Calendula L.
Calendula is a taxonomically complex genus of 10-27 species (although many more have been described). It is native in the Mediterranean region, Macaronesia and southwestern Asia. The center of distribution lies in northwestern Africa with eight species known from northern Morocco (Valdés & al. 2002). The genus is remarkable for the wide range of chromosome numbers across species possibly resulting from the propensity of species with different chromosome numbers to hybridize and form new polyploid taxa (Plume 2008).
Two species are now very widespread and have been recorded in Belgium. Calendula officinalis is a popular ornamental species and C. arvensis a common weed of agricultural fields.
Heads 10-20 mm across, ligules less than twice as long as the involucral bracts, usually yellow === Calendula arvensis
Heads 20-70 mm across, ligules at least twice as long as the involucral bracts, usually orange, more rarely yellow === C. officinalis
Literature:
Gallego M.J. & Talavera S. (1983) Nota sobre las especies espanolas anuales del género Calendula L. Lagascalia 11(1): 100-104.
Heyn C.C. (1987) A systematic-evolutional study in the genus Calendula. Rotem 23: 5-32.
Heyn C.C., Dagan O. & Nachman B. (1974) The annual Calendula species: taxonomy and relationships. Israel J. Bot. 23: 169-201.
Heyn C.C. & Joel A. (1983) Reproductive relationships between annual species of Calíendula (Compositae). Plant Systematics and Evolution 143: 311-329.
Lambinon J. & Verloove F. (avec coll. Delvosalle L., Toussaint B., Geerinck D., Hoste I., Van Rossum F., Cornier B., Schumacker R., Vanderpoorten A. & Vannerom H.) (2012) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Sixième édition. Jardin botanique national de Belgique, Meise: CXXXIX + 1195 p.
Lanza D. (1919) Monografia del genere Calendula L. Palermo.
Meikle R.D. (1976) Calendula L. In: Tutin T.G. & al. (eds.), Flora Europaea vol. 4. Cambridge University Press, Cambridge: 206-207.
Meusel H. & Ohle H. (1966) Zur Taxonomie und Cytologie der Gattung Calendula L. Österr. Bot. Z. 113: 191-210.
Nora S., Castro S., Loureiro J., Gonçalves A.C., Oliveira H., Castro M., Santos C. & Silveira P. (2013) Flow cytometric and karyological analyses of Calendula species from Iberian Peninsula. Plant Systematics and Evolution 299(5): 853-864. [available online at: https://www.researchgate.net/publication/235752882_Flow_cytometric_and_k...
Norlindh T. (1946) Studies in the Calenduleae. II. Phytogeography and interrelation. Botaniska Notiser 4: 471-506.
Ohle H. (1974) Beiträge zur Taxonomie der Gattung Calendula. II. Taxonomische Revision der südeuropäischen perennierenden Calendula-Sippen. Feddes Repert. 85: 245-283.
Ohle H. (1975a) Beiträge zur Taxonomie und Evolution der Gattung Calendula. III. Revision der marokkanischen perennierenden Sippen unter Berücksichtigung einiger marokkanischer Annueller. Feddes Repert. 86: 1-17.
Ohle H. (1975b) Beiträge zur Taxonomie der Gattung Calendula. IV. Revision der algerisch-tunesischen perennierenden Calendula-Sippen unter Berücksichtigung einiger marokkanisch-algerischer Annueller und der marokkanischen und südeuropäischen perennierenden Taxa. Feddes Repert. 86: 525-541.
Plume O. (2008) Systematics of Calendula (Asteraceae): Toward a robust molecular phylogeny and a hypothesis of chemical diversification within the genus. Poster presentation, available online at: http://2008.botanyconference.org/engine/search/index.php?func=detail&aid....
Silveira P.C., Gonçalves A.C., Santos C. & Paiva J. (2013) Two lectotypifications and a new combination in Calendula (Asteraceae) for Flora Iberica. Phytotaxa 145: 47-53. [available online at: http://www.mapress.com/phytotaxa/content/2013/f/p00145p053f.pdf]
Valdés B., Rejdali M., Achhal el Kadmiri A., Jury S. L., Montserrat J. M. [eds.] (2002) Catalogue des plantes vasculaires du nord du Maroc, incluant des clés d’identification / Checklist of vascular plants of N Morocco with identification keys, vol. 2, 670–672. Consejo Superior de Investigaciones Científicas, Madrid, Spain.