TETRAPLOIDS 



109 



Under these circumstances it is not difficult to imagine 

 that the tetraploid originated in a variety that was 

 already parthenogenetic, for, should an egg-cell double 

 the number of its chromosomes by the retention of one of 

 its polar bodies, or become double through the chromo- 

 somes failing to separate after the first division of a 

 nucleus, the double condition might continue to perpetu- 

 ate itself. 



Hoploid 21 ^Diploid 4-2) 



Fig. 64. 



The chromosomes, in reduced number, of the diploid and tetraploid 

 races of Artemia salina. (After Artom.) 



One of the first tetraploids in plants was discovered by 

 de Vries, and named Oenothera gigas (Fig. 42). It was 

 not known, at first, that this giant was a fourfold chromo- 

 some type, but de Vries saw that it was stouter than 

 plants of the parent species (Lamarck's evening prim- 

 rose) and different in many other minor characteristic 

 details. Its chromosome number was later made out. 



Lamarck's evening primrose (Oenothera Lamarcki- 

 ana) has 14 chromosomes (haploid 7). The giant form 0. 

 gigas has 28 chromosomes (haploid 14). The two chromo- 

 some groups are drawn in Fig. 65. 



Gates has made measurements of the cells of differ- 

 ent tissues. The epidermal cells of the anthers of gigas 



