achenium

Achenium is a botanical term used to describe a small, dry, indehiscent fruit that contains a single seed and does not split open at maturity. In many botanical texts this fruit type is called an achene, and achenium is often treated as a synonym or a variant used by some authors to emphasize the unitary nature of the fruit derived from a single ovary. The pericarp forms the outer wall of the fruit, and the seed lies inside, typically attached to the wall at one or few points. The structure is usually one-seeded, though there is variation among species.

Usage of the term varies by tradition and author; in modern floras the word achenium is comparatively rare and many botanists simply refer to the fruit as an achene. When it is used, it generally signals minor morphological distinctions or highlights the very small size of the fruit within a description.

Ecology and dispersal: achenia are commonly dry and do not dehisce, so seed release relies on external processes such as decay of surrounding tissue or dispersal by wind, animals, or other vectors. In many plant families, notably Asteraceae, the fruit is indeed an achene; in others, the term achenium appears in historical or specialized descriptive contexts.

Etymology: the term derives from Greek roots related to the concept of a single-seeded fruit, with the suffix -ium signaling a nominal or diminutive form in some sources. See also Achene; Fruit types; Plant morphology.