236 BOTANY 



their origin are fully known. They arose through hybridisation 

 of distinct species and the subsequent reduplication of their 

 chromosomes made them fertile. It is probable that the numerous 

 polyploid species of Rosa originated in this way, although the 

 reverse process of loss of sets in polyploid species giving rise to 

 lower polyploid sjoecies and ultimately to diploid species is believed 

 by Hurst to be at w^ork in the evolution of the species of Rosa. 

 R. centifolia is one such case in point. The tetraploid species of 

 this plant threw off as bud-sports several triploid mutants in 

 France. 



Taxonomically, polyploidy explains many difficult problems and 

 makes clear the remarkable polymorphism of a genus like Rosa, 

 in which the many sets of chromosomes and characters in the 

 polyploids give a range of variation far exceeding that of 

 diploids. 



" Polyploidy has helped us to realise for the first time w^hat a 

 species really is. It has experimentally demonstrated that a 

 species is no longer an expert opinion, but a real entity which can 

 be experimentally demonstrated by the combined methods of 

 taxonomy, cytology and genetics. True species are entirely 

 discontinuous and isolated from one another by their different sets 

 of chromosomes and characters. A knowledge of the true nature 

 of a species clears the way for an inquiry into its origin and 

 evolution. That polyploidy has played a great part in this is now 

 clear. The evidence shows a rhythmic process of evolution up and 

 down from diploid species to polyploid species by additions of sets 

 of chromosomes, and down from polyploid species by losses of sets 

 of chromosomes. From the experimental evidence there can be 

 little doubt that the evolution of a species is in large measure 

 directed and controlled by the conditions of life acting directly 

 on the chromosome sets and the genes contained therein causing 

 mutational responses and changes of balance which when favour- 

 able create new species." 



Sterility and Polyploidy in Cultivated Fruits. Cultivated fruits 

 are often complex hybrids with a complex chromosome constitu- 

 tion. A number of our fruits are polyploids, and those with the 

 higher chromosome number than the diploid are as a general rule 



