What is a nut, and why is the answer so convoluted? For Thanksgiving, Katherine explores pecans and the very best vegetarian turkey substitute ever: pecan pie.

Traditions
Thanksgiving is all about tradition, and wherever there is tradition, there are entrenched ideas about the right way to do things. Strong opinions can breed discord, judgmental grumbling, or silent rants about how people with so little sense cannot possibly be blood kin or their freely chosen companions. So much for the theory of mind we all developed as toddlers. And so it goes with my feelings about pecan pie.

Pecan pie is properly made according to the recipe on the Karo syrup bottle, preferably by my own father. The recipe does not include bourbon. To be clear, I love bourbon. Bourbon is our only indigenous whiskey. It is made of corn and aged in American oak. I love bourbon, and I respect it enough to drink it neat, from a glass, alongside my pie.

We can all agree that pecan pie should not be rolled in molasses, breaded with crushed pork rinds, and deep fried. Some reasonable people, however, do add chocolate. It might taste just fine that way – even delicious – but it disqualifies the resulting pie from the category under discussion. Sneaking it in under another name doesn’t work either. When the good bourbon-loving people of Kentucky add chocolate to a pecan pie and call it Derby pie, not only are they infringing on a trademark, they are using the wrong kind of nut. Derby-Pie ® is made with walnuts. There is therefore still no excuse for adulterating good pecan pie with chocolate.

What is a pecan?
A pecan half is a rich fat-filled embryonic leaf (a cotyledon) from a pecan tree seed. The flat side of a pecan half bears a pale shield-shaped scar where it was joined to the other cotyledon and where a tiny knobby embryonic root sits waiting for the chance to grow out and start drawing up water. Each pecan half is wrinkled like a brain hemisphere, crammed into its shell. In the natural world, when conditions are right for germination, a pecan seed imbibes water and its cotyledons swell enough to crack open the shell. The cotyledons provide an extremely calorie-dense sack lunch for the seedling to draw upon until it develops leaves and starts photosynthesizing food on its own.

Pecans are the size of your thumb pad and full of fat – about 70% fat by weight – which is all thanks to squirrels. Humans may have selected certain varieties for cultivation, but the irresistible qualities of pecans were in place before we started breeding them. Seeds of many different tree species are large because large seeds give rise to large seedlings which can establish themselves in shady habitats or emerge from deep under leaf litter or the soil surface. Large seeds also invite birds and rodents to cache them with the idea that they will make a good meal sometime later.

When a seed-caching rodent such as a squirrel encounters a nut or seed, he can eat it on the spot or carry it and bury it somewhere. Since squirrels have tiny little hands and tiny little mouths, they can really carry only one seed or nut at a time, even if it is not very large. To make the most of each trip, foraging theory predicts that an animal will choose to carry the larger items if he has a choice. If the item would take a long time to eat right away (because it has a hard shell, for example), and if the squirrel is not all that hungry, then the choice is clear. The squirrel should carry it away to eat later when the harvest is over and he has nothing better to do. He will spend his precious autumn time gathering large nuts but eating smaller items that are easy to open.

Even though squirrels eat a lot of tree seeds, they also lose or forget about a lot of them. Oaks, pecans, walnuts, and other large-seeded trees benefit from having their lost and forgotten offspring buried all around the countryside. In these species, natural selection favors larger seeds with tough shells and calorie-rich embryos. Oaks are famous for another adaptation to cache dispersal: masting, or producing large crops of acorns every few years in synchrony with their neighbors. Pecans do something similar, bearing heavy crops every three or four years. Ecologists hypothesize that in mast years, the caching rodents bury far more nuts than they can possibly eat over the winter, so more are left behind to germinate. There are also far more nuts than can be invaded by the insects that attack pecans. Alternating with leaner years prevents rodent and insect populations from growing to meet the high nut supply of mast years. (Vander Wall, 2010, presents a clear and concise review of the ecological literature on caching.)

Pecans – Carya illinoensis – are in the same genus as other hickories, such as shagbark, mockernut, bitternut, and pignut hickories. They are close relatives of walnuts (genus Juglans) within the Juglandaceae. Pecans are native to the lower Mississippi and Ohio river valleys and into east Texas. The pecan is the state tree of Texas, but Georgia is the largest commercial producer of pecans.

Pecan fruits arise from highly reduced flowers that lack both male parts and distinct petals and sepals. The “tepals” (neither petal nor sepal) hug the ovary at the center of the flower, wrapping it tightly and even forming a landing pad for pollen at the top of the ovary. The ovary and its tepals are in turn wrapped in a layer of modified leaves called bracts. As the fruit develops, those bracts become a leathery husk that will split open when the seed is mature and let fall the pecan in its hard ovary shell. Walnuts also have a husk derived from bracts, but it does not split open on its own.

Husks remaining on the branches of a pecan tree. Photo by David Preston

Are pecans nuts?
Botanical tradition holds that a nut is a large hard fruit that contains a single seed and does not split open when mature. That’s about where the agreement ends.

When we classify organisms into species and higher taxonomic groups, we hope to capture their actual evolutionary history. Contemporary systematics aims to divide organisms (roughly) along the lines cut by the natural biological process we call speciation. Whether species are “natural kinds” remains controversial, but that’s another conversation.

Fruit classification schemes have no such goals. The many versions published over the centuries track the ambitions of their authors and not the natural world. Fruit types (and pies) were invented for our convenience and amusement. If they did fall along phylogenetic lines, there would be almost as many fruit types as species, and they would become useless for describing general plant features.

Fruit types should be fun, and yet people definitely have entrenched ideas about the right way to classify the fruit of a given species. We botanists seem to get particularly worked up over the definition of a nut . Some have given up: “In view of the historical confusion over the meanings given to the term nut, especially when botanists continually try find some way to bring them altogether (e.g., Johnson 1931; Judd 1985), it, therefore, seems best to leave the term nut and its varied meanings in the layperson’s realm.” (noted fruit classifier Richard W. Spjut)

In that part of the layperson’s realm called the kitchen, nuts are dry seeds, usually large and fatty (just like pecans). There are some culinary nuts that all botanists would agree would not be classified as botanical nuts: almonds, cashews, and pistachios (seeds of drupes), Brazil nuts (seeds from a capsule), pine nuts (female gametophytes), and peanuts (legume seeds). Chestnuts and hazelnuts are generally classified as true nuts, although some controversy arises from the extra bits of flower or bract that surround the nut as it develops.

Walnuts are usually called drupes. The classic example of a drupe is a peach, whose ovary wall includes a soft fleshy layer covering a stony “pit” layer surrounding the actual seed. Walnuts also have a thick “fleshy” layer, which is better described as leathery and does not develop from the flower itself. The leather husk is made of fused bracts, not ovary tissue. Most people are willing to overlook that detail and call the whole thing a drupe, but some people withhold drupe status and label it a pseudodrupe.

Pecans are just like walnuts except that their husks split at maturity and the inside part falls to the ground. The stripy football shaped part inside is most definitely a nut, yet we can’t forget those husks. Personally, I love the husks, which cling to branches and look like tiny birds against the winter sky. They help me spot pecan trees from a distance. The husks, however, undermine the pecan’s claim to nuthood. Depending on how loosely you view drupes, you might call a pecan a dehiscent drupe. If you have stricter ideas, you’d probably go with drupaceous nut. To keep the peace at the dinner table, you might try “dehiscent drupaceous nut” and save the debate for things that matter, like pie.

Diet pecan pie
Nobody eats pecan pie for its heart-healthy fats and fiber. Even so, it can be made less catastrophically caloric. The Karo people have figured out a way to replace a third of their corn syrup with cellulose gum, but I’m not about to put that in my pie. My recipe for a lighter pecan pie? Leave out the bourbon. That will save you about 10 calories right there.

References and other reading:
Judd, W.S. et al. (2007) Plant Systematics: A Phylogenetic Approach

Spjut, R.W. A Systematic Treatment of Fruit Types

Vander Wall, S. B. (2010) How plants manipulate the scatter-hoarding behaviour of seed-dispersing animals Phil. Trans. R. Soc. B 365:989-997 doi:10.1098/rstb.2009.0205