Only fruit that has its seeds on the outside

Question: Which is the only fruit that has its seeds on the outside?

Answer: None. There are no fruits in the world that grow their seeds outside of them.

I recently encountered this question in a contest. The answer given was strawberry. I explained them that the question itself is enormously erroneous. Thankfully, in a personal message, they acknowledged the error stating that the question is “somehow ambiguous” and they would have framed the question differently.

In layman's conception of “fruit” and “seed”, we often consider strawberry and cashew apple/nut as two popular examples of “seed” outside of “fruit”. There are also other not so popular examples exist like sunflower seeds, pineapple, pine nuts and so on. But the problem is what we consider as “fruits” in these cases are not “true fruits” at all. Let me explain.

A “true fruit” matures from the Ovary of a flower consisting (A) Pericarp or Ovary Wall comprised of 1) Epicarp/Exocarp, 2) Mesocarp and 3) Endocarp (E.g.: In Mango or Peach, skin, flesh and thick shell containing the kernel) and (B) Seed comprised of 1) Seed Coat, 2) Embryo and 3) Endosperm (E.g.: seed skin, germinating embryo and seed). This structure is placed on a Receptacle (E.g.: the bottom of the dark hard pimple), which is then attached to the main plant by a stem called Pedicel. Other flower organs like Stamens, Calix/Calyces (collection of Sepals) and Corolla(s) (collection of Petals) are attached at the bottom (hypogynous – superior ovary) or in the middle (perigynous – half-inferior ovary) or at the end (epigynous – inferior ovary) of the Ovary.

In Strawberry, the receptacle becomes “fruit”. In the so called “seed”, which is the “true fruit”, all parts of (B) Seed are present but the (A) Pericarp is nearly filled by (B) Seed and thus it looks like another “seed skin”. But at microscopic level all layers are still present. Same as Groundnut/Peanut, when the “true fruits” are young the Mesocarp will be fleshy and the “real seed” will be tiny. When the “real seed” grows, it occupies the entire (A) Pericarp and turns it into a hard shell encompassing the seed.

In Cashew Apple/Nut, the pedicel becomes “fruit”. In the “nut”, which is the “true fruit”, (B) Seed parts are present. (A) Pericarp becomes a double shell containing an allergenic gooey irritant.

In Pineapple, the inflorescence (which holds a flower cluster or flower group) and stem become “fruit”. Cultivators won't allow the “real fruits” to mature. If they mature they will become “seed like”, just like Strawberry, and will be placed in pits out side of the “fruit's surface”.

Roselle

In Roselle, the calyx becomes “fruit”. (A) Pericarp becomes the seed capsule containing (B) Seed. Roselle is not an example of “seed” outside of “fruit”, but is an example of petals becoming “fruit”.

In fact, there are humongous number of species out there in the wild with “seeds” found outside of “fruits”. These are false fruits and are called Accessory Fruits or Pseudocarp. The strange properties of these accessory fruits are, in some cases, unlike Mango or Peach, once cut off from the main plant they won't be getting ripe anymore or the taste won't improve.

Also technically, when we talk about fruits and seeds, it's about flowering plant species, angiosperms, meaning plants that produce seeds within an enclosure (this enclosure is called fruit) as opposed to gymnosperms, meaning “naked” seeds.

So, when the seeds are found outside, they simply become gymnosperms, non-flowering, fruitless, ovary-less species. E.g.: conifers, cycads and ginkgo.