438 Morpho genetic Factors 



cell size and fertility, although its chromosome number was still diploid. 

 This suggests that regulation to a physiologically optimum cell size had 

 taken place. In polyploid series in the higher plants it may be that regu- 

 lation, through natural selection or otherwise, has produced an optimum 

 cell size even with widely different numbers of chromosomes. In poly- 

 ploid series of recent origin, however, such as those produced by colchi- 



> 







w 



V 



4 



* 



V 



% 



Fig. 19-9. Flower of diploid (left) and tetraploid (right) in Antirrhinum majus. 

 ( Courtesy W. Atlee Burpee Co. ) 



cine, there is almost always a close relationship between chromosome 

 number and cell size. 



The increased cell size of polyploids may be reflected in larger plant 

 size (Fig. 19-9). Oenothera gigas, which proved to be a tetraploid, was 

 named for its size, and "gigas" tetraploids of many species are now known. 

 Frequently, however, the tetraploid plant is little larger than the diploid, 

 and it may even be smaller. It usually has stouter stems and thicker 



