66 MITOSIS : THE VARIATION OF THE CHROMOSOMES 



Prunus avium vars. 

 Brassica japonica (race) . 

 Datura Stramonium (race) 

 Anthoxanthum odoratum {^x) 

 A vena sativa (6x — 2) 

 Chrysanthemum ornatum . 

 Taraxacum vulgare . 

 Crepis capillaris (w) 



D., 1930- 

 Fukushima, I93i- 

 Blakeslee, 1929- 

 Kattermann, 1931- 

 Nishiyama, 1931- 

 Shimotomai, 1931- 

 Gustafsson, 1932. 

 Hollingshead, 1930 a. 



There is evidence for somatic doubling in other tissues being also 

 determined by a special racial propensity in some cases. In 

 Kniphofia Nelsonii one plant had independent and repeated doubling 

 in root and stem, giving tetraploid and octoploid tissue (Moffett, 

 1932 ; cf. Meurman, 1933, on Acer). This is doubtless correlated 

 with the spindle abnormalities found in the pollen mother-cells 

 (Ch. X). Octoploid nuclei are also found in the abnormal race of 

 Datura. In Citrus limonum tetraploid seedlings arise vegetatively 

 in a regular proportion, probably through especially frequent 

 doubling in nucellar tissue (Frost, 1925 ; v. nucellar embryony). 



Tetraploid tissue in these cases and in others seems to arise 

 regularly in certain structures as an adaptation of the species. For 

 example, in Cannabis, the periblem cells are generally tetraploid, 

 having 40 chromosomes instead of the 20 found in the plerome (de 

 Litardiere, 1925 ; Breslawetz, 1926, 1935). 



Another source of polyploidy is from failure of reduction in the 

 maturation in both male and female germ cells. This failure is 

 particularly common at meiosis in hybrids, but may also be a 

 racial character (v. Ch. X). The distinction between this method 

 of origin and the first, by somatic doubling, can often be made in a 

 particular case because a somatic change will give a whole tetraploid 

 region with diploid germ-cells, while diploid germ-cells will only 

 arise sporadically by the second method (Ch. VI). 



Finally, tetraploidy may arise by natural apospory and artificial 

 regeneration in ferns and mosses (Ch. XI). 



Tetraploid animal forms have appeared in experiment only in 

 Drosophila melanogaster . In the females of this species Bridges has 

 found tetraploid (and hexaploid) tissue in the ovary. This, giving 

 diploid eggs, yields triploid offspring which in the second generation 

 has given a few tetraploids producing diploid eggs (Morgan et alii, 

 1925) . Polyploids are only known naturally in a few animal species. 



