POLYPLOIDY 235 



the graceful liabit and long drooping fruits of the EE species, 

 In the same way, although with a greater number of complications, 

 the hexaploid and octoploid polyploid species of Rosa show similar 

 combinations of the chromosomes and characters of three and fovir 

 differential species respectively. 



In addition to the two kinds of polyploidy — polyploidy varieties 

 and polyploidy species — a third kind is known among roses, which 

 is really a combination of the two. A number of garden roses are 

 polyploid varieties of the AA species. The triploid tea rose. Lady 

 Hillingdon, according to Hurst, carries three A sets of chromo- 

 somes and characters, while the tetraploid tea rose, Gloire de Dijon, 

 has four sets of A. IVIost of the old-fashioned hybrid perpetual 

 roses belong to the polyploid species carrying two sets of A and 

 two sets of C chromosomes and characters. 



Hybridisation between these two species has given rise to the 

 modern hybrid teas, which are polyploid hybrids carrying three 

 A sets and one C set of chromosomes and characters. So far these 

 have only been found in cultivation, but if diploid gametes arise 

 in cultivation through physiological disturbances due to changed 

 conditions, there is no reason whv thev should not arise in nature 

 under extreme conditions, and no doubt polyploid hybrids of this 

 mixed type will be found in the wild state. 



There is another possibility that must also be considered : 

 Garden roses that were once polyploid varieties may have now 

 become partially differentiated by mutations of their genes under 

 the influence of intensive cultivation. A similar condition may 

 arise from two species hybridising and subsequently reduplicating 

 which were not entirely alike in their chromosome sets. In such 

 circumstances it is to be expected that there would be a mixture 

 of multivalent and bivalent chromosomes at synapsis. In cases 

 of intense cultivation, the sets themselves might be formed of 

 chromosomes from two or more different sets which had become 

 broken up as in the heteroploid hyacinths. 



The experimental creation of polyploid species is now an 

 accomplished fact. Thus Primula kezvensis, Nicotiana digluta, 

 and the new polyploid genera Brassico-raphanus and /Egilo- 

 triticum originated in controlled experiments and the details of 



