156 STRUCTURAL HYBRIDS 



been found in Campanula, but in some crosses the homozygous 

 forms, with simple pairing, are ehminated as mentioned above. 



(vi) Polyploid Interchange Heterozygotes. A relatively inter- 

 changed condition was first found in the odd chromosome of a 

 trisomic Datura (Belling and Blakeslee, 1924). This happens in 

 two ways. First, the extra chromosome may contain two identical 

 segments and may be supposed to be derived from internal inter- 

 change, i.e., between non-homologous arms of the same chromosome 

 or of homologous chromosomes. Thus two chromosomes ahc, def 

 and abc, def might give new types ahc, dob and fee, def, one of which 

 in addition to the normal complement would make a " secondary 

 trisomic " (Rhoades, 1933). Secondly, the extra chromosome may 

 result from interchange of a segment of two complementary chromo- 

 somes. This type gives a " tertiary trisomic." Thus AB and CD 

 will give BC and the trisomic will be made of AB, AB, BC, CD, CD, 

 etc. Such a plant has various configurations, including a chain of 

 five chromosomes. 



In connection with these forms it is convenient to mention 

 certain reduplication homozygotes, i.e., organisms that are 

 tetrasomic in respect of one segment, having four chromosomes that 

 may be represented abed, abed, efgd, efgd. Since all four chromosomes 

 can associate in the d segment, groups of four can be formed and, 

 with terminalisation, a quadruple chiasma. Such configurations 

 are found in Datura hybrids (Blakeslee, 1928) and probably in Rosa 

 (Erlanson, 1931 b), Matthiola (Philp and Huskins, 1931), and 

 Tradescantia bracteata (D., 1929 c). Probably configurations 

 suggesting the same conclusions in (Enothera biennis (Cleland, 

 1926 a) are to be attributed to interlocking {q.v.). 



Polyploid and polysomic interchange heterozygotes combine the 

 large configuration made possible by their hybridity in structure 

 with the variability in forming possible configurations which is 

 characteristic of polyploids. Only the observation of " maximum 

 association " (which is never found in complicated cases) or a 

 knowledge of the ancestry can give the key to the constitution. 

 The determination of their structure, therefore, requires extensive 

 observation, and has only been conclusively carried out in a few- 

 forms. 



