Dahlia (däl′yȧ). A genus of the composite family, containing some of the commonest garden-plants. All of the ordinary garden-forms are derived from a single variable species D. variabilis. The genus contains about ten species, which are mostly Mexican, a few occurring in Central and South America. As an illustration of the varieties which can be made from a single species, it may be said that over 3,000 different garden-varieties of D. variabilis have been named and published in catalogues.