What is about a peanut that makes it not a nut?

Anyone who has watched 3rd Rock From the Sun obviosuly knows that a peanut is not a nut - it’s a legume -HA HA HA HA HA!

My question is - in layman’s terms - what is the difference.

I would appreciate it if the term, “dehisces”, didn’t appear.

Cheers, folks!

Simple layman’s answer: Nuts grow on trees. Legumes grow in pods from plants. Biologically, peanuts are more closely related to peas and green beans than they are to nuts.

I’m sure some botanist will be along and do better but in the meantime.

A legume generally has pods and it seems the the peanut shell is really a pod. They spread their seeds by spliting (dehiscing) the pod along a parting line. the Pods then open up and spread the seeds.

Nuts are described as not splitting their shell (indehiscent) in order to discharge the seed but my source doesn’t say how they actually do the job.

Merriam-Webster Collegiate defines a nut as “A dry indehiscent one-seeded fruit with a woody pericarp.” Does that help?

So nuts have one seed in the shell while legumes usually have more than one seed in a pod.

For the layman, the peanut is a nut. It’s called peaNUT, isn’t it? The same with Cashew Nuts. In both cases, they appear enough like a nut so that the name is not a misnomer.

In scientific terms, a nut is the seed from a shrub or tree, consisting of a shell enclosing a kernal.

Peanuts grow underground on plants, not shrubs. Cashew nuts are an external part of the fruit. So, scientists would not consider them nuts and often lecture laymen for the perfectly legitimate use of the term “nut.”

I read somewhere that in scientific terms a nut is a seed that has rotational symmetry - it looks the same if you rotate it about one axis. So an acorn would be a nut because if you change it’s angle about its vertical axis, it still has the same design. By this definition a walnut or an almond, seeds that grow on trees, would not be nuts.

Don’t have a cite, tho.

The first definition of “nut” on yourDictionary.com is:

“An indehiscent, hard-shelled, one-loculated, one-seeded fruit, such as an acorn or hazelnut.”

“Indehiscent” means not splitting open at maturity. “Loculated” means having, formed of, or divided into small cavities or compartments. So by this definition, a nut is a fruit that has one seed inside a hard shell that doesn’t split open at maturity and that has one compartment or cavity.

Peanuts have more than one seed inside the shell, so they aren’t nuts by this definition. I don’t know whether peanut shells are soft enough to disqualify them as nuts by this usage. I also don’t know whether peanuts split open at maturity, but I doubt it: peanut plants plant their own seeds by forcing their seed-borne stems into the ground. Any splitting would take place underground, which doesn’t seem likely.

This definition is a botanical one. The second definition in yourDictionary.com is:

“A seed borne within a fruit having a hard shell, as in the peanut, almond, or walnut.”

This is the common usage, and it’s perfectly OK to call a peanut a “nut” outside of botany class.

A walnut does have rotational symmetry, but only when rotated through 180 degrees. An almond doesn’t though (the kernel might appear that way, but the shell and the endocarp (the fleshy outer covering, which is not unlike a shrivelled peach) is not symmetrical, as (like the peach) it has a crease down one side.

In any case, the symmetry=nut thing sounds bogus to me - inside the shell, acorns only have rotational symmetry at 180 degrees, as the kernel is naturally split into two halves*. Hazelnuts are often asymmetrically ovoid.

*This is mostly because they are dicotyledons - plants that have two seed-leaves - I’m not sure if there are any nut-producing monocotyledons