The Evolution and Classification of Roses 

 T. D. A. CocKKRrxL 



The paper on Rosa by Dr. C. C. Hurst of England, presented 

 to the Genetics Congress in Berlin in 1927', is of extraordinary in- 

 terest to botanists. It offers a classification of the genus Rosa, 

 based mainly on cytological evidence, or at least the concurrance of 

 external characters with cytological conditions. Starting with the 

 admitted fact that the gametes of roses carry a minimum of seven 

 chromosomes, it is shown that when this number is exceeded we 

 have multipjes of seven, even up to fifty six. The forms with the 

 minimum number, fourteen in the somatic cells, are called diploids ; 

 those with higher numbers polyploids, or more specifically trip- 

 loids, tetraploids, pentaploids, etc. This of course, parallels what 

 has been found in other organisms. In all 1,006 different species 

 and forms of Rosa have been examined cytologically, or had been 

 when Hurst's paper was written. Of these Hurst himself examined 

 674. and the whole series represents all the recognized sections and 

 subsections of the genus. Beginning with the diploids, it is ob- 

 served that they fall into nine distinct groups. Four of these are 

 so distinct that they are excluded from Rosa altogether, under the 

 following generic names : 



Hulthemia, for H. persica (Michx.) of the deserts of Central 

 Asia. 



Platyrhodon, for P. micro phylla (Roxb.) of China and Japan. 



Ernesiella, for E. bract cat a (Wendl.) of China and E. involu- 

 crata (Roxb.) of India and Burma, the latter regarded as a sub- 

 species. 



Hespcrhodos, tor H. niiniitifolia (Engelm.), H. stcllata (Woo- 

 ton), and H. stcllata mirifica (Greene) all of the arid region of 

 North America. Hurst treats Wooton's R. stcllata and Greene's 

 R. mirifica as subspecies of H. minutifolia. 



There now remain five groups, true members of Rosa, which 

 are designated AA, BE, CC, DD, EE, with gametes A, B, C, D, E. 

 The characters of all of these groups are cited; most are morpho- 

 logical, but some physiological, as the late ripening of the fruits. 



' C. C. Hurst. Differential Polyploidy in the Genus Rosa, L. Verh. 

 Internat. Kongresses f. Vererbungslehre, Berlin, 1927. Supplbd. Zeits. f. 

 induktive Abstammungs und Vererebungslehre. 1928. pp. 866-906. 



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