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Euphorbia tridentata, cross-section of a cyathium.

A cyathium (plural: cyathia) is one of the specialised pseudanthia ("false flowers") forming the inflorescence of plants in the genus Euphorbia (Euphorbiaceae). A cyathium consists of:

  • Five (rarely four) bracteoles. These are small, united bracts, which form a cup-like involucre. Their upper tips are free and in the beginning cover the opening of the involucre (like the shutter of a camera). These alternate with:
  • Five (1 to 10) nectar glands, which are sometimes fused.
  • One extremely reduced female flower standing in the centre at the base of the involucre, consisting of an ovary on a short stem with pistil, and surrounded by:
  • Five groups (one group at the base of each bracteole) of extremely reduced male flowers, which each consist of a single anther on a stem.

The flower-like characteristics of the cyathia are underlined by brightly coloured nectar glands and often by petal-like appendages to the nectar glands, or brightly coloured, petal-like bracts positioned under the cyathia. The paired petal-like bracts of Euphorbia section Goniostema are called cyathophylls. here female to male flower ratio is 1:α

The cyathia are sometimes solitary, but are usually in cymes, inflorescences of the second order, in pseudumbels, on dichotomously branched stalks or in so-called simple cymes which consist of one central and two lateral cyathia.

In one group of Madagascan species in the sub-family Euphorbia section Goniostema (E. aueoviridiflora, E. capmanambatoensis, E. iharanae, E. leuconeura, E. neohumbertii, E. viguieri) there is a tendency for a further pseudanthium to grow from the cyme. Probably as an adaptation to pollination by birds, the cyathia have become specialised: Most cyathia have upright cyathophylles which surround them protectively, but render the nectar glands inaccessible. To compensate, between them are naked sterile cyathia whose only job is to produce nectar.