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If you asked a botanist this question, the answer would probably be no, a coconut isn’t a true nut: strictly speaking, it’s a one-seeded drupe. In more general terms, however, a coconut can be a fruit, a seed, and a nut (a nut is technically a type of fruit).
A drupe (such as an olive, almond, or apricot) is a fruit with a central stone containing the seed and has three layers: the exocarp, mesocarp, and endocarp. The coconut is the large, greenish, smooth fruit of a tropical palm tree. This visible part is the outer layer, or exocarp. When this is removed (which usually happens before coconuts reach the shops) you’ll see the next layer, which is the brown, fibrous mesocarp, or husk. This fibre (called coir) isn’t edible, but is valuable in other respects: it’s used in potting compost and to make ropes, mats, sacks, etc.
The final layer is the endocarp, which in the coconut is a hard and woody shell: this surrounds the seed. The white edible ‘flesh’ of the coconut is the layer that forms on the inner surface of the shell and is a component of the seed. In botanical terms, this is known as the endosperm.
Let’s return to the question of whether a coconut is a nut or not. As mentioned, a nut is classified by botanists as a type of fruit comprising a shell and a seed (so, loosely speaking, a coconut is both a fruit and a nut). While a coconut has a hard shell and a seed, however, a coconut isn’t considered to be a true nut: true nuts (such as walnuts and pecans) are indehiscent (that is, they don’t split open to release their seeds when ripe). In the case of a coconut, the seed germinates and then sprouts from the end of the shell, which eventually splits (dehisces).
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Since it's called coconut it must be a nut, right? The answer actually depends on who you'd ask this question.
Coconut: A Fruit, a Nut or a Seed?
by Kasma Loha-unchit
Though its name suggests that it is a nut, I’ve always regarded coconut as a fruit. When the coconut is young, it has properties like fruit, and as it matures, it becomes more nutty. But in fact it is not a nut or a fruit; it is a seed.
Unless it is picked, a matured coconut eventually drops from the tree. The fully developed hard shell does not crack easily. Dry and brown, the coconut may sit underneath the tree for months and appear as if it were dead, until one day a green shoot pushes its way out of the shell. The whole time the old coconut has been sitting under the tree, changes have been slowly taking place inside. At one end of the coconut (where the eyes are), an embryo starts growing, feeding off the juice and nutrition of the thick white flesh. This embryo develops into a creamy mass that gradually fills much of the empty space inside. It is good to eat – sweet, somewhat spongy and less fibrous than the matured meat.
The embryo eventually sprouts out of the shell and becomes a young coconut seedling. At this point, the plant can survive for several more weeks or months on the food and water inside as roots gradually develop and extend out of the shell to anchor the plant in the ground. Nutritious coconut meat can sustain life for a long time; one of my students, who is a horticulturist, has successfully used coconut milk to nurse seedlings of other plants in his greenhouse. Coconut palms lead a long productive life. They begin bearing fruits at the age of five to seven years and continue to do so until they are seventy to eighty years old.
As complete seed packages, coconuts have been known to travel to faraway lands to find new homesteads. Stories abound of coconuts floating their way across seas and oceans to be washed ashore on distant islands, rooting themselves in handsome groves to greet visiting humans in search of paradise. The dehusked coconuts you buy at the supermarket, however, are no longer productive seed packages. Once the husk is removed, the seed dies.
Coconut: A Fruit, a Nut or a Seed? by Kasma Loha-unchit.