MEN DELI SM AND MUTATION 



345 



The ^-chromosome Mutants (gigas group). In the mutant forms of 

 this group, of which (Enothera gigas is a member, the somatic number of 

 chromosomes is 28 rather than 14; the plants are tetraploid (Fig. 135, B). 

 How this condition arises is not certainly known. Stomps (1912) 

 believed it to be the result of the union of two unreduced gametes, whereas 

 Gates (19096) suggested that "the doubling in the chromosome number 

 had probably occurred as the result of a suspended mitosis in the fertilized 

 egg or in an early division of the young embryo." Strasburger (19106) 

 also adopted the latter view. 



FIG. 135. Chromosomes in (Enothera mutants. 



A, interkinesis in CE. Lamarckiana; 7 split chromosomes. B, same in CE. gigas; 14 

 split chromosomes. C, somatic cell of CE. semilata; 15 chromosomes. D, metaphase of 

 homoeotypic mitosis in CE. biennis lata, showing 8 chromosomes on one spindle and 7 on 

 the other. Spores and' gametes with these numbers will result. E, the 21 chromosomes 

 in a mutant from CE. Lamarckiana. (A and B after Davis, 1911; C and D after Gates and 

 Thomas, 1914; E after Lutz, 1912.) 



The mutants of the gigas group are characterized chiefly by an unusu- 

 ally large size, not only of the plant as a whole but also of its anatomical 

 constituents. In the tetraploid mutant (Enothera stenomeres, for instance, 

 Tupper and Bartlett (1916) found that the change from the diploid to 

 the tetraploid condition is concomitant with a 50 per cent increase in 

 the length of the vessel, a 150 per cent increase in the area of its cross 

 section, a 50 per cent increase in the length and diameter of the tracheids, 

 an increase in the three dimensions of the medullary ray cells, and a break- 

 ing up of the^tall multiple ray into a number of thin simple rays. 



