A poinsettia's red petals are not petals, and its flowers have none
Look at a poinsettia and you are being fooled. Euphorbias, the spurge genus it belongs to, strip their flowers down to a single stamen or pistil, with no petals at all. The colourful parts are modified leaves and cups doing a petal's job. It is also the only plant genus using all three kinds of photosynthesis.
Euphorbia is huge, with roughly 2,000 species, from tiny annual weeds to trees; Euphorbia ampliphylla may reach 30 metres or more. All of them bleed a caustic, poisonous milky sap. Linnaeus described the type species, E. antiquorum, in 1753, and the genus shows one of the widest spreads of chromosome counts among flowering plants. Uniquely, its members between them run CAM, C3 and C4 photosynthesis.
The flower head, called a cyathium, is the family's signature. Each individual flower is reduced to bare sexual essentials: a male is just a stamen, a female just a pistil, usually splitting into three parts. Beneath them sits a cup-like involucre holding nectar glands, typically five, which may fuse into a U shape. Below that come paired leaf-like cyathophylls, which can be showy and bright or shrink to tiny scales. The whole cluster mimics a single bloom. Most species carry both sexes on one plant, and stressed or young plants sometimes produce only male flowers until conditions improve.
Many succulent euphorbias from southern Africa and Madagascar look so much like American cacti that people routinely misname them, a case of unrelated plants evolving similar shapes for dry climates. Their spines are often modified stipules. Gardeners prize them for drought tolerance, and the crown of thorns and the Christmas poinsettia are popular ornamentals.
The names carry history. Spurge comes from an Old French word meaning to purge, because the sap was used as a purgative. Euphorbia honours Euphorbos, physician to King Juba II of Numidia and Mauretania, who wrote that a cactus-like species made a powerful laxative. Juba named the plant for him in 12 BC, reportedly because Augustus had honoured the doctor's brother with a statue, and Linnaeus extended the name to the whole genus.
Source: Euphorbia
