SECONDARY POLYPLOIDY 241 



secondary pairing agrees with the assumption that two of the eight 

 chromosomes are present three times, while the rest are present 

 twice (Fig. 80). 



The same is true of Solamim and Nicotiana, which have an apparent 

 basic number of 12, which might be derived from a lower basic 

 number of 6. These assumptions have been verified by the study 

 of secondary pairing (cj. Miintzing, 1933). 



Chromosome pairing in certain hybrids suggests that reduplication 

 of part of the set is responsible for the origin of new complements 

 in Brassica and Papaver, and perhaps Viola. Observations of 

 secondary pairing in Brassica confirm this conclusion (Catcheside, 



1934)- 



Brassica cernua (n = 18, 4x) x B. chinensis [n = 10) and other 



species : 10" + 8' (Morinaga, 1929). 

 Papaver somniferum (n = 11) X P. orientate (n = 21, 6x) : 11" -|- 10' 



(Yasui, 1921 ; Ljungdahl, 1922). 

 Viola tricolor alba (n = 13) X V. elegantula {n = 10) : 8-10", 7-3^ 



(J. Clausen, 1931 c). 

 Brassica Napus {n = 18) x B. rapa {n = 10) : 0-3"', 7-12", 4-8^ 



(Catcheside, 1934). 



The distinction between secondary polyploidy and structurally 

 changed polyploidy (i.e., polyploidy followed by fragmentation or 

 fusion) is important, because the first means a great change in 

 genetic balance, while the second means little or no change of this 

 kind. On the analogy of most trisomic and tetrasomic forms a 

 polyploid with a new balance will, if it breeds true, constitute a new 

 species. The Pomoideae are a sharply characterised group of the 

 Rosacese and Dahlia Merckii is the most sharply characterised 

 species of Dahlia. 



The types so far elucidated are unbalanced modifications of 

 tetraploids, not of diploids. It may be supposed on analogy that 

 tetrasomic diploids would as a rule be sterile, the unbalance being too 

 drastic. This is true also of artificial secondary polyploids. 



Such new forms have been produced in experiment in the following 

 ways. First, from a pentaploid having the complete complement of 

 Nicotiana Tabacum {^x = 48) given by an unreduced gamete 



