98 



The AA forms are numerous, including R. sempennrens L. of S. 

 Europe and N. Africa, R. arvensis Huds. of Europe, R. miiltiflora 

 Thunb. of Korea and Japan, R. setigera Michx. of North America, 

 etc. The BB group includes nine Asiatic roses, but also the 

 American R. gymnocarpa Xutt. CC is the R. rugosa group, in- 

 cluding the American R. nitida Willd. DD is the group of R. 

 cinnamomea, with several American species, such as R. hlanda, R. 

 fendleri, etc. EE, the group of R. inacrophylla LindL, is exclu- 

 sively Asiatic, and almost confined to China. Hurst states that 

 each of these groups may be considered a species, the various forms 

 being ranked as subspecies under that carrying the oldest name. 

 He does not, however, make the trinomials, and botanists are not 

 likely to reduce in this manner a large number of roses hitherto 

 regarded as of specific rank. In addition to the cited forms, it is 

 explained that there are a ery numerous varieties. A subspecies 

 is always homozygous for the subspecific characters, but a variety 

 is frequently heterozygous for the varietal characters. Thus the 

 subspecies ranks with the species rather than with the variety. It 

 was found in the case of the AA roses, that so far as tested in 

 crosses, they were fully fertile in the F^ and Fj generations, — an 

 argument for considering them a single aggregate species. How- 

 ever, the most casual examination of the groups shows great diver- 

 sity among the so-called subspecies. For example R. nitida is ex- 

 tremely unlike R. rugosa, and is usually placed in a different sec- 

 tion. \\t are thus compelled to admit that the five groups are 

 themselves complex, so that AA, BB, etc. are generalized 

 conceptions, not implying genetic uniformity beyond a certain 

 point. An alternate classification might conceivably include over 

 fifty groups, instead of five. Nevertheless, there is a theoretical 

 basis for the limited number of groups or aggregate species, 

 namely that within these groups the sets of chromosomes are 

 homologous in the sense that they are capable of pairing or form- 

 ing synaptic mates, in consequence of which the hybrids should be 

 fertile. Thus A may in fact be A^, Ag, A,, and so forth, through 

 a series of modified forms, which are typically homozygous A^, A^, 

 or Ag, Ag, etc., but can form hybrids Aj, A2, etc. when artificially 

 crossed, or where their ranges meet. 



We now come to the polyploid roses, which are very numerous. 

 First of all are the duplicational polyploids, such as AAAA, which 



