Reproductive relationships between annual species ofCalendula (Compositae)

Breeding experiments were carried out inCalendula species. In the annuals, which are selfers, rarely some outcrossing was observed only in the most peripheral flowers. In experimental crosses fruit was produced in all combinations. Fertile F₁ and F₂ hybrids could be grown from crosses between parents with similar chromosome numbers:C. palaestina ×C. pachysperma and crosses of different morphological forms ofC. arvensis. In crosses of species with different chromosome numbers at least partly fertile F₁ hybrids were obtained fromC. tripterocarpa ×C. stellata andC. tripterocarpa ×C. arvensis and crosses of the latter withC. palaestina. Fertile F₂ plants were grown from the combination ofC. arvensis ×C. tripterocarpa. Considering this information and previously obtained data, a scheme is proposed for explaining speciation in the genusCalendula.