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We've been learning about fruits (and the various categories thereof) in class; among them we have the nut and the drupe.

My textbook differentiates between those terms as:

  • Nut: It is a single-seeded indehiscent, dry fruit.

  • Drupe: A fleshy fruit with a stony endocarp.

How here's the problem.

My textbook lists the walnut (fruit) as a nut. But Wikipedia disagrees on this point, calling it a drupe instead.

I'd like a second opinion, since both my book and Wikipedia have had reliability "issues" in the past.

Q- Is a walnut (fruit) a (botanical) nut or a drupe?

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3 Answers 3

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Short answer
Walnuts are classified both as nuts and drupes ('stone fruits').

background
According to University of Wisconsin - Madison, Department of Botany , hickory and walnut can be classified both as drupes and nuts, but are best classified as nuts.

Nuts fall into the class of indehiscent fruits: dry fruits that do not open when mature to shed their seeds. Many of this group are one-seeded fruits, such as the nuts. In addition, nuts have a thickened and hardened wall. Examples are beech, chestnut, oak, hazel, walnut and hickory. Because of extrafloral bracts, or "husk", the latter two fruits are sometimes called "drupes").

As opposed to indehiscent fruits, the fleshy fruits have a wall that becomes soft and fleshy as it matures. Drupes ('stone fruits') fall into this class. Drupes one-seeded, simple fruits developed from a superior ovary in which the innermost portion of the wall (endocarp) becomes hard and stony, the outermost part (exocarp) becomes a relatively thin skin, and the middle portion between the skin and the stone (mesocarp) becomes either fleshy or fibrous. Examples are cherry, coconut (the endocarp is the thick wood shell that surrounds the fleshy and liquid-filled seed), but also walnut and hickory. The latter two are, however, best called "nuts".

Indeed, according to a popular scientific article in the Guardian, many tree nuts are also drupes, including walnuts and pecans.

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    $\begingroup$ Some nut who thinks he's been duped, I suppose. $\endgroup$
    – rotaredom
    Nov 10, 2017 at 23:31
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    $\begingroup$ Then what's the fleshy part of the walnut? $\endgroup$
    – RonJohn
    Nov 11, 2017 at 2:01
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    $\begingroup$ @Rob The question's about the walnut (fruit), not the kernel (which is commonly referred to as the "walnut"). I had issues wrapping my head around that too at first :3 $\endgroup$
    – paracetamol
    Nov 11, 2017 at 8:37
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Walnut (genus Juglans) fruits are nuts, not drupes.

It is indeed a difficult fruit to classify regarding the traditional classification ("dry" vs "fleshy"), but it is easy to see what's happening here: the fleshy structure is not derived from the pericarp and, therefore, it is just an accessory fruit (just like the cashew fruit, in which the dry fruit is the nut, or strawberries, in which the dry fruits are the black "seeds", not the pinky flesh structure).

This becomes clear when we check the classification of the fruits of the Family Juglandaceae, to which the walnut belongs. According to Milliken (2009):

Fruits are nuts enclosed by fleshy or winglike fused bracts/perianth, giving a samaroid or drupaceous appearance. (emphases mine)

And, according to Manning (1940):

Whole fruit drupelike, but the husk derived from the involucre and the calyx, and the skin from the calyx alone, neither one from the pericarp, hence fruit not a true drupe. (emphasis mine)

The same Manning, in his book titled "The morphology and anatomy of the flowers of the Juglandaceae" (1926), explains it better:

The whole fruit is drupaceous, in that it superficially resembles a drupe, but is not a typical drupe (a true drupe is a fruit in which the outer portion of the pericarp is fleshy, the inner portion hard or stony).

Have in mind that in the traditional classification "dry" and "fleshy" are mutually exclusive, meaning that a given fruit can be one or other, but not both. Therefore, your textbook is correct and the Wikipedia link is wrong.

Sources:

  • Milliken, W. (2009). Neotropical Juglandaceae. In: Milliken, W., Klitgard, B. & Baracat, A. (2009 onwards), Neotropikey - Interactive key and information resources for flowering plants of the Neotropics.

  • Manning, W. (1926). The morphology and anatomy of the flowers of the Juglandaceae.

  • Manning, W. (1940). The Morphology of the Flowers of the Juglandaceae. II. The Pistillate Flowers and Fruit. American Journal of Botany, 27(10), p.839.

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  • $\begingroup$ Those were some pretty heavy comments below! The comments were removed, but they were appreciated nonetheless - I was just wondering what was wrong with my answer. Reading the comments and your answer I'm still not convinced, but +1 for the 1940 reference :) And oh no I would never start a downvote war. I just often leave a note and downvote and this time I was inetersted in what was incorrect about my answer. Also, lately on every post I make, a downvote comes in. I'm just wondering if there's someone out there hating my guts :0 Anyways, I'll remove this comment shortly. $\endgroup$
    – AliceD ♦
    Nov 11, 2017 at 22:35
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    $\begingroup$ @AliceD I removed the comments because I was afraid of initiating a downvote war. Regarding your answer, it's incorrect because a fruit cannot be both A and B in that key, it's simply impossible. Walnuts are dry fruits, not fleshy fruits. Unfortunately, that Wisconsin Uni link has some mistakes. For instance, in my deleted comment (that as a mod you can probably see) I talk about the coconut, which is clearly wrongly classified in that link you provided: coconuts are dry fruits. Unfortunately, university pages do have errors! $\endgroup$
    – user24284
    Nov 11, 2017 at 22:41
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    $\begingroup$ Actually, the book is from 1926! I just edited the answer. $\endgroup$
    – user24284
    Nov 11, 2017 at 23:10
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    $\begingroup$ @paracetamol No, there was no downvotes war! Let me explain to you: After receiving a downvote, Alice asked "why the answer was downvoted?", a legitimate question by the way. I left a comment saying that asking why an answer was downvoted is not a good approach at SE sites because, once the downvoter identifies himself/herself, normally a downvotes revenge war begins. But, after having left the comment, I deleted it, fearing that the very comment could start a downvotes war... that's all. $\endgroup$
    – user24284
    Nov 12, 2017 at 4:56
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    $\begingroup$ Just to make myself clear and finishing this subject, my opinion is that, the same way we don't ask "why the upvote?" when someone upvotes us, we shouldn't ask "why the downvote?" when someone downvotes us. Upvoting and downvoting are the core of SE sites, and they are pretty much self explanatory: if someone likes our answer they upvote, if they don't they downvote. The voter can, in both upvote or downvote situation, leave a criticism/comment, but only if they want to. Have a look here: meta.stackoverflow.com/questions/285081/… $\endgroup$
    – user24284
    Nov 12, 2017 at 5:03
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A true nut, botanically speaking, is a hard-shelled pod that contains both the fruit and seed of the plant, where the fruit does not open to release the seed to the world. Some examples of botanical nuts are chestnuts, hazelnuts, and acorns.

A drupe is a type of fruit in which an outer fleshy part surrounds a shell (what we sometimes call a pit) with a seed inside. Some examples of drupes are peaches, plums, and cherries—but walnuts, almonds, and pecans are also drupes. They're just drupes in which we eat the seed inside the pit instead of the fruit!

Source: http://www.seriouseats.com/2010/04/what-are-the-differences-between-nuts-and-drupes.html

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    $\begingroup$ I would opt against using culinary sources as the basis of an answer. $\endgroup$
    – AliceD ♦
    Nov 10, 2017 at 13:50
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    $\begingroup$ @AliceD: I would normally agree. However, in this case the author is known as the "Peanut Butter Guy" and Founder and President of Peanut Butter & Co. I understand that peanuts are a legume, but I do value his opinion on nuts too. $\endgroup$
    – wanderweeer
    Nov 10, 2017 at 13:54
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    $\begingroup$ I've never seen "drupe" figure in a purely culinary site/blog/book (I'm not much of a cook, but still...). That this source even mentions "drupe", and then goes on to discern between a drupe and a nut, adds to its credibility IMHO. $\endgroup$
    – paracetamol
    Nov 10, 2017 at 15:19
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    $\begingroup$ I wouldn't underestimate the knowledge that a culinarian has with foods. Per wikipedia experts in this field need to have knowledge of food science defined as "the discipline in which the engineering, biological, and physical sciences are used to study the nature of foods, the causes of deterioration, the principles underlying food processing, and the improvement of foods for the consuming public" $\endgroup$
    – wanderweeer
    Nov 10, 2017 at 15:39

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