An achene is a dry, one-seeded indehiscent fruit: it remains closed at maturity, and its seed is not fused to the pericarp. Thus a seed-like object can actually be an entire fruit containing a seed. In the narrow usage of the University of Sydney eFlora, an achene develops from a superior ovary of one carpel.[ref]
Achene in the University of Sydney eFlora. Glossary entry Achene; dry indehiscent fruit and seed free from pericarp
The surface units of a strawberry provide an illuminating example. Under magnification, each small unit has its own enclosing fruit wall and seed; the large fleshy region beneath the units is the receptacle. Calling every surface unit a seed hides this extra anatomical layer.[ref]
Genome-scale transcriptomic insights into early-stage fruit development in woodland strawberry Fragaria vesca by Chunying Kang and others. Abstract and Introduction; dry achenes on the enlarged receptacle
Terminology varies between floras. PlantNET also includes comparable fruits from an inferior ovary under achene, while noting that these may be called a cypsela. The source's convention should therefore be checked when identifying a specimen.[ref]
Glossary of Botanical Terms by PlantNET. Entry achene; superior/inferior ovary distinction and cypsela terminology; plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?glossary=yes&ill=Fig.+18+Q%2C+R+YY+W&page=nswfl&term=achene
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