Crown of thorns (Euphorbia milii)
|Size||Height: 1 - 2 m (2)|
Classified as Data Deficient (DD) on the IUCN Red List (1), and listed on Appendix II of CITES (3).
Varieties: Euphorbia milii var. bevilanensis and Euphorbia milii var. vulcanii are classified as Endangered (EN), Euphorbia milii var. roseana is classified as Vulnerable (VU), and Euphorbia milii var. hislopii, Euphorbia milii var. longifolia, Euphorbia milii var. milii, Euphorbia milii var. splendens, Euphorbia milii var. tenuispina and Euphorbia milii var. tulearensis are classified as Data Deficient (DD) on the IUCN Red List (1).
The crown of thorns is a low growing, heavily branched, woody shrub, with oval shaped leaves and dense purplish-brown stems covered by many sharp, spiny thorns (2) (4) (5). Although the crown of thorns flowers throughout the year, flowering is particularly profuse during the winter months, with clusters of between two and eight flowers produced at the tips of green stems (5). Brightly coloured bracts (modified leaves) resemble petals and give the crown of thorns its colourful appeal, while the true flowers of the plant are small and unobtrusive (2) (4) (5) (7). Most crown of thorns varieties have stunning red bracts, although pink, yellow or whitish varieties are also known (2) (4).
Native to Madagascar, the crown of thorns is now found all over the world as a widely grown ornamental species. Originating mainly in south and central Madagascar, most varieties of the crown of thorns were described from collected specimens, and the original locations were not always properly recorded. As such, the wild locations of some crown of thorns varieties are no longer known (8).
The crown of thorns occurs mainly in areas containing granite rock formations, although different varieties exhibit different habitat preferences. E. m. roseana is known to occur on humus (partially decomposed organic matter) in shady areas of the Zombitse forest, E. m. bevilanensis occurs in bush habitat, while E. m. volcanii only occurs on mountainous plateaus (1). In cultivation, the crown of thorns has been found to thrive in sun or shade, on nutrient poor, sandy, well-drained soils (9).
The crown of thorns belongs to the genus Euphorbia, which contains nearly 2,000 species, and is characterised by a specialised inflorescence, called the ‘cyathium’ (10) (11). The cyathium of the crown of thorns is made up of cup-like whorls of the brightly coloured (usually red), modified leaves, known as bracts, which enclose several male flowers and a single, female flower. The flowers are unusually simple, with the male flowers reduced to single stamens and the female flower composed of an ovary and greatly reduced perianth (7) (10). The flowers are pollinated by flies (from the genus Diptera), which are attracted to the plant by nectar-producing glands on the cyathium (7).
Another characteristic feature of all Euphorbia species, including the crown of the thorns, is the presence of milky latex, or sap, which is secreted by the plant though broken stems, or damaged roots and leaves. Found in all parts of the plant, the latex is usually poisonous and probably developed in order to protect the plant from herbivores (4). Ingestion of the plant is known to cause severe irritation of the mouth and digestive systems, and may induce nausea, diarrhoea and swelling, while direct contact with the sap causes skin irritation, inflammation and blistering (4) (5) (7) (10) (11).
The greatest threat faced by all varieties of the crown of thorns is likely to be the destruction and degradation of habitat caused by expanding agricultural development and overgrazing. Fires (usually resulting from charcoal burning activities) are a significant threat, and collection for the horticultural trade may also pose a danger to some varieties (1) (10).
Currently, all varieties are listed on Appendix II of the Convention on International Trade in Endangered Species (CITES) (3), meaning that international trade in wild grown plants should be carefully monitored. Several varieties are also known to exist within protected areas; for example in Madagascar, the endangered Euphorbia milii var. bevilanensis is thought to occur in the Andohahela Reserve, while the vulnerable Euphorbia milii var. roseana is restricted to an area within the protected Zombitse forest (1).
Further investigation into Data Deficient varieties is desperately needed, in order to ensure that the proper conservation measures can be taken to adequately protect the species.
To find out more about the crown of thorns and other Euphorbia species, see:
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- Bract: modified leaf at the base of a flower.
- Genus: a category used in taxonomy, which is below ‘family’ and above ‘species’. A genus tends to contain species that have characteristics in common. The genus forms the first part of a ‘binomial’ Latin species name; the second part is the specific name.
- Herbivore: an animal that consumes only vegetable matter.
- Inflorescence: the reproductive shoot of a plant, which bears a group or cluster of flowers.
- Ovary: in plants, known as the gynoecium, the female reproductive organs of a flower.
- Perianth: the outer envelope of a flower, typically comprising of an inner whorl (calyx) of sepals or floral leaves, and an inner whorl (corolla) of petals.
- Stamens: the male reproductive organs of a flower. Each stamen is comprised of an anther (the pollen-producing organ) and a filament (stalk).
- Variety: In taxonomy, (the science of classifying organisms), variety is the rank below subspecies. Members of a variety differ from others of the same species in relatively minor ways.
IUCN Red List (September, 2010)
Cactus and Succulent Society of America (September, 2010)
CITES (September, 2010)
UCC Biology Department (September, 2010)
University of Illinois at Urbana-Champaign Veterinary Medicine Library (September, 2010)
Nova Scotia Museum: Poisonous Plant Patch (September, 2010)
- Heywood, V. H. (1978) Flowering Plants of the World. Oxford University Press, Oxford.
- Eggli, U. (2002) Illustrated Handbook of Succulent Plants: Dicotyledons. Springer-Verlag, Berlin, Germany.
Floridata (September, 2010)
- Oldfield, S. (1997) Cactus and Succulent Plants – Status Survey and Conservation Action Plan. IUCN/SSC Cactus and Succulent Specialist Group. IUCN, Gland, Switzerland and Cambridge, UK.
PlantZAfrica (September, 2010)