Black walnuts husked (orange are the recently husked ones that are oxidizing)

The Fruit
When you buy a walnut at the grocery it’s likely an english walnut (Juglans regia) that’s been pre-shucked for ya. And lucky for you. It’s already hard enough to crack a walnut, but it’d be too much work for most people to have to first remove the husk. Optimal foraging theory in action. When fresh, the husk (the outer 2 layers of the pericarp) can be a real hassle to rend free of the nut and it stains your fingers a deep brown color. Ross claims otherwise, but dry husks can still be a pain to remove. Regardless, if you harvest from a forest and not a grocery store, to get at the ‘meat’ on the black walnut (Juglans nigra) you’d have to remove the husk (wear gloves) before cracking the shell (the inner layer of the pericarp).

With the nut cracked, you then begin the time consuming process of extracting the nutmeat. Unlike an English walnut, the black walnut’s shell is extremely convoluted, making it exceedingly difficult to extract the tasty nutmeat. It is definitely worth it though. With a stronger taste than english walnut, almost minty, it’s quite tasty.

Black walnut flowers

Botany of the fruit
While describing individual selection in his behavioral ecology lecture, Stanford professor extraordinaire, Robert Sapolsky invokes the Samuel Butler quote: “A chicken is an egg’s way of making another egg.” And so it is that a plant is just a seed’s way of making another seed. And a seed, once made, needs to get from the plant to the ground, and hopefully a hospitable patch of ground. And it has to avoid nasty predators like flying squirrels, tree squirrels (red and gray), and chipmunks, oh my.

Some fruits have a palatable outer rind to entice animals to consume the fruit and digest the outer layer and excrete the hard coated seeds before digesting them. The seeds are then scratched, exposing them to air and water which are key for initiating the growth of the seed, and deposited in a healthy pile of fertilizer. With walnuts, however, the seed is doing everything it can to prevent itself from being eaten. Rather than relying on animlas to provide the fertilizer, the seeds are very large, with a large amount of energy stored in oil and carb rich cotylendons (the nutmeat). This is about as close as it gets to parental investment in the plant world. That thick rind, hard shell, and gross unpalatable (and noxious) chemical defenses of the fruit are deterrents to squirrels to eating the seeds. Or at least they’re barriers that slow down the squirrels and increase the squirrel’s inefficiency at consuming all of the walnuts seeds.

Walnut in cross section. The green rind is the exocarp, the orange softer layer is the mesocarp, and the hard shall is the endocarp. The light yellow on the interior is the seed.

In the image below you can see the same anatomy on an apple, which is a pome, not a drupe. But the same basic features are all there. The red tough skin is the exocarp, the white sweet flesh that makes the fruit a tasty snack is the mesocarp, and that tough layer surrounding the seeds is the endocarp. All this is made by the tree so its genetically identically to the branch on which it grows. The seeds, as we’ll see below are made from a fertilized ovary so its genome is derived from to genetically different individuals. The result is a genetically unique seed. This is why apple seeds don’t produce fruit like their parents and we have to graft branches onto root stocks to get identical fruits to what we want.

An apple in cross section showing the same basic anatomy as a walnut. Technically this is a pome, not a stone fruit as it lacks the stone (but the tough core is the equivalent of the shell)

– Seed
For walnuts, the edible part is on the inside of that tough shell. The seed - the fertilized embryo – is responsible for developing these parts of the fruit. Since teh seed is a result of sexual reproduction, the fusion of male and female gametes. The seed consists of a seed coat (a very thin papery covering in walnuts, the tough black shell on aple seeds), endosperm (the carb storage that will fuel early growth of the plant; this is the nutmeat), and an embryo (the future leaves and roots and next generation of walnut). It’s a lot more complicated on a genetic level (angiosperms have double fertilization, what?!), but that’s the anatomy of walnuts in a nutshell :)

And as a last note, plants work with the anatomy of their ancestors to cobble together structures that suit their ecology. The distant ancestor of apples and walnuts had pericarps and seeds. Both species have over time morphed these anatomical structures in different ways. The apple uses the pericarp as an edible offering and protects its seeds with a tough seed coat and a heavy dose of cyanide. The walnut modified the pericarp to be an outer inedible (even toxic) layer and inner strong shell to protect a totally helpless seed without any chemical defenses or tough seed coat.

More on the topic

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