Experimental taxonomy of the genus Arabis.

The taxonomy of some species of the genus Arabis L. (including Cardaminopsis Hayek) has been investigated experimentally with particular reference to the British taxa of Arabis hirsuta s.l. and Cardaminopsis petraea (L.)Hiitonen. Cytological studies, hybridization experiments, ecological investigations, population analyses and observation of the behaviour of plants in cultivation, have contributed to an understanding of the origins of variation and the relationships of taxa. The A. hirsuta complex contains diploid and tetraploid taxa. The complete sterility of hybrids between the diploids and tetraploids effectively prevents gene-exchange between chromosome-levels. The wide-spread and variable A. hirsuta (L.)Scop. is suggested to have an allo-tetraploid origin from A. corymbiflora Vest. and A. sagittata DC. In Britain, A. hirsuta is differentiated into distinct, but interfertile, local biotypes as a consequence of facultative autogamy and the recent disjunction of its populations. The plants of the Atlantic calcareous- -dunes constitute an ecotype for which subspecific status is proposed. The A. alpina complex is a group of geographically isolated taxa which are little differentiated morphologically. Incompatibility barriers have developed between some taxa. The status of the components of the circum-boreal C. petraea complex is obscured by the extreme plasticity of the response of the plant to its environment. This plasticity is more obvious than the genetic variation of the species in Britain and is the origin of the varieties grandifolia (Druce) and faeroensis (Hornem.). The synthesis of a sterile hybrid between C. petraea (L.)Hiitonen and C. halleri (L.)Hayek and the possession of similar modes of vegetative reproduction indicates the close relationship of these species. C. arenosa (L.)Hayek, because of its morphological differences from these species and its ability to hybridize with species of Arabidopsis Heynhold, has greater affinities to the latter genus.