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Firstly, I understand that it is classified as a drupe and its mesocarp is fibrous hence its hardness. However, how did botanists conclude such? Putting both mango and coconut side by side, we can at least see that the mango has fleshy mesocarp and soft exocarp. I would argue that the coconut's structure bears more similarity to the durian yet the latter is a capsule (or dry).

With that in mind, I cannot comprehend how the pericarp is distinguishable in 3 layers in coconut but not in durian yet they seem to be same? If coconut is like a deviant under drupes due to its fibrous mesocarp, (1) how'd they even distinguish that it was a mesocarp and (2) what is the clincher that made them conclude that coconut is indeed a fleshy and drupe despite seemingly deviant from common examples such as mango?

Apologies for not being articulate enough to explain.

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The drupe definition from other answers is correct: it is a fruit with a fleshy outer part (exocarp and mesocarp), and a single hard endocarp. A drupe does not split open on its own (it is indehiscent).

A coconut fruit matches this description. The exocarp and mesocarp are fibrous (which botanically counts as fleshy), and the endocarp is hardened.

A niu kafa coconut split longitudinally showing the fibrous mesocarp, white endosperm, and solid endocarp between the two. The seed coat is too thin to be visible.

Two shucked coconuts, one intact and one split open. The hard shell of a shucked coconut is the endocarp. The seed coat is barely visible in the split coconut between the white endosperm and the dark brown endocarp.

The hard shell of the shucked coconut is the endocarp, and is derived from the ovary wall. The white flesh is part of the seed, called endosperm, and the outermost layer of the seed is the seed coat. The seed coat is a very thin layer between the hard, dark brown endoderm and the endosperm.

A coconut also only contains one locule, or chamber of the ovary.

A durian, on the other hand, is a very different fruit architecture. A durian fruit contains multiple locules, and these will eventually split apart once the fruit is ripe enough (it is dehiscent). This makes it a type of fruit called a capsule. Capsules are normally described as "dry fruits" by botanists, but that descriptor is a little stretched here since the exo- meso- and endocarp that make up the walls and structure of the ovary chambers are, if not soft and pulpy, not exactly hard or papery.

The soft fleshy part of the durian that people eat is a part of the seed called an aril. Not all seeds have an aril—it's an extra fleshy bit on the outside of the seed that usually enhances seed distribution. (The red juicy bit of a pomegranate seed is also an aril.)

Finally, often the layers of a fruit can be hard to distinguish because they are compressed or of a similar structure. Botanists can identify fruit layers by tracking the development of the fruit and what the young tissues become.

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A [drupe][1] is defined as a fleshy fruit, that does not open at maturity, with a single seed enclosed in a hard shell. Against all evidence, a coconut is a fleshy fruit - that corky brown husk around the seed is botanically the same as the flesh of a mango or a grape. Coconut fruits never contain more than one seed, and that seed is contained within a hard, woody shell. So, coconuts meet all of the technical criteria to be drupes.

EDIT:The fleshy part of a pomegranate seed is a sarcotesta, a fleshy outer seed coat, not an aril, which is a growth separate from the seed coat attached to the seed's hilum. [1]: https://en.wikipedia.org/wiki/Drupe

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