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(also head) a monopodial inflorescence in plants whose main axis has thickened into a patelliform or globular surface on which small flowers are usually distributed. The flowers are tubular, ligulate, or pseudoligulate. This type of inflorescence is primarily characteristic of plants of the family Compositae (daisy, dandelion, and sunflower). It is also found among plants of the families Umbelliferae (eryngo and sanicle) and Campanulaceae (sheep's-bit). The capitulum is surrounded by a many-leaved involucre, formed by the bracts of the outer flower (in sheep's-bit) or by modified apical leaves (in Compositae). This structure aids in the pollination of the flowers. In the process of evolution, several biological adaptations occurred to prevent self-pollination. For example, the flowers open gradu-ally, and the stamens develop earlier than the pistils. The capitulum developed from the umbel as a result of the reduction of the length of the flower stem.