Ever wonder what the only edible botanical nut is?  It's the hazelnut!

The culinary definition of a nut differs from the botanical definition of a nut. In fact the definition of nut in different botanical dictionaries and glossaries lack common "must have" criteria.

So what is a nut? That is a great question.

The Oxford English dictionary defines a nut as, "A fruit consisting of a hard or tough shell around an edible kernel."

The Encyclopedia Brittanica defines a nut in botany as, "A dry hard fruit that does not split open at maturity to release its single seed."

The culinary use of the word nut is more lenient than the botanical use of the word. The definition of nut is unclear and often very confusing.

The editors of Encyclopedia Brittanica explain that nuts are similar to achene, but grow from 2 or more carpels, are frequently bigger, and have a fibrous woody pericarp.

What did you just describe? Phew! We thought we were the only ones stuck on all the technical terms - achene, carpel, pericarp - botany experts, bear with us - we thought this might help:

achene: a pocket-sized, moisture-less fruit with a single, contained seed (mature fruit does not split to free the seed); Example: sunflower seeds carpel: the ovary, stigma, +/- style (existing as one of a group or individually) of a female flower's reproductive tract
pericarp: the component of fruit made from a mature ovary's wall

Encyclopedia Brittanica writes that the chestnut, hazelnut, and acorn are all examples of true nuts.

The use of the word nut is used loosely in everyday talk - but despite part of the word being _____ -nut does not actually mean it is a nut!  For example, peanuts are legumes! Peanuts are in the same class as garbanzo beans (chickpeas) and other beans.

In botanical terms nuts are actually fruits. The definitions of the fruit types are given below so you can associate the fruit type with the common name for that nut.

berry: a mere, fleshy fruit that typically contains multiple seeds where the middle and inside layers are frequently indistinguishable from one another
Examples: bananas, cranberries, and tomatoes; dates are one-seeded berries; watermelon, cucumber, and gourds are berry-like fruits called
pepos; the berry of citrus fruits (oranges, lemons, limes) are called hesperidium
Note: blackberries, raspberries, and strawberries are NOT true berries -- they are aggregate fruits!

capsule: fruit that lacks water and is unenclosed when mature; valves are the result of the fruit dividing from top to bottom into detached sections and each section bursts open (seen in the iris); some create pores at the apex (seen in the poppy), while others divide around the perimeter, shedding the top (seen in pigweed and plantain)
Examples: brazil nuts

corm: upright stem that is beneath the ground and contains a lot of soft, more thickened parts than usual and flesh-like tissue; serves as a storehouse for select plants with seeds; contains slender, clear, and bendable or a reduced or poorly developed, non-functional leaves and buds; when sliced in two, no rings can be seen (different from bulbs); contain a stringy outer covering (tunic); the roots arise from the level area at the bottom (basal plate); corms are storehouses for starches to aid in plant development and help them sustain life in adverse environments; lots of corms make baby corms via the type of asexual reproduction where the new plant grows from part of the original plant
Examples: Chinese water chestnuts

drupe: fruit whose ovary has a slender outside wall, thick and often fleshy (or at times tough or fibrous) middle, and a stony, hard pit on the inside; there is only one seed inside the pit (infrequently there are 2 or 3 seeds in the pit and in these situations only one of the seeds fully matures)
Examples: almonds, cherries, coconuts, dogwoods, mangoes, olives, peaches, and walnuts
Note: Raspberries and blackberries are aggregate fruits (not true berries); they contain several drupes grouped together.

follicle: a waterless fruit that bursts open only on one side and has ≥1 seed(s)
Examples: Bambara groundnuts, Moreton Bay chestnuts, oak acorns, sweet chestnuts

legume: also referred to as a pod; belongs to the pea family Fabaceae; many are fruits that naturally burst open along an inherent line of weakness to release their seeds; the exceptions include carobs and peanuts which do not open naturally; although legumes come in an assortment of shapes and sizes, lots of them have one line of seeds and are long and thin; they are typically firm and wood-like when mature, although edamame, green beans, and snow peas are green and fleshy when they are picked
Examples: edamame, green beans, peanuts, snow peas
Interesting Fact: the biggest legumes come from Entada gigas (the monkey ladder) and may be up to 6.6 ft long!

tuber: functional storehouse for the stalk of select plants with seeds; they are commonly small and dense, cultivated beneath the dirt, and contain and store an abundant amount of starchy fundamental tissue; often representing the dormant state for many plants which allow many species to live through the winter; as adapted stalks, the majority of tubers have small modified leaves each containing a sprout and having the ability of growing into another plant
Examples: tiger nuts (aka Chufa in Spain)

Now that we've established nuts are really fruits, lets take a look at the different criteria experts have created for defining a nut.  Oh by the way - many examples do not meet all the criteria and often clash with the definition of nut.

The Nut Criteria:
Dry
Tough/woody pericarp
Indehiscent
Single seed
Two or more carpels

What is the only edible nut that meets ALL the above criteria?
Hint: We told you aboveHazelnut (good job if you remembered without looking!)

What does indehiscent mean?
It means that the fruit or pod does not break open to free its seeds when matureSo in a nutshell - botanically speaking nuts are mostly fruits.

References:
Berry: Plant Reproductive Body. (1998, July 20). Retrieved June 20, 2018, from https://www.britannica.com/science/berry-plant-reproductive-body

Contributors: The Editors of Encyclopedia Britannica

Corm: Plant Anatomy. (1998, July 20). Retrieved June 20, 2018, from https://www.britannica.com/science/corm Written By: The Editors of Encyclopedia Britannica Revised: March 19, 2018 by: Melissa Petruzzello

Drupe: Plant Reproductive Body. (1998, July 20). Retrieved June 20, 2018, from https://www.britannica.com/science/drupeContributors: The Editors of Encyclopedia BritannicaFruits. (n.d.). Retrieved June 24, 2018, from http://theseedsite.co.uk/fruits.html

Fruits with Dry Pericarp and are Indehiscent [Digital image]. (n.d.). Retrieved June 27, 2018, from http://slideplayer.com/slide/6996641/

Indehiscent. (n.d.). In English Oxford Living Dictionaries. Oxford University Press. Retrieved June 27, 2018, from https://en.oxforddictionaries.com/definition/indehiscent.

JB Steel & JB Wilson (2011) Is there only one kind of edible nut?, New Zealand Journal of Botany, 50:1, 71-76, DOI: 10.1080/0028825X.2011.615938

Legume: Fruit of Fabaceae Plants. (1998, July 20). Retrieved June 20, 2018, from https://www.britannica.com/science/legume Written By: The Editors of Encyclopedia Britannica Revised: Jan 20, 2017 by: Melissa Petruzzello

Nut. (n.d.). In English Oxford Living Dictionaries. Oxford University Press. Retrieved June 24, 2018, from https://en.oxforddictionaries.com/definition/nut.

“Nut: Plant Reproductive Body.” Encyclopedia Britannica, Encyclopaedia Britannica, Inc, www.britannica.com/science/nut-plant-reproductive-body. Written by: The Editors of Encyclopedia Britannica Revised: May 25, 2017 by: Melissa Petruzzello

Tuber: Part of a Plant. (1998, July 20). Retrieved June 20, 2018, from https://www.britannica.com/science/tuber Written By: The Editors of Encyclopaedia Britannica Revised: March 06, 2018 by: Melissa Petruzzello