The chromosome numbers in Dahlia (Asteraceae, Heliantheae) are surveyed, and counts made on species grown in the Botanic Garden, University of Copenhagen, are given. Next a biosystematic study of species within Dahlia sect. Dahlia is presented. It is concluded that taxa with the same chromosome number can be brought to hybridize without specific barriers (some diploids possibly excepted). Based on this conclusion, it is demonstrated that the original European introduction of dahlias from Mexico consisted of two tetraploid (2n = 64) hybrids and a native, diploid (2n = 32) D. coccinea with scarlet rays. One of the hybrids was named D. pinnara which designation is consequently not applicable for the native species, currently known under that name. Instead, the new designation D. sorensenii is formally established. The other hybrid, D. rosea, is a synonym of D. pinnara.
Dahlia campanulata and D. cuspidata (Asteraceae, Coreopsideae), two new species from the Mexican states of Oaxaca and Hidalgo, respectively, are described and illustrated. Dahlia campanulata is distinguished by its exceptionally large, pendulous, and campanulate flower heads; leaf petioles with enlarged clasping bases that are much thicker than in any other species of Dahlia; and enormous tubers that exceed 0.5 m in length. Dahlia cuspidata is distinguished by its large outer involucral bracts and cuspidate leaflets, particularly at the terminal end of the rachis and those at the ends of the lateral rachillae; the terminal leaflet on mature leaves is often much more rounded (versus elliptic) than the lateral leaflets.Key words: Asteraceae, Coreopsideae, Dahlia, Heliantheae sensu lato, Hidalgo, Mexico, Oaxaca.
RESUMENSe describen e ilustran Dahlia campanulata y D. cuspidata (Asteraceae, Coreopsideae), dos nuevas especies de los estados mexicanos de Oaxaca e Hidalgo respectivamente. Dahlia campanulata se distingue porque las cabezuelas de las flores son excepcionalmente grandes, campanuladas y péndulas, las bases de los pecíolos son alargadas y amplexicaules, mucho más gruesas que en cualquier otra especie; los tubérculos son enormes y exceden 0.5 m de largo. Dahlia cuspidata se distingue por sus brácteas involucrales externas grandes y sus folíolos cuspidados, especialmente el que remata el 1 Current address:
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