“What is the cyathium, this astounding performer?” wondered
León Croizat in his “On the classification of Euphorbia” (1937).
The (literally) most beautiful Euphorbia, E. pulcherrima
(from the latin pulcher / pulcherra = beautiful; pulcherrimum / pulcherrima = the most beautiful), stars in this festive season. It
seems to be therefore an appropriate occasion to unfold the Euphorbia topic.
The story of E.
pulcherrima begins from a very far distance to Christianity or Christmas
traditions. The plant is native to Mexico, and has been well known to the
Aztecs who used it to make red dye and as a fever cure.
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| Wild Euphorbia pulcherrima in Mexico. Picture: Mark E. Olson |
It was only in 1825 that it was first brought to the US by
the ambassador Joel Roberts Poinsett – hence their common name, Poinsettia. Its
undeniable extravagance due to the vibrant contrast between the intense red
bracts and the deep green leaves rapidly made E. pulcherrima a popular species in horticulture. Being a December bloom,
it soon became a favourite in Christmas decorations, which very much increased
its popularity.
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| Joel Roberts Poinsett |
The structure of Euphorbia
flowers is a trend topic in botany classes because of their striking strangeness.
This striking strange appearance is assigned to their being strikingly simple
and complex at the same time. Euphorbia flowers
are actually too simple to be called complex
for the complexity does not lie in the flower itself, but on the organization
of flowers within the inflorescence. It is so exquisite and unique that they
gained a name for themselves – cyathium.
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| The structure of Euphorbia cyathium Source: Euphorbiaceae.org |
The cyathium is derived from a cymose-type inflorescence, and consists of a terminal female flower surrounded by prophylls (bracteoles) subtending the male flowers and forming the involucrum. Each flower consists of a single organ, so female flowers consist of a single pistil with the very typical 3-locular ovary (three carpels) and bifid styles, and male flowers consist of a single stamen. As there are no petals or sepals associated to the flowers, the cyathium develops bracts (cyathophylls) and large nectar glands for pollination purposes.
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Cyathium of Euphorbia pulcherrima.
Picture: Marc Perkins
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But the cyathium structure is itself diverse, after all Euphorbia is a huge genus with around
2500 described species scattered around the globe (!!) – no wonder diversity is
enormous. Some species (e.g., E. milii)
have single cyathia with their own cyathophylls (two per cyathium). Others
(e.g., E. albomarginata) have glands
with petaloid appendages, resembling the classic structure of angiosperm flowers.
E. pulcherrima, on the other hand, forms
synflorescences, which in botanical
language simply means a group of inflorescences (botanists really fancy throwing
complicated names just because). So, E.
pulcherrima forms cyathia lacking cyathophylls, which are grouped in
synflorescences; therefore the red bracts so characteristic of poinsettias are not cyathophylls. Some authors don’t
even consider these as being bracts, but simply as leaves that turn color, so
you may find them described as bracteate leaves…
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| Left picture: Euphorbia milii (Source: Subhin) Right picture: Euphorbia albomarginata (Source: Jason Penney) |
I could continue endlessly writing about Euphorbias, but before getting too extended
on the subject, I shall stop this post at once to wish all readers a fantastic
holiday and a 2016 blooming with happiness.
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| Picture: Pauline Brock |
