Poinsettias are the most popular Christmas house plant in Europe, the US and other countries. Native to Mexico, they were first introduced into the US in the 1820's by the US ambassador (and keen botanist) Joel Roberts Poinsett, from whom they take their common name.
Euphorbiaceae Plant Family
The Latin name is Euphorbia pulcherrima, meaning "very beautiful", and there are hundreds of cultivars. As members of the Euphorbiaceae, a large and complex plant family, they have small, unisexual flowers (flowers are either male or female) and superior syncarpous ovaries (ovaries that are fused and above the insertion point of floral parts such as petals and stamens).
Evolution of the Inflorescence
With over 2000 species Euphorbia itself is one of the largest plant genera, and has extremely reduced apetalous flowers (the continuation of floral structure reduction throughout evolution) clustered into cyathia (singular: cyathium). This is a structure that may have evolved from the contraction of a larger branching inflorescence and from overlap in the expression of regulatory genes in the flower and inflorescence. Both morphological and molecular analyses indicate that it evolved only once.
Features of the cyathium are used to differentiate species within the genus. For example, they may have nectar glands which are attractive to insect pollinators; the cyathia of poinsettias typically have a singular greenish gland.
Euphorbia and related genera have male flowers with one stamen and female flowers with three carpels. Flowers are yellowish. Plants contain a latex which is often caustic and thought to be a defence against herbivory.
False Flowers of the Poinsettia
The brightly coloured bracts of poinsettia are modified leaves. Plants that have reduced flowers without petals often evolve alternative showy structures to attract pollinators, known as pseudanthia ("false flowers"). Bracts that perform this function are a common feature in the Euphorbiaceae plant family, as are various types of pseudanthia.
Short Day Plants
Poinsettias are "short day plants", which means they flower (and bracts change colour) in response to shorter day lengths. This is an example of photoperiodism and is an adaptive process to ensure that the plant flowers when pollinators are available.
The Colour and Form of Poinsettias
Poinsettia bracts in the wild are red, and initially all cultivated forms had red bracts too. Modern cultivars are often pink and white, and other colours such as gold, peach and variegated or spotted forms are now available too. Poinsettias grow to ten feet in nature, but as house plants are bred to be more compact and bushy, with wider bracts.
To make a poinsettia bloom again for next Christmas, cut it back after the leaves have dropped and allow it to go dormant. From late spring repot in new compost, keep in a mild shady place and feed regularly. From the end of September keep in total darkness for 14 hours each night, for eight weeks. By the beginning of December it should again be ready to give some Christmas colour.
References
Hessayon, D.G. (2003) The House Plant Expert. Expert Books, New York
Heywood, V.H., Brummitt, R.K., Culham, A. and Seberg, O. (2007) Flowering Plants of the World. The Royal Botanic Gardens, Kew, Richmond
Park, K. and Backlund, A. (2002) Origin of the Cyanthium-Bearing Euphorbieae (Euphorbiaceae): Phylogenetic Study Based on Morphological Characters. Botanical Bulletin of Academia Sinica 43: 57-62
Prenner, G. and Rudall, P.J. (2007) Comparitive Ontogeny of the Cyathium in Euphorbia (Euphorbiaceae) and its Allies: Exploring the Organ-Flower-Inflorescence Boundary. American Journal of Botany 94: 1612-1629
Prenner, G., Ivalu Cacho, N., Baum, D. and Rudall, P.J. (2011) Is LEAFY a Useful Marker Gene for the Flower-Inflorescence Boundary in the Euphorbia Cyathium? Journal of Experimental Botany 62: 345-350
Ronse De Craene, L.P. (2010) Floral Diagrams. Cambridge University Press, Cambridge
University of Illinois (2011) The Poinsettia Pages: History & Legends.
Wurdack, K.J., Hoffmann, P. and Chase, M.W. (2005) Molecular Phylogenetic Analysis of Uniovulate Euphorbiaceae (Euphorbiaceae sensu stricto) Using Plastid RBCL and TRNL-F DNA Sequences. American Journal of Botany 92: 1397-1420
