achene [ ey-keen, uh-keen ] noun: a one-seeded, indehiscent, dry, simple fruit
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 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.
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.
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). https://doi.org/10.1590/S0100-84042008000300018.