pair 

 from one species and 24 pairs from N. tabacum. By another series of 

 crosses, alien substitution races were formed, whereby a pair of 

 chromosomes were substituted in the N. tabacum genome.^^ Other 

 species carry factors that can be traced by successive crosses into the 

 interspecific hybrid, then by a backcrossing procedure through a 

 number of generations. The monosomic method of analvsis lias been 

 \\orked out with good success in Nicotiana}- 



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