U I'ellkw and F. y\. Durham 17i» 



P. K/:\VEA'.S/S COMPARED WITH OTHER TKTUAl'l.OI I)S. 



The genetics of two other tetraploid f'orins, viz. (Enuthera ijiijus'' and 

 Primula sinensis'-, have been studied. Neither of these teti'aploids 

 arose as the result of crossing two species, and therefore they ai-e not 

 parallel cases with Kewensis, still it is of some interest to compare 

 them. 



Althougli these three tetraploid forms are alike in possessing 

 double the number of chromosomes of the types from which they come, 

 we do not know if th(>y resemble each other as regards the comparative 

 size of the chromosomes. As shown by Farmer and Digby ', " the total 

 amount of chromosome substance in the nuclei of each of the two types 

 of hybrids known as P. Kewensis is the same. The nuclei of the one 

 form of hybrid contain t\vice as many chromosomes as the nuclei of the 

 other type, but the increase in number is a.ssociated with a corre- 

 sponding diminution in size." Although the size of the chromosomes in 

 the tetraploid is thus diminished, the size of the nuclei and cells are 

 larger than in the diploid. In their discH.ssion of these facts, Farmer 

 and Digby conclude that the doubled number of chromosomes may be 

 attributed to a transverse fission of the "imrmal" chromosomes. They 

 suggest that the increase in size of nuclei and cells may be due to 

 a correlation between these sizes and the superficial area of the 

 chromosomes, for the transverse fission of the chromosomes would add 

 to the normal amount of free surface. The volume of the chromosomes 

 in (E. gigas and in the tetraploid P. sinensis have not been determined, 

 but another giant form of P. sinensis has been investigated by Gregory*. 

 This giant was found to have the diploid number of chromosomes, but 

 nuclei and cells larger than in the normal form. 



It remains to consider to what extent the doubling in the number 

 of chromosomes is correlated with an increase in the size of the plants. 

 In the case of (E. gigas the evidence is not perfectly consistent, the 

 work of Geerts and Stomps, and that of Miss Lutz, giving different 

 results. Heribert-Nilsson* (refs. given) concludes that the doubling 

 of the chromosome number is not the primary cause of the peculiar 

 habit of gigas. In the tetraploid form of P. sinensis the correlation 



' H. de Vries, Grupponccise Artbilduny, 1913. 

 - B. P. Gregory, Proc. R. S., B. 87, 597. 

 3 Farmer and Digby. Phil. Tram. li. S., B. 205. 

 * E. P. Gregory, Proc. Camh. Phil. Soc. Vol. xv. Part in. p. 239. 



° N. Heribert-Nilsson. Spultuvrisentclieimiiii/eii der Oenothera I.aijuirckiana, 1915; 

 Lunds Uuiversitets Arskrift. 



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