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Is it a nut... or a seed, fruit, drupe or legume? 

Assortment of nuts in their shell
Warning: may not contain any actual nuts Credit:  iStockphoto

As a rant 
by David Mitchell on Radio 4’s The Unbelievable Truth (episode five, series 21) demonstrated recently, there is much hilarity to be extracted from the collision between the technical and popular use of botanical terms. I’ve alluded in the past to the numerous grasslike plants that turn out not to be grasses, and the very many reproductive structures that would make a pretty dire crumble but that are, nevertheless, fruits. In the latest version, comedian Richard Osman essentially went through a long list of “nuts”, none of which (of course) are technically nuts. Some other panel members nearly wet themselves, and I’m not surprised – he was very funny.

It’s also very true. Botanically, a nut is a single seed contained within an indehiscent (not splitting to release the seed when mature), hard, woody pericarp (fruit wall). Simple enough, but nearly every popular “nut” contravenes at least one of those criteria.

To take Osman’s first example, a peanut does not have a woody pericarp and contains more than one seed. It is in fact a legume, the characteristic fruit of the pea and bean (legume) family. These days we normally eat peas in an immature state, but a mature, dry pea has just as much (or as little) right as a peanut to be called a nut. The fact that most peas and beans could be called nuts if we chose illustrates the rather elastic nature of the popular use of the word “nut”.

An almond looks at first sight like it qualifies as a nut, but that’s only because the outer, fleshy part of the pericarp has been removed before it reaches you; in a nut the pericarp must be entirely woody, and in an almond it isn’t. Things become clearer when we look at the very closely related apricot, where we eat the fleshy outer parts of the pericarp and discard the woody inner part along with the seed.

Other “nuts” that suffer from the same problem (pericarp at least partly fleshy, spongy or fibrous) include the pecan, walnut, cashew, pistachio and even coconut. All are technically drupes.

I recently received a gift of a necklace of kukui nuts (or candlenuts) from Hawaii. They certainly look nutty to me, but here no part of the pericarp is woody or even dry, the hard shell being part of the seed itself, technically the testa. The same is true of a Brazil nut.

Only the hazelnut is really a 'nut' Credit:  Kevin Summers

Pines aren’t flowering plants and therefore don’t even have a pericarp, so a pine nut couldn’t be less of a nut if it tried. Mind you, there are “nuts” that are even further from the definition, not being reproductive structures at all. The Chinese water chestnut (no relation to the true water chestnut, which is another drupe) is not a fruit at all but an underwater corm, and the tiger nuts I used to buy from the corner shop when I was at school are stem tubers.

The sweet chestnut and acorn come closest to being nuts, but in neither case can the pericarp be described as genuinely woody. Anyway, paradoxically, we don’t call an acorn a nut and there would no doubt be uproar if we insisted on calling it an “acorn nut”. Only the hazelnut, almost uniquely among edible “nuts”, really is a nut.

Ken Thompson is a plant biologist with a keen interest in the science of gardening. He writes and lectures extensively; his most recent book is Darwin’s Most Wonderful Plants. Visit books.telegraph.co.uk