224 BOTANY 



4n 



5n 

 6n 



8n 

 All plants with a hig 



Tetraploid 



Pentaploid 

 Hexaploid 

 Octoploid 

 ;'her chromosome number than 2n are termed 

 polyploids. Polyploids are divided into two classes according as 

 their different sets of chromosomes are derived from the same 

 species, in which case they are termed autopolyploids, or from 

 different species, when they are known as allopolyploids. The 

 genetics of the two classes is very different. 



The earliest experimental case of polyploidy was that of 

 CEnothera gigas, which was first recorded by Gates and Lutz some 

 years ago. This plant is a tetraploid mutant and is a cell giant. 

 It was first suggested that this mutant owed its origin to a 

 longitudinal split in the chromosomes at division of the fertilised 

 Ggg, a view opposed by de Vries, who considered that it arose 

 through the union of two diploid gametes. Bridges has termed 

 the doubling of the diploid number of chromosomes by the process 

 of splitting and non-separation, " non-division." It has generally 

 been thought that such a process occurs most frequently in the 

 zygote, with the result that a tetraploid individual is produced. 

 The actual evidence for such a view is only indirect, but neverthe- 

 less very strong. De Litardiere, for example, found in Spinacia 

 oleracea not only somatic cells with 12 chromosomes, the normal 

 number for this species, but also cells with 24 and 48 chromosomes. 

 As a general rule the daughter chromosomes remained closely 

 associated at prophase and separated normally at anaphase, but in 

 other cells they became independent and underwent a second split- 

 ting process, with the result that 24 chromosomes passed to either 

 pole of the spindle. 



A doubling of the chromosome number through nuclear fusion 

 may also occur. Gates has reported such a case for CEnothera. 

 The nuclei may apparently fuse during the telophase of the 

 homotypic division of mitosis. Similarly Blackburn and Harrison 

 found one giant spindle instead of two normal ones in the second 

 sporocyte division of Rosa, thereby showing that the mitotic 

 figures may unite. Winkler has reported tetraploidy in graft 



