6o PLANT GROWTH SUBSTANCES 



ample evidence that genes may be caused to mutate by radiation treat- 

 ment. Ratios of purple to white flower color in the offspring of our 

 induced type Nubbin showed it to be a convenient form with which to 

 separate the A and B chromosomal types in races from nature which 

 we had earlier distinguished by tedious breeding procedure and by the 

 fact that B races occasionally threw certain abnormal in + i types. 

 It was in one such type that Belling found two large chromosomes 

 attached to a small chromosome to form a trivalent at metaphase. The 

 fact that like chromosomal ends become attached, which he had earlier 

 used in interpretation of Secondary in + i types, he again put to work 

 in explaining our B races. He concluded that two of their chromosomes 

 could have been derived from the A race by interchange of segments of 

 two nonhomologous chromosomes. As shown by Bergner and Satin, 

 segmental interchange has been responsible for the formation of so- 

 called Prime Types in all the species of Daiura of which an adequate 

 number of races have been investigated. It has accompanied the forma- 

 tion of species in this genus. The theory of segmental inrri^hange laid 

 the basis for interpretation of the peculiar genetic end c ; ^- j'jrcal be- 

 havior of the Oenotheras which had baffled students of hei cu 'y '^ince 

 de Vries first published on their mutability. 



Through the utilization of Secondary chromosomes and TertuTy 

 chromosomes which had been formed by segmental interchange, it 

 was possible to synthesize pure-breeding types with predicted ch?rac- 

 teristics. As examples the following types all with similar characters due 

 to the extra 2 half chromosome may be mentioned: in — (11-12)2 + 

 (2- II- 12)2: 2«- (1-2)2+ (-1)2+ (2-2)2:2^- (13-14)2- (23-24)2 + 

 (2-14)2 + (13-23)2 + (-24)2. It has been pointed out that such types 

 should not be called synthesized new species since they possess no 

 isolating mechanism which would prevent them from crossing with the 

 normal type. However, if their chromosome number is doubled (and 

 this has been done through treatment with colchicine) such crossing 

 is almost completely prevented. 



Since tetraploids are a source of triploids and thus of the full range 

 of the Primary in + i types, methods have been sought for doubling the 

 chromosome number. In 1937 we stumbled on the use of colchicine for 

 this purpose. Colchicine apparently prevents the process of spindle 

 formation without interfering with chromosome division. Hence the 

 chromosomes may divide several times before the effect of the drug 



