Cyathium


A cyathium is one of the specialised pseudanthia forming the inflorescence of plants in the genus Euphorbia. A cyathium consists of:
  • Five bracteoles. These are small, united bracts, which form a cup-like involucre. Their upper tips are free and cover the opening of the involucre. These alternate with:
  • Five nectar glands, which are sometimes fused.
  • One extremely reduced female flower standing in the centre on a stalk at the base of the involucre surrounded by:
  • Four or five groups 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 subfamily Euphorbia section Goniostema 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 cyathophylls 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.