Thursday, 24 December 2015

Cyathium, the astounding performer

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.

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.

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.

The structure of Euphorbia cyathium

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.

Cyathium of Euphorbia pulcherrima. Picture: Marc Perkins

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…

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.

Picture: Pauline Brock