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The cyathium is a characteristic feature of the genus, and consists of a number of male flowers, each reduced to a single stamen, surrounding a central female flower which consists only of a stalked pistil; the group of flowers is enveloped in a cup formed by the union of four or five bracts, the upper part of which bears thick, conspicuous, gland-like structures, which in exotic species are often brilliantly coloured, giving the cyathium the appearance of a single flower.

In Mercurialis and others with inconspicuous flowers pollination is effected by the wind, but in many cases insects are attracted to the flower by the highly-coloured bracts, as in many Euphorbias and Dalechampia, or by the coloured calyx as in Manihot; the presence of honey is also frequently an attraction, as in the honey-glands on the bracts of the cyathium of Euphorbia.

A partial inflorescence, cyathium, bearing the petaloid glands.

A cyathium without petaloid glandular appendages.

Diagram of the inflorescence of Euphorbia, illustrating the dichasial cymose arrangement of the ultimate branches. b, Bract subtending the central terminal cyathium I. a′b′, Bracteoles of the first order subtending the secondary cyathia II. a″b″, Bracteoles of the second order subtending the tertiary cyathia III.