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Fothergilla gardenii Witch Alder
Fothergilla gardenii (Witch Alder). Photo © Mary Free
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Achene

achene [ ey-KEEN, uh-KEEN ] noun: a one-seeded, indehiscent, dry, simple fruit

Aggregates of achenes of invasive Clematis terniflora
Achenes of invasive Clematis terniflora (sweet autumn clematis) are aggregated. When they break apart, the long feathery style acts like a kite tail and helps each seed sail on the wind. Photo © Elaine Mills

When you hear the word achene, you may think of the seeds of sunflowers. Although many botanical works still refer to the fruits of Asteraceae (commonly called the aster, composite, daisy, or sunflower family) as achenes, some botanists consider them to be cypselae. One main difference between the two fruits is the location of the ovary from which each originates. An achene develops from a superior ovary, which is situated above the attachment of the petals, sepals, and stamen to the receptacle, the part of the plant that connects the flower to the stem. A cypsela originates from an inferior ovary that lies below the attachment of these floral parts.

The seed of an achene is attached to the ovary wall (pericarp) of a flower at a single point. The pericarps of some achenes extend into papery, wing-like tissue longer than the seeds to aid in wind dispersal. On Acer species (maple), these samaras are paired and may hang from branches alone or in clusters. On native Liriodendron tulipifera (tulip tree), the samaras are aggregated (an etaerio) and resemble a cone. Some achenes are surrounded by fine cottony hairs or attached to silky tufts or other structures that help their seeds to be carried by the wind away from the mother plant. In many Carex (sedge) species, the achene is covered by a perigynium, a sac-like bract (modified leaf) whose different structural aspects are useful in species identification.

Acer rubrum (red maple) achene with an extended, wing-like pericarp
Clustered pairs of winged achenes (samaras) of native of Acer rubrum (red maple)
Aggregated winged achenes (samaras) of native Liriodendron tulipifera (tuliptree)
The cottony hairs surrounding the achenes of native Anemone virginiana (thimbleweed)
The 3-sided achenes of Carex comosa (bottlebrush sedge)
In Fragaria (strawberry), achenes are distributed in a Fibonacci spiral pattern on the outside of the receptacle.
Achenes inside the rose hip of a Rosa (rose)
Left to right: Acer rubrum (achene, cluster), Liriodendron tulipifera, Anemone virginiana,
Typha latifolia, Carex comosa, Fragaria, Rosa.    

Some achenes are aggregated into accessory fruits, in which the fleshy portion does not develop from a flower’s ovary. The achenes of Fragaria (strawberry) are embedded on the outside of the red flesh that forms from the receptacle. Rosa (rose) achenes are contained inside a fleshy, cup-like structure known as a rose hip.

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Reference

Marzinek J, De-Paula OC, Oliveira DMT. 2008. Cypsela or achene? Refining terminology by considering anatomical and historical factors. Brazilian Journal of Botany. 31(3).