Origin stories: spices from the lowest branches of the tree

Why do so many rich tropical spices come from a few basal branches of the plant evolutionary tree?  Katherine looks to their ancestral roots and finds a cake recipe for the mesozoic diet.

I think it was the Basal Angiosperm Cake that established our friendship a decade ago.  Jeanne was the only student in my plant taxonomy class to appreciate the phylogeny-based cake I had made to mark the birthday of my co-teacher and colleague, Will Cornwell.  Although I am genuinely fond of Will, I confess to using his birthday as an excuse to play around with ingredients derived from the lowermost branches of the flowering plant evolutionary tree. The recipe wasn’t even pure, since I abandoned the phylogenetically apt avocado for a crowd-pleasing evolutionary new-comer, chocolate.  It also included flour and sugar, both monocots.  As flawed as it was, the cake episode showed that Jeanne and I share some unusual intellectual character states – synapomorphies of the brain – and it launched our botanical collaborations.

Branches at the base of the angiosperm tree
The basal angiosperms (broadly construed) are the groups that diverged from the rest of the flowering plants (angiosperms) relatively early in their evolution.  They give us the highly aromatic spices that inspired my cake – star anise, black pepper, bay leaf, cinnamon, and nutmeg.  They also include water lilies and some familiar tree species – magnolias, tulip tree (Liriodendron), bay laurels, avocado, pawpaw (Asimina), and sassafras.

It would be a mistake to call these basal groups “primitive,” since they have been evolving ever since their divergence.  When a lineage branches off of the trunk of the main tree, it’s not like it is getting off a train and standing still while everyone else keeps moving through evolutionary space.  “Basal” or “early-diverging” are much more accurate terms.  Still, members of these groups have some features that we think were present in the first flowering plants, and we could call those features primitive – or more accurately, ancestral or plesiomorphic: most are woody, with simple leaves and simple unfused floral parts that are often neither sepal nor petal (so are called tepals).  Think bay leaves and magnolia flowers.  Some basal species make special cells packed with flavorful ethereal oils.  These were not present in the very first flowering plants but apparently evolved fairly early, maybe 150 million years ago. Again, think bay leaves, sassafras, and star anise.black pepper and star anise plants from the National Arboretumavocado seedling and flowering branch of california bay

Bay laurel and cinnamon from the National Arboretum

Selection of basal species showing similar simple leaves

The basal groups are sometimes awkwardly called the “nonmonocot noneudicot angiosperms” because they do not belong to either of the two very large main branches of flowering plants: the monocots (e.g. grasses, orchids, lilies, palms) or the “true” dicots (eudicots, e.g. grapes, beans, roses, broccoli, tomatoes, sunflowers).  We more often use a much more charming label: ANITA and the Magnoliids.

BasalAngiosTreeANITA is an acronym of orders and families within the three lowermost branches of the angiosperm tree.  Taken together, these groups do not form a single discrete branch, or clade; instead, they represent three different clades arising from the main tree and are collectively called a “grade.”  (Now that the families represented by I, T, and A have been grouped within the order Austrobaileyales, we sometimes collapse the acronym to ANA.) A fourth branch, the Magnolia group, is larger and younger and is sometimes treated separately from the more basal groups.  We include it here because it shares important features with the ANITA grade.  And if we didn’t allow the spices from the magnoliids, star anise would be the only truly basal ingredient.

A new Basal Angiosperm Cake
Spices from the basal groups taste good together and I have been experimenting with them ever since that first cake.  My new recipe is almost completely phylogenetically pure and really captures some of the essential flavors of the grade.  I love that the spices come from different plant parts and are beautiful up close.  A cinnamon stick is a dusty roll of inner bark, and bay is a smooth simple leaf.  Black peppercorns are entire fruits, nutmeg is a seed with a convoluted interior, and star anise has flavorful fruit walls.

Cinnamon, bay, star anise, black pepper, and nutmeg

Cinnamon, bay, star anise, black pepper, and nutmeg

To minimize monocots and eudicots in the recipe, I replaced cane sugar with honey.  The honey undoubtedly contains nectar collected from many flowering plant species, but I’m willing to live with this one imperfection.  If you can raise a hive on only California bay, let me know.  Seriously.

In place of wheat flour, another monocot, I folded in an outgroup: pine nuts.  Pines represent the gymnosperms, the closest living relatives to the flowering plants.  In systematics, the ancestral state of a character is judged by looking at an outgroup, a taxonomic relative just outside of the tree of interest.  Just as the outgroup anchors the base of the tree, the matrix of ground pine nuts anchors this cake.  There’s no gluten in it, and the resinous flavor of the pine resonates with the aromatic basal spices.

Before talking about the basal ingredients, I’ll take a short detour to mention Amborella, the most famous species you’ll never see.

The Amborella story
In the years since I made that first cake, evidence has been mounting that the most basal branch on the whole angiosperm tree is represented by a single surviving species, Amborella trichopoda, a small-flowered shrub endemic to the South Pacific islands of New Caledonia.  It exists naturally in only a dozen or so populations on isolated slopes in the wet tropical forest of the main island, Grande-Terre.  We don’t eat it, but it’s worth noting here because it starred in one of the biggest botanical stories of the year.  In late December 2013 its genome sequence (actually both its nuclear and mitochondrial sequences) was published in Science Magazine (summaries here and here).  What the authors report about the sequence and its functional structure tells us a lot about plant evolution in general, and it opens the door for many more extremely important discoveries.  No wonder #amborella dominated my Twitter feed for a few weeks.

Star anise (Illicium verum)
Star anise belongs to the family Illiciaceae – the “I” in the ANITA grade – in the order Austrobaileyales.  Whole star anise is the tough woody fruit of a simple flower.  The flower makes six or more carpels that become the fin-shaped points of the star.  Each contains a single seed, which is usually still in place when we buy them.  In nature, seeds explode out of the fruits, propelled by the drying of unevenly thickened cell walls within the carpels (Romanov et al. 2013).  The seeds are extremely smooth and shiny, which aids their ballistichoric launch into space.

Star anise (Illicium verum) fruits

Star anise (Illicium verum) fruits

The genus Illicium is distributed broadly across the moist tropics and subtropics, and I. verum comes from southeast Asia.  To use the spice, grind it, grate the fruit walls on a microplane, or cook the whole fruits with the dish and remove them before serving.

Black and White pepper (Piper nigrum)
Piper nigrum belongs to the magnoliid clade, in the order Piperales.  It grows as a woody understory vine and probably originated in moist forests at the very southern tip of India.  A black peppercorn is an entire fruit, a single seed surrounded by an aromatic fruit wall full of ethereal oil cells.  White pepper is harvested from the same species, but is treated differently.  Its outer wall is soaked (and sometimes rotted) off before the seeds are dried, which contributes to its complex smell.  A recent paper reports over 40 volatile compounds emanating from white pepper (Liu et al., 2013).  The scent has always reminded me of wet dog, but odor experts call it “fecal” or “barnyard.”

The distinct flavors of both black and white pepper are complemented by a separate physical sensation produced by piperine, an alkaloid that trips heat receptors in our mouths.  This compound has to be present in the seed itself, not just the fruit, because chewing white peppercorns definitely triggers the receptors at least as much as chewing black ones does.  I guess the polite way to describe the combination of white pepper flavor and sensation would be “burning barnyard.”

Bay (Laurus nobilis)
Bay, cinnamon, and avocado all belong in the Laurel family (Lauraceae), along with sassafras and camphor.  Laurus nobilis is the Mediterranean plant we know as bay leaf.  The simple shape and smooth edges of bay leaves illustrate some of the leaf characteristics common within the Magnolia group.  California bay (Umbellularia) is closely related, but has a different and very strong flavor.  It can also be used in the kitchen, but very sparingly.  Just the smell of its leaf litter in the woods can give me a headache, and a recent study showed that its volatile oils repel mosquitoes (Tabanka et al., 2013).

Bay leaves are superficially simple and ubiquitous – and we toss them into stews with hardly a thought – yet their scent is complex, with a hint of genuine antiquity.  Bay trees can be found growing among the ruins of the Forum in Rome.  For centuries, wreaths of Laurus nobilis have crowned victors, graduates, and poets.  Apollo wore a laurel wreath, and when Daphne couldn’t outrun him, she had no choice but to turn herself into a laurel tree.  Even now we seek refuge in the scent of bay leaves wafting from a huge pot of vegetable bean soup in a warm kitchen.  Antique stores filled with old wood and furniture polish actually smell a bit like bay.

Cinnamon (Cinnamomum verum)
“True” cinnamon is derived from the inner bark of a tree native to Sri Lanka, where the vast majority of the species is still grown.  Other species (especially C. cassia) are grown elsewhere, also for export as cinnamon.  (Cassia cinnamon was at the center of the second biggest Basal Angiosperm story this year).  In addition to ethereal oils such as cineole, cinnamon contains the strongly flavored cinnamaldehyde.

Nutmeg (Myristica fragrans)

Nutmeg seed showing brown seed coat folded within the ruminate endosperm

Nutmeg seed showing brown seed coat folded within the ruminate endosperm

Grating a fresh nutmeg seed reveals beautiful dark lines running through it like veins in cream colored marble.  Those veins are places where the seed coat has folded in and been caught by the endosperm (the storage tissue of the seed).  It is common in this part of the phylogenetic tree for the endosperm to be ruminate, meaning that its surface is covered in deep folds, like the villi lining an intestine.  The seed coat falls between those folds.  A striking closeup can be seen here.

A nutmeg seed develops as the only seed within a dense fleshy fruit.  At harvest, the seed is covered by a bright red aril, an outgrowth of the funicle which attaches the seed to the inside of the fruit.  The aril is removed, dried, and ground to become the spice we call mace.  Myristica fragrans is native to Indonesia where it grows in wet lowland forests, and it is also cultivated in southern India.

Ethereal oils and evolution
Star anise and the magnoliid spices owe some of their strong and complex flavors to abundant ethereal oils held in specialized spherical cells in their leaves and some other tissues.  Specifically, these are volatile mono- or sequiterpenoids that generally have a resinous, citrusy, peppery, or camphor scent.  Their flavors and pungency are also influenced – sometimes dominated – by other types of compounds, such as phenylpropenes and alkaloids.  Ethereal oils seem to be broadly effective at repelling insects and retarding fungal and bacterial growth.  All of the spice-bearing species described here, except bay, live in warm wet forest understories, where defense against predators and pathogens would be extremely useful, so it’s reasonable to suppose that ethereal oils play a defensive role.  There’s nothing ethereally dainty about either the flavor or function of ethereal oils.

Evolutionarily, ethereal oil cells disappear somewhere along the branch leading to the eudicots, only to show up again in scattered families: myrtle (allspice, clove, eucalyptus), citrus, mint, carrot (caraway), and sunflower (tarragon) families, for example.  Some of the monocots either retained or regained ethereal oils, notably ginger and cardamom from the ginger family.  Extracted ethereal oils also show up in mouthwash and shampoo.  The botanist in the shower can douse herself with cineole, limonene, menthol and thymol, among other more floral scented terpenoids.

For now, though, we’ll stay in the kitchen and take advantage of the spices we can collect from the lowest flowering branches.

BA cake

The cake that makes the grade
1.5 cups pine nuts, finely ground, but not a nut butter
4 eggs, separated
0.5 cup honey
0.5 t baking soda
1 t salt (only half is used)
1/8 t ground star anise or half of one “fin” (carpel) of a whole fruit
1/4 t freshly ground nutmeg, loosely packed
3 small bay leaves
0.5 t ground white pepper (black pepper is also good)
0.5 t ground cinnamon

Preheat the oven to 350º F.  Line the bottom of a 9” springform pan with parchment and butter its sides.

With a mortar and pestle, grind a teaspoon of salt into the bay leaves to extract their fragrant oils.  If you are using whole star anise, add the half fin and grind lightly.  A little star anise goes a long way.  The salt will be reduced to powder.  Sift the salt through a very fine sieve or microplane grater and measure out 0.5 t to use here.

Beat the yolks lightly to loosen them and add to them the honey, soda, salt, and spices.  Stir in the ground pine nuts.

Beat the whites until they are very foamy but not quite forming soft peaks.  Stir them into the rest of the batter.

Bake the batter in the pan for 40 minutes.  If the center does not seem set, cook for another five minutes.  Allow to cool for 10 minutes and remove the sides of the pan.

Optional avocado topping
Heat a cup of whipping cream to just below simmer.  Turn off the heat and add a whole star anise, a cinnamon stick, a heaping teaspoon of black peppercorns, half a bay leaf, and a good grind of nutmeg.  Steep until the cream cools to room temperature and strain it.

Meanwhile, mash and then beat one medium-small avocado with a fork until it is smooth.  Stir in a generous tablespoon of honey.  Add about half of the cream and beat it with a mixer.  If the topping isn’t whipping up well, add a bit more cream.  Keep beating until it is light and holds its shape.  Chill.  Serve with the cake and a dash of cinnamon.  (The leftover cream can be used to make chocolate truffles.  Add a bit of avocado flesh or leftover topping to the warm truffle mixture for the full phylogenetic package.  It actually tastes really good.)


Amborella Genome Project (2013) The Amborella Genome and the Evolution of Flowering Plants Science 342: 1241089

Gershenzon, J., and N. Dudareva (2007) The function of terpene natural products in the natural world Nat. Chem. Biol. 3:408–414.

Liu et al. (2013) Studies on the chemical and flavor qualities of white pepper (Piper nigrum L.) derived from five new genotypes. Eur. Food Res. Tech. 237: 245-251.

Massoni, J., F. Forest, and H. Sauquet (2014) Increased sampling of both genes and taxa improves resolution of phylogenetic relationships within Magnoliidae, a large and early-diverging clade of angiosperms, Molecular Phylogenetics and Evolution 70:84-93 .

Romanov, M., A. Bobrov, and P. Endress (2013) Structure of the unusual explosive fruits of the early diverging angiosperm Illicium (Schisandraceae s.l., Austrobaileyales) Botanical Journal of the Linnean Society 171: 640–654

Soltis, D. and many others (2011) Angiosperm phylogeny: 17 genes, 640 taxa Am. J. Bot. 98:704-730; doi:10.3732/ajb.1000404

Tabanca et al. (2013) Comparative Investigation of Umbellularia californica and Laurus nobilis Leaf Essential Oils and Identification of Constituents Active against Aedes aegypti. Jour. Agr. Food Chem. 61: 12283-12291.

Let’s get it started with some black-eyed peas (and rice)

You don’t have to be superstitious to believe in the power of hoppin’ john on New Year’s Day.  Katherine’s recipe is below, but first, she takes this good excuse to talk about the structure of beans, the magical fruit (really seeds).

The magic of beans
Beans are extremely satisfying seeds.  They are large and germinate easily.  They can be harvested young and eaten soft – like limas, favas, and green peas – or in their fresh pods, like green beans and sugar snap peas.  They are most beautiful and useful when allowed to mature and dry naturally.  They are creamy white, chestnut, blue-black, or pink; mottled, speckled, cow-spotted, or black-eyed; fat and reniform, or shaped like a lens or a ram’s head.  They can weigh down pie crusts or fill bean bags.  Food co-ops everywhere are built on the cornerstones of bulk bins full of colorful dried beans.  Running your hands through a bowl of cool dried beans is an inexplicably simple joy.

This time of year news outlets like to report that beans are a traditional food for the new year because beans look like coins.  Maybe they do a little bit, if you lack depth perception, but most people would mistake beans for coins about as easily as they would cabbage for a dollar bill.  I think beans are perfect for the new year because a good collection of dried beans makes a person feel both rich and lucky.  Just like optimism, beans are cheap enough that anyone can have them.  They are easy to store and last longer than a human lifetime (at least 200 years, according to research from Kew Gardens).  If they aren’t cooked, they can be planted to make more of themselves.  They may be simple and earthy, but they generate wealth.

Fabaceae black-eyed peas in a pot

Do these look like coins?

Beans are also perfect for eating this time of year.  After weeks of elaborate rich food, something easy and healthy is very appealing.  Beans have been paired with rice in just about every culture that has adopted farming.  It’s a great combination, and one of my favorite traditional versions for the New Year is hoppin’ john.  It may have originated in some Caribbean cultures as a dish made with pigeon peas (pois au pigeon, very roughly pronounced “hoppin’ john”), but most people I know use black-eyed peas (Vigna unguiculata), and that’s what I use in my recipe below.  Black-eyed peas probably evolved in Africa (Delgado-Salinas et al., 2011), and they are close relatives of mung beans, cowpeas, and Chinese long beans.  All of these species have more box-shaped seeds and longer narrower pods than common Phaseolus beans, the New-world group that includes string beans and black, white, and kidney beans, among others.

Beans as seeds
If you remember learning anything about plants in elementary school, you probably remember growing beans, and it was probably boring.  It’s a wonder anyone goes on to study plants anymore.  But should old acquaintance be forgot?  I say absolutely not.  Dried beans are amazing creatures, and you likely have a pile of them alive but sleeping soundly in in a jar in your cabinet right now.  Of all the things living in there, beans may be the only things you really want to know about.

Dried beans are seeds of legume plants, and seeds are far more complicated than they look.  Like two-faced January, they bridge the winter gap between last fall’s harvest and next spring’s planting.  Seeds also kicked off one of the most important periods in plant history.  When the first seeds evolved, plants finally gained a bit more freedom to reproduce without staying wet all the time, which turned out to be a great strategy.  Seed plants have flourished in the 370 million years since then, and now the vast majority of plant species are seed plants (flowering plants and gymnosperms).  Legumes (family Fabaceae) evolved fairly recently, just over 60 million years ago, yet is now one of the largest and most diverse plant families in the world (approx 18,000 species).  Jeanne will turn her fabulous phylogeny machine on the beans in a future post and bring you more details about their origin and distribution.  For now, I’ll focus on bean structure.

Black-eyed pea soaked and split.  The seed coat is on the right, showing the black eye around the hilum.  The two cotyledons are on the left.  The uppermost is still attached to the rest of the embryo.

Black-eyed pea soaked and split. The seed coat is on the right, showing the black eye around the hilum. The two cotyledons are on the left.

Seeds are more than just a plant embryo and its sack lunch, bundled together and tossed optimistically into the wild.  Seeds contain three distinct generations and a jumble of genetic material: the seed coat belongs to the mother plant; the nutritive tissue (endosperm) contains an unequal mixture of maternal and paternal genes, with the balance tipped towards the mother plant; the embryo itself gets half of its genes from each parent; and in gymnosperms the seed contains a generation intermediate between the parents and the embryo, called a gametophyte.  (In flowering plants, unfertilized seeds – ovules – contain gametophyte cells too.)  These various players have distinct interests, and you might even say that there is a war raging inside our peaceful looking little seed.  Many of those genes direct resource allocation and the synthesis of storage materials, which should align the interests of the embryo itself and both of its parents.  The maternal plant, however, has more than just this one selfish seed to provide for and must spread resources accordingly; while the paternal parent takes the side of the embryo, since it may not be the father of any of the other seeds on the plant.  These conflicting genetic interests seem to have driven some important aspects of seed evolution (see work by Friedman for discussion).

What’s in a bean?
Seeds pack a lot of resources into a small space to fuel the transition of the embryo to an independent seedling.  Orchid seeds are one exception.  They are minuscule – think of the specks in vanilla bean ice cream – and when they germinate, they recruit fungi to bring the resources to them.  Most seeds, though, contain a lot of starch, protein, and some fats (e.g. “nuts”).  Many also contain defense compounds, such as lectins, tannins, and cyanide precursors.  Bean seeds also contain a lot of oligosaccharides, sugars like raffinose and stachyose, that make some people fart.  That’s because mammals lack the enzyme α-galactosidase (α-GAL) needed to digest the oligosaccharides ourselves.  We pass them along to the bacteria in our large intestines, which gratefully accept the sugars and break them into CO2, methane, and other gases.  We then pass these along to the outside world.

If you buy your beans at a burrito stand, you have to take what you get.  If you cook your own beans, though, you can control the oligosaccharides and the gas.  The sugars are water soluble and come out in the soaking water and the cooking water.   The slime around canned beans contains a lot of oligosaccharides, so it should obviously be washed off.  Any flavor you lose is probably mostly salt anyhow.  I always defy any recipe that directs me to use the bean cooking water, and in the recipe below, I suggest changing the soaking water as well.  (On this point, see also Sally and Martin Stone’s terrific book The Brilliant Bean.)

Protein is not cheap for plants to make because it contains a lot of nitrogen, which is often limiting for plants since they cannot get it from the air.  Carbohydrates, by contrast, are relatively cheap because carbon dioxide is abundantly available to be fixed through photosynthesis.  The reason beans and other legumes can afford to be particularly high in protein is because their roots house symbiotic bacteria that turn atmospheric nitrogen into something a plant can use.  Beans are generally about 22-25% protein by weight, whereas even whole grains contain only one-third to half that much.

Seeds becoming plants
Should old acquaintance with elementary-school botany have been forgot, the diagram below may bring it back to mind.  It maps the structure of a bean seed onto the seedling it will become.  Fabaceae BEP germination

The relationship between a bean seed and the plant it will become

The bulk of a bean seed is a pair of cotyledons, or “embryonic leaves.”  These leaves serve the dual purpose of storing the starch and sugars, proteins, and fats needed by the young plant before it becomes independent; and turning green and making some new sugars once the seedling emerges from the soil.  The cotyledons enclose the main axis of the embryo, with the nubbin of a root (radicle) at one end and a little leafy shoot at the other.  It is not easy to see, but the cotyledons are actually attached to the axis between the root and the shoot.  Soaking a bean in water will soften it enough to be split, revealing the main axis of the embryo inside.

I should take a moment to note that not all flowering plant species rely on their cotyledons for storage.  The seeds of most flowering plants make a specialized nutritive tissue called endosperm, which results from double fertilization and contains genes from both parents, but mostly the maternal plant (see above).  In some plant species, the endosperm is present in the mature seed.  The white part of popcorn is endosperm, as is coconut water and coconut meat.  In beans and some other plant groups, the contents of the endosperm are taken up by the cotyledons and stored there.

Soaking bean seeds is the first step to sprouting them for eating or planting.  When a bean seed takes up water, it breaks its dormancy and starts a complicated physiological process.  Starch grains swell, cells enlarge, and the seed coat is broken.  Enzymes begin to digest complex molecules to be used by the growing seedling. Seeds are full of mRNA molecules (instructions from the DNA for making proteins) that may allow very rapid synthesis of proteins needed early in germination.

The root is the first part of the embryo to emerge through the seed coat.  It starts taking up water and growing down in response to gravity.  The tender shoot with its tiny leaves does not have to push its way through the soil.  In beans, the less vulnerable region between the root and the cotyledon takes on this job, and the shoot tip follows safely behind.  Once free of the soil, the stem straightens out and the true leaves expand and start photosynthesizing.  The cotyledons don’t last long after the seedling has withdrawn the stored resources, and they soon shrivel and fall off.  (Roger Hangarter at Indiana University has a terrific series of germination videos.)

The eye of the pea
As far as I know, the black eye of the black-eyed pea does nothing for the seed.  It simply surrounds the hilum, which is the plant version of a belly button, marking the point where the seed was attached by a funicle to the inside of the pod, or ovary wall.  For me, the purpose of the eye is to be striking against a melange of white rice and red peppers.  It also reminds me to be grateful for my harvest while looking forward to a new season of growth.

Now, let’s get it started in here.

A pan of hot hoppin' john for New Years Eve

A pan of hot hoppin’ john for New Years Eve

Hoppin’ John

The ratio of ingredients may vary, but I like to use equal parts dried beans and rice.

  1. Soak two cups of dried black-eyed peas in water to cover by several inches.  After a couple of hours, drain and replenish the soaking water.  After a couple more hours, drain the soaking water, rinse the beans, and cook them with several bay leaves for about half an hour.  The beans should be cooked through, but not too soft.  Drain the cooked beans and rinse with cool water to stop the cooking.  When they are thoroughly drained, return the beans to the empty pan and cover them with about 2 T of good olive oil plus salt and fresh black pepper to taste.  You can multitask as you do this first step.
  2. Sauté a large onion and two or three red bell peppers, chopped, in olive oil with some crushed red pepper and a generous amount of thyme.  Fresh thyme is best, but about a tablespoon of dried will work.
  3. Add two cups of nice white rice to the pan and stir to coat with oil.  Add about 2t of salt and more black pepper.  A dash of liquid smoke is nice.  Cover with four cups of water and cook until done, about 20 mins.
  4. Gently stir in the black-eyed peas.  Adjust seasonings if necessary.  Serve with tabasco sauce.  Hoppin’ john goes really well with kale (or more traditional collard greens) and some hard cheese.

Leftover hoppin’ john is good reheated, room temperature, or thinned into soup.  You can make fritters by mixing it with enough egg to hold the mixture together, breading small patties, and frying them in a skillet.

References and additional reading

Delgado-Salinas et al. (2011) Vigna (Leguminosae) sensu lato: The names and identities of the American segregate genera.  American Journal of Botany 98:1694-1715 doi: 10.3732/ajb.1100069 http://www.amjbot.org/content/98/10/1694.full 

Friedman , W. E. , E. N. Madrid , and J. H. Williams . 2008 . Origin of the fittest and survival of the fittest: Relating female gametophyte development to endosperm genetics. International Journal of Plant Sciences 169 : 79 – 92 .http://www.oeb.harvard.edu/faculty/friedman/Friedman_Harvard/Friedman_Publications.html

Stone, S. and M. Stone (1988) The Brilliant Bean: Sophisticated Recipes for the World’s Healthiest Food. http://www.amazon.com/The-Brilliant-Bean-Sophisticated-Healthiest/dp/0553344838

Hollies, Yerba maté, and the botany of caffeine

Yerba maté, the popular herbal tea from South America, is a species of holly. It’s also caffeinated, a characteristic shared by only a small number of other plants.


English holly. Photo by K. Bills

Along with conifer trees and mistletoe, hollies are a botanical hallmark of the winter holiday season in Europe and the United States. Most hollies are dense evergreen shrubs or small trees and produce beautiful red fruits that stay on the plant through the cold winter months. Sprays of the dark green foliage grace festive decorations, and wild and cultivated hollies punctuate spare winter landscapes. Especially popular in winter, too, are warm beverages. One of the most popular, at least in South America but increasingly elsewhere, is yerba maté. It is a seasonally appropriate choice because the maté plant is a holly. Unlike the decorative hollies, usually American (Ilex opaca) or English (Ilex aquifolium) holly, maté (Ilex paraguariensis) is caffeinated. This puts it in rare company, not only among hollies, but among all plants.

Dried, crushed yerba maté

Dried, crushed yerba maté

Of the 400-600 holly species in the genus Ilex (family Aquifoliaceae; order Aquifoliales), four are known to contain caffeine, humanity’s favorite stimulant: yerba maté, guayusa (I. guayusa), yaupon (I. vomitoria), and té o’ maté (I. tarapotina) (Crown et al. 2012). The genus is cosmopolitan, but the caffeinated species belong exclusively to the Americas. Yerba maté, guayusa, and té o’ maté are native to the Central and South American highlands. Yaupon ranges from the southern United States down into Mexico, making it the only caffeinated plant native to the United States. All of these caffeinated hollies have a long history of human use as beverages (infusions or “teas”). Traditionally maté is consumed in a mug made from a hard gourd shell. Boiling water is poured over a small amount of the shredded dried (and often lightly smoked) leaves in the bottom of the gourd. The infusion is drunk through a special straw, called a bombilla, with a filter on the bottom of it. Many non-caffeinated holly species throughout North American, Europe and Asia historically have also been—and in some places still are—employed in pleasant or medicinal beverages. Green Deane has great tips for preparing your own infusions from edible hollies.


English holly. Photo by K. Bills

The four caffeine-containing hollies also boast appreciable levels of theobromine and/or theophylline, two other xanthines (purine alkaloids) that are chemically similar to caffeine and also affect mammalian physiology. Although only a small fraction of the hundreds of holly species have been tested for xanthines, most of the species that have been tested completely lack xanthines. Xanthine content is shown on the Ilex phylogeny below, containing species for which xanthines have been measured and for which there was also phylogenetic placement within the sprawling genus. Guayusa and té o’ maté have not been phylogenetically placed within Ilex, but the other two caffeinated hollies, yaupon and yerba maté, cluster together on the phylogeny as sister species. Theobromine especially, and theophylline less so, are widely distributed throughout the genus.

Phylogenetic relationships among Ilex species for which both xanthine and phylogeny data exist. Red branches indicate caffeine; blue branches indicate theobromine; green branches indicate theophylline. Ilex phylogeny data from Manen et al. (2010). Ilex chemistry from Alikaridis (1987), Ashihara and Crozier (1999, 2001), Filip et al. (1998), Reginatto et al. (1999), Hao et al. (2013).

Phylogenetic relationships among Ilex species for which both xanthine and phylogeny data exist. Red branches indicate caffeine; blue branches indicate theobromine; green branches indicate theophylline. Click to enlarge. Ilex phylogeny data from Manen et al. (2010). Ilex chemistry from Alikaridis (1987), Ashihara and Crozier (1999, 2001), Filip et al. (1998), Reginatto et al. (1999), Hao et al. (2013), and Crown et al. (2012).

When caffeine is detected in any plant, theobromine and/or theophylline (and some other minor xanthines) are often also present. Coffee (Coffea sp.; Rubiaceae; Gentianales) has theobromine as well as caffeine, and tea (Camellia sinensis; Theaceae; Ericales) and kola nut (original flavor and caffeine source for cola sodas; Cola nitida; Malvaceae; Malvales) have all three xanthines, as does guarana (Paullinia cupana; Sapindaceae; Sapindales), a caffeine-containing plant that is popular in the tropics and gaining a following elsewhere. Theobromine is famous as chocolate’s (Theobroma cacao; Malvaceae; Malvales) primary drug, but chocolate also contains caffeine and theophylline. Theobromine and are more prevalent than caffeine. Theobromine and theophylline have been detected in a few more Ilex species beyond the caffeine-containing four (see Alikaridis 1987, Filip et al. 1998). This pattern holds for Coffea (Silvarolla et al. 2004) and Camellia (Ishida et al. 2009) species as well, which may be because theobromine is an intermediate step in the caffeine production metabolic pathway (Ashihara and Suzuki 2004, Ashihara and Crozier 1999, 2001). If the last enzyme in the process is somehow disrupted, either by natural genetic mutation or genetic engineering, theobromine can accumulate.

In total, Ashihara and Crozier (1999, 2001) report that around 100 plant species have confirmed caffeine content. Annoyingly, though, I couldn’t find a comprehensive list of caffeinated plants, even by following the trail of literature cited by otherwise highly competent people reporting these 100 caffeinated species from “13 orders” (e.g. Ashihara and Suzuki 2004). Turns out most of those “13 orders” (Ashihara and Crozier 1999) are obsolete names from a 1950s-era taxonomic scheme, so without an actual list of species, it will be impossible to know the modern phylogenetic pattern of caffeine. This seems to be a gross oversight in the known biology of such an economically important compound. Ashihara and Crozier (1999, 2001) and Ashihara and Suzuki (2004) do helpfully note that all caffeinated plants are eudicots except Scilla maritima, a monocot. They also mention that caffeine has recently been discovered in the flowers and pollen (but not other structures) of several citrus-family (Rutaceae; Sapindales) species. Caffeine is also manufactured by at least one fungus, Claviceps sorhicola, an ergot-type pathogen of sorghum (Ashihara and Crozier 2001). Also annoying is rampant disagreement among published studies about whether or not some Ilex species have caffeine or other xanthines. This disagreement is only over very low levels of the stuff, though, so I’ve erred on the side of “no caffeine present” for controversial cases here. So, wherever it may be, the list of caffeine-containing species is clearly a work in progress.

The popular caffeine-containing plants, though, come from five different orders on the plant phylogeny (see phylogeny below; go to our Food Plant Tree of Life for a brief phylogenetics refresher). The picture of the phylogenetic distribution of caffeine in our food plant orders, as far as I can determine it, suggests caffeine production arose by convergent evolution, in which the same character evolves multiple times independently in different lineages (like lemon flavor). Three of the five orders containing most caffeinated plants, however, are from the large asterid clade, and Sapindales and Malvales are fairly closely related orders within the malvids. This suggests that the genetic environment in which caffeine synthesis could evolve is fairly old and somewhat conserved through evolutionary history. While purine nucleotides provide the raw material for caffeine production in all these plants, different plant lineages use unique metabolic pathways to synthesize the stuff (Ashihara and Crozier 1999, 2001, Ahihara and Suzuki 2004).

Phylogeny of orders with edibles. Caffeinated branches in red, with the caffeinated genera labeled.

Phylogeny of orders with edibles. Caffeinated branches in red, with the caffeinated genera labeled. Click to enlarge.

Xanthines are only one class of the hugely diverse group of alkaloids widely present in plants all over the phylogeny. Most plant species don’t produce alkaloids, but alkaloids are widespread and often have spectacular pharmacological properties (see our nightshade post for a few and Amy Stewart’s Wicked Plants for a rundown of the most notorious). Like so many other phytochemicals that humans find flavorful, fun, medically beneficial or poisonous, plants use alkaloids to defend themselves against herbivores and pathogens. Xanthines may also be allellopathic, as they are retained in large quantities in seed coats and abscised leaves that fall to the ground, whereupon the xanthines are released to the soil and may inhibit seed germination of other species (Ashihara and Crozier 1999). Alkaloids are rich in nitrogen, which is an element often in short supply to plants and limits plant growth. Nitrogen fertilization of yaupon has been shown to dramatically increase its caffeine content (Palumbo et al. 2007). Soil fertility likely contributes to the dramatic differences in xanthine content of yerba maté plants grown in different plantations (Marx et al. 2003).

English holly. Photo by K. Bills

English holly. Photo by K. Bills

Humans the world over must have cautiously respected the physiological effects and addictiveness of caffeine from their earliest experiences with it, as the long history of human consumption of all caffeinated plants always begins with judiciously incorporating the plants into religious ritual or reserving their use for royalty (Fredholm 2011, Jamieson 2001). Later caffeinated plants were used primarily for medicine, or regarded simply as delicious (and variously otherwise beneficial or harmful). Males from tribes indigenous to the southern United States historically used a very strong brew of yaupon to induce vomiting to supposedly induce purity and clarity in advance of religious rites or important meetings, which is why the plant’s Latin name is I. vomitoria (Crown et al. 2012). Modern fans of the plant find a dilute infusion of yaupon quite pleasant and rightly tout it as a local caffeinated beverage (Palumbo et al. 2009), but that history and Latin name inspire me to tip my hat to a company marketing the stuff. Given that warm teas from various species, coffees, and chocolate are important parts of modern holiday celebrations, it feels appropriate during this time to remember the role of the caffeinated plants in rituals throughout history. Whether you mark the season with reflection, spending time with people you love, religious devotion, or slinging your pumpkin spice latte around town in a mad shopping rush, we wish you a warm and happy end of the year.


Alikaridis, F. 1987. Natural constituents of Ilex species. Journal of Ethnopharmacology 20: 121-144.

Ashihara, H., and A. Crozier. 1999. Biosynthesis and metabolism of caffeine and related purine alkaloids in plants. Advances in Botanical Research 30: 118-205.

Ashihara, H., and A. Crozier. 2001. Caffeine: a well known but little mentioned compound in plant science. Trends in Plant Science 6: 407-413.

Ashihara, H., and T. Suzuki. 2004. Distribution and biosynthesis of caffeine in plants. Frontiers in Bioscience 9: 1864-1876.

Crown, P. L., T. E. Emerson, J. Gu, W. J. Hurst, T. R. Pauketat, and T. Ward. 2012. Ritual Black Drink consumption at Cahokia. PNAS 109: 13944-13949.

Filip, R., P. Lopez, J. Coussio, and G. Ferraro. 1998. Mate substitutes or adulterants: study of xanthine content. Phyototherapy Research 12: 129-131.

Fredholm, B. B. 2011. Notes on the History of Caffeine Use. Methylxanthines. Edited by B. B. Fredholm. Springer. Pages. 1-10.

Hao, D., X. Gu, P. Xiao, Z. Liang, L. Xu, Y. Peng. 2013. Research progress in the phytochemistry and biology of Ilex pharmaceutical resources. Acta Pharmaeutica Sinica B 3: 8-19.

Ishida, M., N. Kitao, K. Mizuno, N. Tanikawa, M. Kato. 2009. Occurrence of tehobromine synthase genes in purine alkaloid-free species of Camellia plants. Planta 229: 559-568.

Jamieson, R. W. 2001. The essence of commodification: caffeine dependencies in the early modern world. Journal of Social History. Winter: 269-294.

Manen, J.-F., G. Barriera, P.-A. Loizeau, and Y. Naciri. 2010. The history of extant Ilex species (Aquifoliaceae): Evidence of hybridization within a Miocene radiation. Molecular Phylogenetics and Evolution 57: 961-977.

Marx, F., M. J. J. Janssens, P. Urfer, and R. Scherer. 2003. Caffeine and theobromine composition of mate (Ilex paraguariensis) leaves in five plantations of Misiones, Argentina. Plant Foods for Human Nutrition 58: 1-8.

Palumbo, M. J., F. E. Putz, and S. T. Talcott. 2007. Nitrogen fertilizer and gender effects on the secondary metabolism of yaupon, a caffeine-containing North American holly. Oecologia 15: 1-9.

Palumbo, M. J., S. T. Talcott, and F. E. Putz. 2009. Ilex vomitoria Ait. (yaupon): a native North American source of a caffeinated and antioxidant-rich tea. Economic Botany 63: 130-137.

Reginatto, F. H., M. L. Athayde, G. Gosmann, and E. P. Schenkel. 1999. Methylxanthines accumulation in Ilex species—caffeine and theobromine in Erva-Mate (Ilex paraguariensis) and other Ilex species. Journal of the Brazillian Chemical Society 10: 443-446.

Silvarolla, M. B., P. Mazzafera, and L. C. Fazuoli. 2004. A naturally decaffeinated Arabica coffe. Nature 429: 826.

Cranberries, blueberries, and huckleberries, oh my! And lingonberries, billberries…

Flavorful and juicy thought it may be, Thanksgiving turkey, for me, is merely the vehicle for the real star of the meal: cranberry sauce. And cranberry is in the same genus as blueberries, lingonberries, huckleberries, and billberries. And they all make their own pectin. Let us give thanks this holiday season for Vaccinium.

Cranberry sauce is my favorite staple item at our big holiday dinners. Long-prized by indigenous North Americans, cranberries would have been in the diet of those Native Americans participating in the first Thanksgiving if not part of the meal itself. When the fresh cranberries hit the stores in late fall, we stock up. Cranberries, however, are not the only member of their genus that is perennially in our freezers or in our annual diet: blueberries, many huckleberries, lingonberries, and billberries are all in the large genus Vaccinium (family Ericaceae, order Ericales).

Two cranberry sauces at Thanksgiving last week: a raw sauce (cranberries, an entire orange, sugar and salt thrown into the food processor); and a cooked sauce (cranberries simmered with sugar until gooey).

Two cranberry sauces at Thanksgiving last week: a raw sauce (cranberries, an entire orange, sugar and salt thrown into the food processor); and a cooked sauce (cranberries simmered with sugar until gooey).

Eastern North American natives V. macrocarpon and V. oxycoccos are sometimes called the “true” cranberries, and are what are marketed as cranberries in North American stores. Cranberries grow as low mat-forming evergreen shrubs in acidic bogs. Slightly uphill from those bogs on acidic soils, multiple species of blueberries and and huckleberries often co-occur.  Many blueberry, billberry, lingonberry and huckleberry species are variously commercially and recreationally domesticated, cultivated, and wild harvested throughout the cooler regions of the Americas and Eurasia. I’d say Vaccinium would be one fruit genus I would take to a deserted island with me, but there’s a decent chance it’s already there. Some of the 150-450 species (yup, the range is that large) in the poorly understood genus are endemic to (found only on) various islands, including Hawaii, evolutionary descendants of plants grown from seeds deposited from migrating or wayward birds.

The phylogeny of the complex genus is far from well resolved but undoubtedly includes multiple subgenus clades and is, as it currently stands, definitely paraphyletic (Kron et al. 2002). That is, the things currently called Vaccinium did not all derive from a most recent common ancestor and are from different clades. There appear to have been a lot of whole genome duplication events in the evolution of this group, which makes teasing out the history using genetic sequence data kind of hard. Kathleen Kron will likely be the one to finally figure it out, and her lab website is a fun place for Vaccinium nerds.



The phylogeny will be useful because there is huge morphological and ecological variation within the large group. Some species are evergreen, like cranberries, while others are deciduous, like most blueberries and huckleberries. Some tropical species are epiphytic vines. Some species are very mat-like and love bogs, like cranberries, and some are large sprawling shrubs. All their fruits are known for their brilliant anthocyanin flavonoid pigments—bright cranberry or lingonberry red, or deep blues and purples in the blueberries and huckleberries. Some geographic areas, including the Pacific Northwest in the United States and the mountains of southeast Asia, have a huge proliferation of charismatic Vaccinium species. The flowers of some species, like blueberries, are shaped like little frilly bells, while others in the genus have more open flower morphologies. Cranberries are in this latter open-flower category, and early European colonists to eastern North America thought that the flowers looked like the heads of cranes, so they named the fruit “craneberry,” of which cranberry is derivative. The complete phylogeny will help us understand the selection pressures and geographic factors that shaped this diverse group.

As Vaccinium’s chief pollinators, bees undoubtedly act as a selection pressure affecting flower morphology and fruitset in the group. Many Vaccinium can self pollinate, but bee pollination does increase fruit set, so wild bees are encouraged around Vaccinium fields, and commercial honeybees are sometimes brought in during flowering. Bumblebees and other wild bees, some even called “blueberry bees” (e.g. Habropoda laboriosa and Osmia atriventris), however, are more effective pollinators of Vaccinium than honeybees because Vaccinium benefits especially from what is called buzz pollination, a pastime at which bumblebees and blueberry bees excel. To buzz pollinate the bumblebee hugs the pollen-bearing anthers, which are sometimes fused into a tube with the pollen in the middle, and vibrates intensely by rapidly moving its flight muscles. This releases a huge cloud of pollen that lands on the bumblebee’s body.  I don’t know if more pollen from previous flowers gets on the stigmas during the buzzing phase or when the bee gathers nectar. Buzz pollination is useful in the nightshades, too, which is why bumblebees are commercially available for hothouse tomato pollination.

The cell walls of many Vaccinium species, cranberries notably, are rich in pectin, a name for a group of polysaccharides that provide primary cell wall structure and help bind cells together. When heated with an amount of acid and sugar specific to a particular pectin, pectin turns the liquid in which it is dissolved into a gel. This is why pectin is added to cooked fruit macerated with sugar to make jam or jelly. Some fruits, cranberry among them, make enough pectin on their own to gel sufficiently when cooked with sugar—hence the jiggly stuff at the Thanksgiving table. Membrillo, the classic Spanish quince paste, and the Latin American guava equivalent Dulce de guayaba, gel themselves, too. Naturally-occurring pectins gel citrus marmalades and are responsible for the pleasant chewiness of candied citrus peel. Cranberries and citrus make enough acid to set the gel. Lemon juice or some other acid must be added in order to get the natural or added pectins on low-acid fruit to gel. One of the acids in cranberries that promotes gel production is hippuric acid, which acidifies urine after cranberry consumption and may be one of the factors in cranberry that dislodges E. coli from the urinary tract wall, contributing to the scientifically demonstrated ability of cranberries to clear urinary tract infections.

I went through a period when I made a lot of apple paste, in which apple puree mixed with sugar and lemon juice is dramatically reduced to the point at which it will gel itself in its own abundant pectin. Despite cooking it down enough, sometimes it wouldn’t gel. I’d end up with really thick, amazing apple butter. Fruit over-ripeness may have been a contributing factor. As fruit ripens enzymes called pectinases break down pectins to soften the fruit, so I might not have had enough pectin in there in old apples. In a few batches I added cranberries to the apple pulp, and this solved the problem and was tasty (and pink!).


Kron, KE, EA Powell, and JL Luteyn. 2002. Phylogenetic relaitonships within the blueberry tribe (accinieae, Ericaceae) based on sequence data from MatK and nuclear ribosomal ITS regions, with comments on the placement of Satyria. American Journal of Botany. 89:327-336.

The irrational nature of pie

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.

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.Juglandaceae pecan halves close

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

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

Nasturtiums and the birds and the bees

Hummingbirds and ancient bees are responsible for the color and shape of nasturtium blossoms and have a unique view of them, explains Jeanne over salad. 

Nasturtium flowers cut into tomato salad with parsley

Nasturtium flowers cut into tomato salad with parsley

Fall frost hasn’t yet claimed our nasturtiums (Tropaeolum majus; Tropaeolaceae family). The large, colorful blooms amidst the round leaves are still spilling over planting boxes.  All parts of the plant are edible and boast spicy mustard oil glucosinolates, betraying the plant’s membership in the order Brassicales, along with the cruciferous vegetables and mustard in the Brassicaceae family, capers (Capparaceae), and papaya (Caricaceae; try the seeds, as suggested here). I’ve heard that the immature flower buds and immature seed pods can be pickled like capers, but I haven’t tried it yet. Mostly I use the flowers, throwing a few in a salad or chopping them coarsely with other herbs and stirring them into strained yogurt or butter to put on top of roasted vegetables or lentils. In addition to the mustardy kick, the sweet flower nectar adds to these dishes.

Nasturtium spur

Nasturtium spur

The nectar is located in the conspicuous long spur descending from the receptacle, the base of the flower that supports the sepals, petals, and reproductive organs (Decraene and Smets 2001).  It turns out nasturtium nectar is exceptionally sweet: its sugar is especially concentrated and consists mostly of sucrose, instead of the fructose and glucose in nectars of most species.

Yellow nasturtium with red nectar guide patterns

Yellow nasturtium with red nectar guide patterns. Photo by T. Wesiger

For the sweet nectar, handsome spur, bright yellow-to-red colors, and large flowers, we can thank hummingbirds, the main pollinator of nasturtiums.  Hummingbirds typically visit a much narrower range of flowers than do bees or butterflies, so catering to hummingbirds means that the pollen of a relatively small number of species will fall from a hummingbird’s body onto a nasturtium stigma, increasing the chances that pollen from another nasturtium plant will land there. Even among hummingbird-pollinated flowers, nasturtium nectar is especially sweet (Hainsworth and Wolf 1976). The nectar hidden in the long spur is only accessible to animals with tongues or proboscises long enough to reach it, another mechanism to favor hummingbirds and winnow down the pollinator field. Hummingbirds perceive red better than bees do and respond to bright colors and large floral displays.

Radially symmetrical purple coneflower (Asteraceae)

Radially symmetrical purple coneflower (Asteraceae)

For the beautiful striped pattern at the center of the flower and bilateral symmetry of nasturtium flowers, however, we can likely thank ancient bees. Sunflowers (Asteraceae) are radially symmetrical, or actinomorphic. Any line that passes through the center of the flower splits it into two equal mirror-images of one another. Like snapdragons (Scrophulariaceae), basil (Lamiaceae), and pea (Fabaceae) flowers, however, nasturtium flowers are bilaterally symmetrical, or zygomorphic. Only the vertical line passing through the center from top to bottom splits the flower into equal halves.

Orange nasturtium. Dark nectar guide stripes visible on upper petals. Photo by T. Wesiger.

Orange nasturtium. Dark nectar guide stripes visible on upper petals. Photo by T. Wesiger.

Both zygomorphy and the petal stripes, called nectar guides, are adaptations to assist pollinators. Zygomorphy evolved numerous times in disparate plant lineages, from radially-symmetric ancestors, in response to behavior of pollinators, especially bees (Citerne et al. 2010, Neal et al. 1998). Bird pollination likely evolved from bee pollination, so many species predominantly pollinated by birds are zygomorphic.

Zygomorphy may improve plant reproductive success by improving the efficiency of pollination by bees (and maybe other insects that pollinate during the day) and birds. The vertical orientation of the flowers matches the plane of approach of flying visitors, and for bees especially, the lower lip of zygomorphic flowers may serve as a convenient landing pad. There is usually only one narrowly-specified location for a pollinator to sit in order to access nectar from zygomorphic flowers, which allows the flower to more precisely match the contact points of pollen-bearing anthers and pollen-collecting stigmas to the pollinator’s body as it collects nectar (and/or pollen). Zygomorphic flowers also may present a more complex visual search image for pollinators, which may encourage pollinator fidelity to a particular plant species, which increases the chance that pollen from different individuals of the same species will make its way around a population (Neal et al. 1998). The upper and lower lobes of some zygomorphic flowers (like in a snapdragon—Antirrhinum) may be pressed together such that only very strong, large bees can weigh down the lower half of the flower, revealing the nectar. This limits the range of possible pollinators capable of visiting those flowers and is another mechanism to increase the chance that any particular pollinator visitor will carry pollen from the same species.

Yellow coneflower (Asteraceae) as seen by us in visible light, under UV light, and a composite "bee view" image. Image copyright Dr. Klaus Schmitt, Weinheim, Germany.

Yellow coneflower (Asteraceae) as seen by us in visible light, under UV light, and a composite “bee view” image. Image copyright Dr. Klaus Schmitt, Weinheim, Germany.

Zygomorphic flowers typically have conspicuous nectar guides visible to us (humans) as well as animal pollinators: lines and/or dots or color differentiation leading in towards the flower center, where the nectar is. Nectar guides may help animal pollinators, especially bees (Anderson 1977), identify appropriate flowers and quickly position themselves to collect nectar and perform the pollen transfer.  Many flowers also have nectar guides that are only visible under ultraviolet light, making them visible to insect and bird pollinators, but not to us (Osorio and Vorobyev 2008). Captured by special ultraviolet photography, these patterns can be quite striking and different from those visible to us. Bjørn Rørslett and Klaus Schmitt have utterly engrossing websites devoted to these images.  Interestingly, ultraviolet-only nectar guides are far more prevalent in radially symmetrical flowers than in bilaterally symmetrical flowers, and as far as I can tell, the reason for this is not well understood. Curious if the nasturtiums in the garden looked even more spectacular to hummingbirds and bees than they do to me, I asked Klaus Schmitt if he had any bee-vision ultraviolet images of nasturtiums, and he went out and took some!

Nasturtium under visible and UV light. Image

Nasturtium under visible and UV light. Image copyright Dr. Klaus Schmitt, Weinheim, Germany.

True to form, while still beautiful and extra-velvety looking, only very weak ultraviolet patterns are apparent.

Snail vine (Fabaceae) flowers

Snail vine (Fabaceae) flowers

Most of my favorite edible flowers are bilaterally symmetrical: violets, nasturtiums, snail vine (Fabaceae), various mints (sage, basil, thyme, lavender). Next time you see or harvest them in your garden, look for the nectar guides, symmetry, and other adaptations for animal pollination. And toast your pollinators!

Many special thanks to Klaus Schmitt for taking the nasturtium UV photos and for lending us his amazing flower images for this post.


Anderson, A.M. 1977.  Parameters determining the attractiveness of stripe patterns in the honey bee. Animal behavior 25: 80-87.

Citerne, H., F. Jabbour, S. Nadot, and C. Damerval. 2010. The evolution of floral symmetry. Advances in Botanical Research 54: 85-137.

Decraene, L.P.R., and E.F. Smets. 2001. Floral developmental evidence for the systematic relationships of Tropaeolum (Tropaeolaceae). Annals of Botany 88: 879-892.

Hainsworth, F.R, and L.L. Wolf. 1976. Nectar characteristics and food selection by hummingbirds. Oecologia 25: 101-113.

Neal, P.R., A. Dafni, and M. Giurfa. 1998. Floral symmetry and its role I plant-pollinator systems: Terminology, distribution, and hypotheses. Annual Reviews of Ecology and Systematics 29: 345-373.

Osorio, D., and M. Vorobyev. 2008. A review of the evolution of animal colour vision and visual communication signals. Vision Research 48: 2042-2051.

Okra – what’s not to like?

What is hairy, green, full of slime, and delicious covered in chocolate? It has to be okra, bhindi, gumbo, Abelmoschus esculentus, the edible parent of musk. Katherine explores okra structure, its kinship with chocolate, and especially its slippery nature. What’s not to like?

Okra flower with red fruit below

Okra flower with red fruit below

People often ask me about okra slime. Rarely do they ask for a good chocolate and okra recipe, which I will share unbidden. With or without the chocolate, though, okra is a tasty vegetable. The fruits can be fried, pickled, roasted, sautéed, and stewed. Young leaves are also edible, although I have never tried them and have no recipes. Okra fruits are low in calories and glycemic index and high in vitamin C, fiber, and minerals. The plant grows vigorously and quickly in hot climates, producing large and lovely cream colored flowers with red centers and imbricate petals. The bright green or rich burgundy young fruits are covered in soft hairs. When they are sliced raw, they look like intricate lace doilies. In stews, the slices look coarser, like wagon wheels. And yes, okra is slimy. And it is in the mallow family (Malvaceae), along with cotton, hibiscus, durian fruit, and chocolate.

Why is okra slimy?
Okra is slimy because it produces a water-soluble acidic polysaccharide (in the galacturonorhamnan group) more generally called mucilage, that forms a complex mixture with proteins and minerals. It’s not snot: animals make a glycoprotein-based complex mucus, whereas plants make mucilage. Okra mucilage is produced in large specialized cells scattered throughout the plant body, and it pours forth from chopped pods, clinging persistently to knives and cutting boards. Although okra slime is soluble in water, it does not dissolve quickly, and it leaves an odd slick feeling on your hands and dish sponge. Some people add okra mucilage to homemade paper to make it strong and smooth. Others have been testing its use as a vegan snot substitute, useful for delivering drugs nasally. Mucilage from the root of a close relative, the marsh mallow Althaea officinalis, was an original ingredient of marshmallows. Now marsh mallow root extract sometimes shows up in cosmetics and throat-soothing teas.

Two varieties of okra

Two varieties of okra

Why does okra produce all that slime? You might suspect that mucilage repels herbivores, since plant defense is often behind otherwise inexplicable plant compounds. We have talked about defense compounds in the cabbage and nightshade families, among others. In this case, though, the slime seems to deter only some humans, and not animals or microbes more generally. Instead, mucilage is all about lubrication and moisture. Basically all plant species produce a little bit of mucilage at the tips of their roots to ease movement through the soil, cultivate a microbial community, and prevent delicate root tips from drying out. The seeds of many species produce mucilage when they get wet, apparently to buffer moisture fluctuations and to bind themselves more tightly to the soil as they germinate. Flax, chia, and basil seeds make especially thick and slimy jackets when wet. Mucilage within leaves and roots of some species seems to help plants regulate the water content of their cells, although I have found no robust studies specifically testing its role in okra plants. This is a ripe area for research, since okra is an important food plant in south Asia and west Africa and is reputed to tolerate drought and salt well.

Okra plant

Okra plant

Working with the slime
We may know very little about the functional role of mucilage in the life of an okra plant, but the scientific community knows a lot about its behavior under various culinary conditions (e.g. Woolfe et al. 1977). For example, viscosity peaks at neutral to alkaline pH. Adding a bit of baking soda to your okra soup will thicken the slime, whereas stewing it with acidic tomatoes – a classic pairing – will thin it. Heating over 90º C (close to boiling) reduces viscosity even when the dish is cooled again, probably because it denatures some associated proteins. So if you love your okra thick and slick, steam it lightly and serve it on its own or sauté it briefly and add (alkaline) eggs to the pan to make an omelette. If you care less about the slime than about the flavor and beauty of okra wheels, sauté your okra in a hot pan with onion and tomato or cook it in a tomato-based soup or stew. The ultimate acid treatment for okra is pickling. In New Orleans, even the vinegar-soaked and salted okra fruits are paired with tomato juice (and lemon juice and vodka, etc).

Some cooks recommend working with absolutely dry okra and keeping the fruits whole to reduce the slime, as if slime were a misfortune visited upon the okra from somewhere outside. As we have seen, though, slime is made by cells within the fruits, and keeping okra whole will not prevent its spilling its mucilage all over your tongue when you bite through it. Cutting the okra and exposing it to water distributes the mucilage and increases its volume by thinning it, so the whole dish may seem more slippery. However, cutting it also exposes more of the mucilage to the slime-busting action of acid and heat.

Okra structure
Okra flowers are showy but so ephemeral that gardeners have been known to miss them, finding overly mature okra fruits a day too late, hidden among their plant’s large leaves. Right away the fruits start pushing relentlessly towards their natural end state – a woody capsule – so they are edible only for their first few days of life. Many other fruits we eat, from avocados to eggplants, are soft at maturity and stay soft. If we didn’t eat them, they’d keep getting softer until mold and fruit flies took them over. These fruits never harden and split to spill their seeds. Okra fruits are different. As capsules, technically loculicidal capsules, okra

Dry okra capsules

Dry okra capsules

fruits left on a plant will grow to be about 8 inches long, develop woody fibers, dry out, and split lengthwise along and between the chambers that hold the seeds (locules). If you have ever tried to cut an okra fruit that is even just a tiny bit too old, you may have been surprised by the violent sound that comes from sawing through the tough flesh.

Young tender fruits are an entirely different story. They are softly fuzzy, beautiful inside, and a joy to prepare if you can handle a slippery knife. The little cap on one end is the base of the flower where the sepals, petals, and pollen-bearing stamens were attached. (Very fresh okra may still bear on the cap margins a few narrow pointed green bits that resemble sepals, but which are really bracteoles.) Some people eat the cap, but I find it tough and remove it. The fruit itself is five-sided, with thick walls and five open channels (locules) running from cap to pointed tip. The locules contain the seeds, which are spherical and typically fill the open space.

Malvaceae- okra cross section

Okra slices look lacy in cross section because of their heart-shaped locules, large mucilage cells, and obvious vascular and support tissue. Mucilage cells are more or less scattered throughout the tissue, while the bundles of water- and sugar-conducting vascular tissues stand out against the pale background as c-shaped darker green areas. Young supporting fibers surround vascular bundles and lie just below the outer skin of the fruit.

Chocolate and okra?
Among okra’s closest edible relatives are hibiscus (specifically their fleshy sepals) used to make tea. There is also the marsh mallow, and wild food foragers celebrate a colorful variety of other mallows as well. See Green Dean’s treatment for several examples. Cotton is another close relative, but we don’t eat it on purpose. Slightly more distantly related is the durian, a notoriously smelly southeast Asian fruit with a cult following. But everyone’s favorite okra relative has to be chocolate, the drink of the gods, Theobroma cacao. Chocolate deservesMalvaceae: Theobroma_cacao_fruit3 its own long discussion, and many paeans, odes, histories, and scientific findings can be found with little effort. I will simply point out that chocolate, like okra, produces slimy polysaccharides in its fruits and has a distinct and complex flavor. You might counter that cacao flowers do not look much like okra flowers, and

Chocolate flowers.  Click to enlarge.

Chocolate flowers

they grow directly from the trunks of large trees, not on small, fast-growing herbaceous plants as okra does. Indeed, cacao and okra used to be classified in separate closely related families, but genetic evidence has brought them together where they belong (Alverson et al. 1999), making sense of their obvious culinary affinity. In all seriousness, the flavor profile of okra includes bright floral notes along with something darker, like clove or wood smoke, and a hint of a bitter edge that resonates well with high-quality dark chocolate.

Fried Okra with mole sauce
My favorite pairing of okra and chocolate is fried okra with mole, and specifically Deborah Madison’s life-changing mole – a rich savory and sweet concoction of dried red pepper, onions, toasted spices, and dark chocolate.

To begin, cut fresh young okra into pieces about as long as they are wide. Discard the cap-shaped receptacles and peduncles that attached the fruit to the plant. Count about a cup of okra per person. Toss the okra in flour.

In a large bowl, beat with a fork enough egg to coat the okra. A large egg will barely coat a cup of okra. Add a bit of milk to thin the eggs if you need to stretch them. Toss the okra in the egg, making sure the egg sticks to all sides.

In another large bowl, mix approximately equal parts flour, cornmeal, and bread crumbs. Add a pinch of salt to taste, but be conservative. Toss the egg-coated okra pieces in the cornmeal to cover.

In a large cast iron skillet, heat enough canola or sunflower oil to cover the bottom of the pan about half the depth of an okra piece. Heat until a bit of coating sizzles rapidly in the oil. Cook the okra in the oil, turning the pieces, until the okra is fork tender. If the coating is burning, turn the heat down a bit. The okra does need to cook through, which can take over 5 mins. Drain on a paper towel. You may need to cook these in batches to keep them in a single layer.

Serve the fried okra with a drizzle of mole and a nice dollop on the side. Fried okra is often served with hot vinegary tabasco sauce, which nicely complements the sweet corn and cuts the slippery okra, but can also overwhelm the okra flavor. Madison’s mole is not hot at all. Rather it is rich and shows off the fruit, smoke, and pleasant sourness inherent in good chocolate. It is strong enough to balance the okra goo while letting the unique okra flavors shine through.

Alverson et al. (1999) Phylogeny of the Core Malvales: Evidence from ndhF Sequence Data. American Journal of Botany , Vol. 86, No. 10 (Oct., 1999), pp. 1474-1486.

Madison, D. Vegetarian Cooking for Everyone. Ten Speed Press

Sharma, N., et al. (2013) Development of Abelmoschus esculentus (Okra)-Based Mucoadhesive Gel for Nasal Delivery of Rizatriptan Benzoate. Tropical Journal of Pharmaceutical Research 12: 149 - 153

Woolfe, M. L.; Chaplin, M. F.; Otchere, G. (1977) Studies on the mucilages extracted from okra fruits (Hibiscus esculentus L.) and baobab leaves (Adansonia digitata L.). Journal of the Science of Food and Agriculture  28: 519-529.

Figs and Mulberries, inside and out

A shorter version of this essay appears in the Autumn 2013 issue of the beautiful, creative online magazine Soiled and Seeded.  Here Katherine and Jeanne explain the topological relationship between figs and mulberries and do a little investigative journalism.



Figs and mulberries are both gorgeous, sexy fruits, but in very different ways. At first blush a mulberry could be the fragile hot-mess cousin of a blackberry, while figs are classically sensual fruits, like marble nudes teetering on the edge of vulgar. For all their fleshy assertiveness, both fruits keep their secrets; and it takes more than a long, intense gaze to uncover their close relationship and know what makes them sweet.  Mulberries may look like blackberries (and share a taxonomic order), but they are built from different plant components. The true siblings are mulberries and figs (both in family Moraceae), and at heart they are very much alike, although figs are clearly the more introverted of the two.

Anatomy of the Moraceae
To understand a mulberry or a fig, you first have to recall the basic structure of a flower and imagine the various ways flowers can be grouped on a plant. Both figs and mulberries cluster their tiny flowers together into dense and well-defined inflorescences. And, in both, all the flowers on an inflorescence then develop into a single fused unit, which we casually call a fruit. Before offering the details of what we eat we’ll need to look at the individual flowers and fruit.

An idealized flower has four concentric rings of parts, or whorls. From the outside in, they are:
1) usually green, modified leaves, called sepals (collectively the calyx), which are prominent in the nightshade family and persimmons;

2) often colored or otherwise showy petals (together called the corolla);

3) the “male” stamens, consisting of a filament holding aloft a pollen-filled anther; and

4) one or more “female” pistils, anchored by an ovary.  The pistil catches pollen grains, which then grow down through a style to the ovary and the seeds within. The ovary matures into a fruit.

Not all flowers have all of these parts.  Figs and mulberries make separate flowers with only one or the other sex: “female” flowers lack stamens, and “male” flowers lack pistils.  Both female and male flowers also lack petals.  So, for example, a “female” mulberry flower will have only sepals and a pistil, and a “male” flower will have only sepals and stamens.  Male flowers therefore cannot make fruit just as female flowers cannot make pollen.

Moraceae: Mulberry Inflorescences labeled

Mulberry inflorescences

Mulberry flowers are not large or showy, but they are visible. Fig flowers, by contrast, cannot be seen without opening up the structure that encloses them. As it turns out, being hidden from view also means being hidden from all but the most specialized pollinators, which is a big part of the fig story (see below).

Under a magnifying lens, mulberry and fig flowers are remarkably similar.  However, their stories diverge as the flowers are pollinated and their fruits develop.

Fruiting bodies
Tiny mulberry flowers make miniscule fruits; by sticking together across the entire inflorescence, they masquerade as one large fruit and probably enhance their power to entice birds and improve their dispersal efficiency. But that’s not the mulberry’s only beauty secret. Strictly speaking, mulberry fruits are not that attractive. They are hard dry achenes with the appeal of a grain of sand.  To make themselves sweet and juicy, they plump up the only other flower parts they have—their sepals.

Mulberry flowers becoming fruits

Mulberry flowers becoming fruits

This is in contrast to blackberries, denizens of the rose family (Rosaceae), whose  flowers, fruits and seeds are  structurally distinct from those of mulberries (see figure below).

Click to enlarge.  A-C Blackberry.  D-F Mulberry.  Mulberries resemble blackberries, but blackberries derive from a single flower with multiple fleshy ovaries, whereas mulberries derive from multiple flowers, each with a single hard ovary and fleshy sepals.

Click to enlarge. A-C Blackberry. D-F Mulberry. Mulberries resemble blackberries, but blackberries derive from a single flower with multiple fleshy ovaries, whereas mulberries derive from multiple flowers, each with a single hard ovary and fleshy sepals.

A fig is essentially an inside-out mulberry, an entire edible flower cluster hidden down inside its own stalk.  This stalk, the peduncle, is the delicious bulk of what we enjoy when we eat a fig. To understand how we get from mulberry to fig, it is useful to reconstruct (approximately) evolution’s recipe for figs. Imagine a mulberry inflorescence, give it a long peduncle, and mentally expand the entire axis into a globe. All of the flowers should be above the equator, with the peduncle below it.  Pull outwards on the equator, flattening the globe into a disk with all the flowers on top and the peduncle constituting the underside.  There are members of the mulberry family (e.g. Dorstenia) whose inflorescences take this shape.  Finally, curve the disk upwards into a bowl, then into an urn, and finally into a sack. All the flowers will be inside the sack, completely surrounded by peduncle tissue. The entire structure has a specialized name: syconium.  (Interested readers may look up the not-safe-for-work etymology of “sycophant,” from the Greek for “showing the fig.”)

Cutaway view of a fig with a closeup of a female flower on the left.  Flowers within the fig are shown without a calyx, which is not apparent anyhow.

Cutaway view of a fig with a closeup of a female flower on the left. Flowers within the fig are shown without a calyx, which is not apparent anyhow.

Fig flowers are even smaller than mulberry flowers and likewise make gritty little achenes for fruit. They are attached to the inside of the syconium by flower stalks (pedicels), which get very soft as figs ripen. The sweet part of the fig is a combination of peduncle, pedicels, and sepals.  The crunchy parts are the achenes.  But what about the old legend that wasp parts add a little something to the texture?

Are there wasps in my Newtons?
Usually, before fruit can ripen, flowers must be pollinated. Mulberries are simply wind pollinated. However, in most of the more than 800+ fig species, pollination is courtesy of small wasps from the Agaonidae family. Figs and agaonid wasps have required one another for existence for at least 60 million years. And like many co-dependencies, this one isn’t pretty. Fig seeds feed wasp larvae. A large family of newborn wasps synchronously emerges from the seeds within a syconium. The wingless, blind males have two quick duties before dying: inseminating their sisters and chewing escape holes for them. Before leaving home, young females gather pollen from male flowers. A winged female has 48 hours to find and enter a new receptive fig, pollinate the flowers, and lay eggs. The fig doesn’t help her. The only opening, the narrow ostiole, is defended with sharp bracts. With specialized jaws and a strong head, she chews her way past this gauntlet and into the fig, but tears her wings and antennae in the process.

The ostiole defended by sharp bracts.

The ostiole defended by sharp bracts.

Figs make two kinds of female flowers: long-styled and short-styled. Wasps can lay eggs only in short-styled flowers, but they are able to transfer pollen to all flowers.  The short-styled flowers thus make wasps, whereas the long-styled flowers make fertile seeds.  Under this arrangement, both the plant and the pollinator may reproduce.  The mother wasp dies after her tasks are complete and fig enzymes devour her body during ripening.  The spent male offspring meet the same fate, while their gravid sisters fly off to other figs.

About half of fig species come in two different sexes: “male” plants are similar to those described above, and their syconia bear both male and short-styled female flowers.  (The ovaries of the female flowers serve mostly as wasp nurseries, so they tend to be forgotten by fig sexers.)  “Female” plants produce syconia containing only long-styled female flowers, and the poor wasp entering one of these cannot lay eggs before dying.  Although her own reproduction is thwarted, she brings pollen from a “male” plant that triggers seed and syconium development.  Some favored varieties (e.g. Calimyrna) fall into this category and are called gynodioecious.  Because they are pollinated, they produce viable seeds and a large sweet fig syconium, but their long-styled flowers preclude egg-laying, and we avoid a mouthful of baby wasps.

Mission fig with ostiole

Mission fig showing its ostiole

Some mutant fig varieties can ripen syconia without pollination.  These parthenocarpic (“virgin fruit”) plants have been propagated asexually by humans for over 11,000 years and comprise most of our edible figs (e.g. Mission and Kadota).  They may lack well-developed seeds, but their achenes provide some crunch and their flesh is free of liquified female wasp body.

So are there wasp bits in your Newtons?  It depends on which fig variety the good people at Nabisco use for their iconic cookie.  This seems to be a trade secret, though, as the label simply lists “figs.”  We decided to investigate.  The most common varieties used for fig paste include the wasp-free parthenocarpic Mission and Kadota and the female-trapping gynodioecious Calimyrna.  Calimyrna figs reportedly have larger achenes than other varieties, and we further reasoned that their achenes would contain well-formed seeds, whereas Mission and Kadota achenes would not.  We did not have any Kadota figs handy but were able to compare Newton filling to fresh Mission figs.

We discovered right away that the image on the box, which shows a filling rich with achenes, does not match the grit-free filling in an actual cookie.  The filling seemed suspiciously smooth, and after extensive searching, we found only 19 achenes in an entire cookie.  It is possible, then, that the achenes we found slipped through the (postulated) strainer mesh and don’t represent the full achene size distribution of the mystery variety.  In any case, they were no larger or smaller than Mission fig achenes.  When we opened up the Newton achenes with a razor blade, we found what looked like a small seed inside each one.  We were getting somewhere, as this suggested the Calimyrna variety.  When we opened up the Mission achenes, though, we found them filled with something very seed-like as well.  As it turns out, at least according to a few older papers, parthenocarpic figs such as Missions can make seeds full of nutrititive tissue (endosperm), even though they lack actual embryos.  Alas, we found no evidence in favor of one fig variety or another.

It may be comforting to know that there would be no baby wasps in filling made with any of these varieties.  If the filling came from Calimyrnas, any female wasps would have been digested before the figs were even harvested.  We probably should be more worried about a fruit fly floating in our chardonnay than a wasp in our Newtons.  While perhaps small comfort to the squeamish, the rest of us fig and mulberry enthusiasts can toast the wasps and the wind before digging into a plate of figs roasted with basil and cheese–recipe below.

Acknowledgement: Thanks to Quentin Cronk for correspondence about the morphological delimitation of peduncles.

More information about fig pollination may be found at the wonderful site Wayne’s Word

Roasted figs with basil and cheese

Slice fresh figs lengthwise and bake at a high temperature (400º F) until they bubble.  When they have cooled just slightly, top each with a small basil leaf and a nubbin of rich soft cheese (a triple creme, for example).


Evolution of Lemon Flavor

A batch of lemon balm-lemon verbena syrup reminds Jeanne of the multiple evolutionary origins of lemon flavor.

DSC00796The citrus lemon itself is only one of many plant species that lends its namesake flavor or fragrance to our food and drinks.  Lemon flavor primarily comes from a few terpenoid essential oils:  citral (also called geranial, neral, or lemonal), linalool, limonene, geraniol, and citronellal.  The production of one or more of these essential oils has independently evolved multiple times in species on widely separated branches of the plant phylogeny (see figure).

Phylogeny of plant orders with edibles (click the tree to enlarge). Orders with species with lemony essential oils are highlighted in red.  For a refresher on reading phylogenies, please see our food plant tree of life page.

Phylogeny of plant taxonomic orders with edibles (click the tree to enlarge). Orders with species with lemony essential oils are highlighted in red. For a refresher on reading this phylogeny, please see our food plant tree of life page.

Lemon vebena and lemon balm leaves, pre-syrup

Lemon vebena and lemon balm leaves, pre-syrup

I was reminded of this extraordinary evolution of lemoniness today when I made a syrup (recipe below) from lemon balm (Melissa officinalis), a mint (family Lamiaceae), and lemon verbena (Aloysia citrodora), in the verbena family (Verbenaceae).  The mints and the verbenas are closely related families within the order Lamiales, however, so combining them together did not produce a particularly phylogenetically extreme lemony syrup.   Many other species in the mint family produce lemony essential oils, such as lemon basil (Ocimum spp.), lemon thyme (Thymus spp.), and lemon mint (Monarda spp.). And the sand verbenas (Abronia spp.) growing on the sand dunes on the California coast have a lemony aroma.

Lemongrass (photo from Wikipedia)

Lemongrass (photo from Wikipedia)

Moving away from the Lamiales, lemony essential oils are most spectacular, of course, in the citruses (family Rutaceae, order Sapindales).  Several of the Australian Myrtaceae species (order Myrtales) are lemony and used as a spice, including lemon myrtle (Backhousia citriodora), lemon gum (Corymbia citriodora), and lemon tea tree (Leptospermum polygalifolium).  The lemony leaves and fruit of Litsea cubeba, in the bay family (Lauraceae, order Laurales), are locally popular as a spice in its native tropical Asia.  Popular now the world over, however, are other tropical Asian natives the lemon grasses, some 55 species of grasses in the genus Cymbopogon (family Poaceae, order Poales).  Cymbopogon citratus is the most popular culinary species.  Commercial citronella essential oil (the real stuff in natural insect repellent) is distilled from C. nardus and C. winterianus.  In all of these diverse taxa, the lemony essential oils serve as defense compounds against pathogens and herbivores.  This is not to say that these plants all taste the same, which of course they don’t.  They all have their own unique flavor compounds as well, and the relative proportions of the lemony terpenoids vary.

Lemon balm

Lemon balm

While the lemony essential oil compounds themselves have independently evolved numerous times, the structures that plants use to store and deploy them against marauders are highly variable across the plant tree of life.  Citrus leaves and fruit concentrate their essential oils in oil glands.  Specialized cells cluster together, synthesize the oils, and secrete them into the space between the cells, forming a chamber (gland) filled with oil (Thomson et al. 1976). The oil glands are so large that you can see them as translucent spots if you hold a citrus leaf up to the light.



The oil glands in citrus are found in almost all plant structures but are most abundant in the outer fruit rind (zest).  Many species in the Myrtaceae also have oil and resin canals or specialized storage cells, although these are structurally different from those in citruses.  The essential oils in lemongrass leaves are stored in specialized cells nestled between the vascular bundles in the spongy mesophyll, beneath the photosynthetic tissue.  The cell walls of these lemongrass oil cells are lignified (woody), perhaps contributing physical as well as chemical defense against marauding herbivores (Lewinsohn et al. 1998).

Lemon verbena

Lemon verbena

The verbenas and the mints in the Lamiales have my favorite essential oil delivery system—trichomes.  Trichomes are hair-like growths on the outside surface of the leaf.  Like epicuticular wax, trichomes take on particular shapes and forms in different plant lineages and species.  The hooked trichomes on the leaves of bean plants (Fabaceae), for example, feel like sandpaper if you rub your hand over them.  These hooks may serve the plant in part by slowing insect herbivores down, if their efficacy at catching bedbugs is any indication.  The leaves of mints and verbenas have two kinds of trichomes.  The first kind is hair-like, with either no branches, as in lemon balm, or with multiple branches, as in lavender ( Lavandula spp.).  These trichomes may help cool the leaf by deflecting excess solar radiation.  The other trichomes on these leaves are glandular hairs.  These trichomes fill with essential oil and sit like squat little water balloons on the surface of the leaf.  The flowers, too, are covered in glandular hairs.  See great scanning electron microscope pictures of the two kinds of trichomes in mint family species here.

Lemon thyme on the left, French thyme on the right

Lemon thyme on the left, French thyme on the right

When you rub a verbena or mint-family leaf between your fingers, you rupture the glandular trichomes and release some of the most fragrant substances this planet has to offer.  They may be repellent to bugs, but hardly to us.

Fresh herb syrup

There are two different ways I make a sweet syrup out of fresh herbs.  In the first, I start by making a strong infusion (“tea”) out of the herbs.  I usually barely cover them with water in a pot, bring it to a boil, then turn the heat off and let it steep for however long it needs.  The steeping is definitely up to you, but you should start tasting it after one minute.  Steeping leaves like lemon balm and lemon verbena for more than 5 minutes or so will start to draw out some of the bitter and dark tannins from the leaf (remember the essential oil is on the surface), which may be fine with you, but you should just keep checking and tasting.  Then, I strain out the leaves and add an equal volume of sugar to the remaining liquid in a saucepan and stir to dissolve the sugar while I bring it to a boil.  The second method is to measure or guess how much liquid I’ll need to cover the herbs packed into the bottom of a saucepan, then add an equal volume of sugar to that amount of warm/hot water, stir it to dissolve the sugar, then pour it over the herbs, bring it back to a boil, turn off the heat, let it steep, then strain.  Either way, stirring in about a tablespoon of vodka or brandy per cup of syrup will help preserve it for about a month in the fridge if you’re not going to can it or freeze it.  I like stirring a little of this syrup into club soda to make a grown-up soda, and it’s also good incorporated into or onto ice creams or sorbets or drizzled over an almond cake.


Gattuso, S., C. M. van Baren, A. Gil, A. Bandoni, G. Ferraro, and M. Gattuso. 2008. Morpho-histological and quantitative parameters in the characterization of lemon verbena (Aloysia citriodora palau) from Argentina. Boletín Latinoamericano y del Caribe de Plantas Medicinales y Aromáticas 7: 190-198.

Lewinsohn, E., N. Dudai, Y. Tadmor, I. Katzir, U. Ravid, E. Putievsky, and D. M. Joel. 1998. Histochemical localization of citral accumulation in lemongrass leaves (Cymbopogon citratus (DC.) Stapf., Poaceae). Annals of Botany 81: 35-39.

Stewart, A. 2013. The drunken botanist: the plants that create the world’s great drinks.  Algonquin Books of Chapel Hill, Chapel Hill, N.C.

Svoboda KP and RI Greenway. 2003. Lemon scented plants. International Journal of Aromatherapy 13: 23-32.

Thompson, W.W., K. A. Platt-Aloia, and A. G. Endress. 1976. Ultrastructure of oil gland development in the leaf of Citrus sinensis L. Botanical Gazette 137: 330-340.).

Making ratatouille like a botanist

The story of the nightshades is usually told as a tale of European explorers, New World agriculturalists, and a wary bunch of Old World eaters.  But what about the birds?  And the goji berries?  Jeanne and Katherine introduce you to the Solanaceae family and walk you through the botany to be observed while making ratatouille, the classic French collision of Eastern and Western nightshades.

Can you imagine Italian cuisine without tomatoes? The Irish without potatoes? Chinese cuisine without spicy, fruity chiles?  Such was the case prior to the discovery of the New World nightshades (family Solanaceae) by sixteenth-century Spanish explorers.  And they couldn’t help but run into them.  Solanaceae is a huge family, with over 100 genera and nearly 2500 species, most of which are in Central and South America.

Around 1700 of those species are in the genus Solanum, one of the ten largest (most speciose) genera in the world.  The Solanum species potatoes and tomatoes were already American staples when the conquistadores arrived, and they are now first and fourth on the USDA’s list of most-consumed vegetables in the United States.

Ground cherries and tomatillos (Physalis) have impressive calyces!

Ground cherries and tomatillos (Physalis) have impressive calyces!

Ships coming back from the Americas carried lots of nightshades: tobaccos (Nicotiana spp.); tomatoes (Solanum lycopersicum); potatoes (S. tuberosum); and chiles and “peppers” (Capsicum spp.).  They might have also borne ground cherries (Physalis peruviana), tomatillos (P. philadelphica), flowers (Petunia, Datura), and other fruity Solanum species cultivated at small scales in Central and South America (see phylogeny below).

We often hear that all these beautiful and carefully domesticated plant foods were initially rejected by Europeans because their own local nightshades were intensely toxic, or at least psychotropic. While there are very nasty European nightshades (see below), they can’t be blamed entirely for the lukewarm reception.

Chili pepper plant

Chili pepper plant

Despite being obvious nightshades, spicy chile “peppers” were instantly popular with a western Europe that had sent Columbus out in the first place to look for black pepper (Piper nigrum) (they’re both “hot”— right?).

Eggplants, another obvious nightshade, were already well integrated into Italian cuisine by the time the Spanish ships returned home. Eggplants (otherwise known by the much nicer name aubergines) are Solanum species that were domesticated independently in both India and China long ago.  At least some of the resistance to New World solanums seems to have come from religious leaders, who initially objected to them because they are not in the Bible (Madison 2013, Rupp 2011, Wong), whereas the genuinely dangerous middle-eastern nightshade known as mandrake (Mandragora) gets its stamp of approval in Genesis.

Old World nightshades
The scary-toxin story does raise the question, however, of how Europeans could have known nightshades before the 15th century.  Solanaceae: eggplant collageLong before European ships began transporting charismatic Solanaceae species home from the Americas, a few American nightshades made the transoceanic voyage on their own, probably assisted by birds.  Something like 15 to 17 successful dispersal events are necessary to account for the present-day distribution of Solanaceae native to Pacific Islands, Africa, Australia, and Eurasia (Olmstead 2013).  Some modern descendants of three of these dispersal events are shown on the phylogeny in red: Chinese goji berries (Lycium barbarum and L. chinense); European deadly nightshade (Atropa belladonna); and, of course, the eggplants.

Evolutionary relationships among domesticated nightshades.  Phylogeny data from Knapp (2002)

Evolutionary relationships among some domesticated nightshades. Phylogeny data from Knapp (2002).  Click to enlarge (to review reading phylogenies, please see our Phylogenetic Food Plant Tree of Life).

After being unceremoniously deposited by the birds in their new homeland, the Solanaceae species that successfully gained a foothold in Eurasia and Africa were undoubtedly aided by their potent blends of alkaloids and other defense compounds.  People are among the herbivores these toxins repel, and with precious few exceptions, the Old World nightshades earn their reputation as a very dangerous group.  Amy Stewart’s excellent Wicked Plants, a compendium of toxic plants and their sins against humanity, is full of great quotes from literary giants—Shakespeare, Hawthorne, Donne—whose devils, witches and medicine men wielded alkaloid-stuffed European (and some American) Solanaceae species: henbane (Hyoscyamus niger), mandrake (Mandragora officinarum), and deadly nightshade (Atropa belladonna).

Atropa’s main alkaloid, atropine, dilates pupils.   In Columbus’s day, society women in search of a wide-eyed look employed small amounts the stuff before it was adopted by the medical community.  Hence the species name belladonna, meaning “beautiful woman.”  Famous every-day alkaloids from the New World include nicotine from the tobaccos (Nicotiana spp.); the fiery capsaicin in chiles (Capsicum spp.); and solanine, which ramps up when potatoes turn green and is why you should never eat them when they do.  In your garden you might find jimson weed (Datura) or angel’s trumpets (Brugmansia spp.) making scopolamine, which has an infamous medical history and is still used in very small doses to treat motion sickness.

Offense or defense?
In fiction, wickedness usually masks a deeper vulnerability, and so it is with the nightshades and their poisons.  As with most defense compounds, herbivores and pathogens can eventually overcome alkaloids through evolution.  When high genetic variation for defense chemicals exists in a population, like among the thousands of potato varieties in their native Peruvian Andes, each sequential victory by a pest over defenses leads to limited damage, and there is a good chance that cross-breeding can reinstate chemical roadblocks. Relatively few native American varieties made it to Europe, however; and the limited genetic diversity allowed pathogens and herbivores to do more damage there. A fungal-like blight (actually an oomycete) figured out how to circumvent the particular defensive chemical cocktail in the most widespread variety of potatoes adopted by the Irish and wiped out nearly every potato tuber on the island during the Irish Potato Famine of the mid-1800s.  Europe’s potato blight woes ended with fungicide sprays, which are still heavily used in European and North American potato crops.   Blight-resistant (for now) potato varieties that don’t require sprays (or as much), however, are increasingly available, thanks to many more trips to the Andean potato fields by plant breeders.  Blight-resistant potatoes are a huge argument in favor of preserving genetic diversity of agricultural crops and their wild relatives.

Solanaceae parade of spuds

While we scoff now at European’s initial reluctance to embrace the New World nightshades, we might feel more empathy for the skeptics if we remember that all parts of the cultivated plants – besides the parts we now eat (tomato and pepper fruit, potato tubers) – are potentially very toxic.  More than a few rich 16th-century western Europeans became violently ill because their chefs, cooking potatoes for the first time, didn’t realize or believe that the tuber was the only edible structure on the plant.  Maybe this explains why we have found a way to eat only a dozen or so species from one of the largest plant families on the planet.

La botanie de ratatouille
Ratatouille is a beautiful showcase for the nightshade family, which is represented by three of its starring ingredients: peppers, eggplants, and tomatoes.

clockwise from top left: unknown wild, cherry tomato, potato, jalapeno chile, eggplant

clockwise from top left: unknown wild, cherry tomato, potato, jalapeno chile, eggplant

This recipe also allows the cutting board botanist to compare and contrast the structures of these vegetables (technically fruits).  Like their flowers (see picture), the similarity of the fruits and seeds may overcome any initial surprise you have about the close relatedness of these summertime gems.

There are at least as many variations on ratatouille recipes as there are grandmothers in France, and some ratatouille recipes are more elaborate than this one.  Julia Child layers her vegetables carefully and other authors roast theirs instead of sautéing them.  Feel free to combine the botany below with your favorite fussy recipe if you so desire.

Olive oil for sautéing
A medium onion
2-3 bell peppers (Capsicum annuum)  – yellow or orange ones make a nice color contrast
One large globe eggplant or a few smaller eggplants (Solanum melongena), about 1.5 lbs
2-3 zucchini, or slightly less than the weight of the eggplant
2 or 3 lbs of fresh tomatoes (Solanum lycopersicum), the amount depending on how tomato-laden you like your ratatouille
Generous pinch of herbes de Provence, optional
A bunch of fresh basil, enough to make a loose half cup when julienned
Salt and black pepper to taste

Berry: A fruit that is uniformly fleshy throughout (no pits or hard papery bits) and does not naturally split open when ripe.  Tomatoes, eggplants, and peppers are berries.
Calyx: The sepals as a group, comprising the outermost whorl of a flower
Fruit: A mature ovary, normally bearing seeds. Cooks treat the ingredients above as vegetables because they are savory, but if they contain seeds, they are botanical fruits.  Only the basil, some of the herbes de Provence, and onion are true vegetables (in this case, leaves).  Even the black pepper is a dried fruit.
Locule: A compartment within the fruit (ovary).  Often, the number of locules corresponds to the number of carpels (leaf-derived subunits) making up the ovary.
Placenta: the tissue that connects seeds to the fruit, either at the ovary wall, and thus the fruit wall, or to a central axis.

You will sauté the vegetables separately in a skillet or cast-iron pan and then transfer them to a Dutch oven or large stock pot.  You should not need a magnifier to see structures described here.

1. Chop the onion and sauté it in olive oil over low enough heat that it becomes soft and sweet without much browning.  We will defer a lesson on onion structure for another post.

2. Meanwhile, prepare the peppers.  Remove the top of each pepper where the calyx sits.  Set one top aside.

Pepper calyx barely showing its lobes

Pepper calyx barely showing its lobes

Look down into the hollow pepper and notice that most bell peppers have three or four lobes.  Notice also the soft and thin white sheets of tissue running lengthwise down the inside of the fruit.  That is the placenta, and it carries some of the seeds.

Chili peppers cut to show thin placenta dividing the fruit into two locules

Chili peppers cut to show thin placenta dividing the fruit into two locules.  Click to enlarge.

In a smaller pepper, such as a jalapeño, the placenta would divide the cavity into two compartments, or locules.  This is the ancestral condition.  In larger varieties of peppers, there appear to be three or four locules (corresponding to the lobes), and the placenta does not go all the way across the fruit.  In these varieties, most of the seeds are attached to the placenta on a mound inside the top of the pepper.
Cut the peppers lengthwise, open them up, remove the placenta, and cut the fleshy ovary wall of the pepper into squares about ¾ to 1 inch on a side.

When the onions are done, transfer them to the larger pot (not heated yet) and use the newly emptied skillet to sauté the peppers gently.  Add additional olive oil if needed.  When these are done, transfer them to the cool pot along with the onions.

3. Now address the eggplant or “aubergine,” which is our representative of the Old-World nightshades.  Observe its calyx next to the pepper calyx.  Solanaceae: EggplantPairIt is similarly thick and fleshy and not very leaf-like.  Often eggplants have five calyx lobes, corresponding to the five sepals that match the five petals on the eggplant flower.

Remove the calyx, setting it aside to compare to the tomato calyx.  Notice that an eggplant is solid and spongy, whereas a pepper is open.  Solanaceae: EggplantSliceThe fruit wall and placenta both contribute to the solid insides of an eggplant.  Cut the eggplant crosswise into slices about an inch thick.  Cut these slices into cubes.

Some recipes recommend salting the eggplant to “remove bitterness.”  I have never found the flesh of cooked eggplant to be bitter; and its distinctive flavor is what I like about it.  Old skins can sometimes be harsh, so if your eggplants are not fresh, or the skins seem tough, you might peel them.  Unfortunately, by removing the black-purple skin you will lose some of the color variation in the finished dish.  Seeds in overly-mature fruits can also be a little harsh.  If your seeds are very dark when the eggplant is freshly cut, you might avoid using the very center of the fruit.  Even young seeds will darken as the fruit sits out.  Speaking of seeds, notice how closely they resemble the seeds of peppers and tomatoes.

Sauté the eggplant cubes in ample olive oil over slightly higher heat than used for the onions and peppers.  The goal is to brown the sides of the cubes but not to let them get very soft.  When they are done, add them to the pot along with the onions and peppers, but do not stir them in yet.

4. Chop the zucchini into cubes about the same size as the other vegetables.  There are no calyces to notice here since they will have fallen off.  Zucchini are in the squash/cucumber family, and we won’t go into details about them here, except to note that zucchini’s ancestors, the American hard “winter” squashes, arrived in Europe on the same ships as tomatoes and chiles.  It took a few centuries of plant breeding after that for the Italians to create zucchini.  Unlike eggplant, zucchini do sometimes benefit from being salted and drained, not to remove bitterness but to draw out some of the water.  I don’t bother here.  Sauté the zucchini cubes like the eggplant, to brown the sides but not soften them to mush.  If you are using herbes de Provence, add them at the beginning of the sauté with additional oil if needed.  Dump the finished zucchini on top of the eggplant in the pot.

5. Prepare the tomatoes.  Some tomatoes retain their calyces and others don’t.  If you have a tomato with a calyx, compare it to the peppers and eggplants.

Note thin leaflike calyces.  Click to enlarge.

Note thin leaflike calyces. Click to enlarge.

The sepals making up a tomato calyx look much more like leaves than pepper and eggplant calyces do.  They are thin and narrow and well separated from each other.  Although the ancestral state for Solanum flowers is to have five calyx lobes (sepals), breeding has irregularly proliferated those of large tomatoes.  You will probably find a slightly different number on each tomato.  (Cherry tomatoes usually have only five sepals).

Remove the “core” of the tomatoes, which is equivalent to the top and inner mound of the peppers.  Slice a tomato crosswise and examine its internal structure.

Tomato halves, showing the compartments called locules filled with seeds attached to viscous placenta.

Tomato halves, showing the compartments called locules filled with seeds attached to viscous placenta.

Cherry tomatoes still have the ancestral state of two locules separated by placenta.  Larger tomatoes have many locules of irregular number and shape.  They are neither completely filled with flesh, as the eggplant, nor open and dry, like the pepper.  They have solid septa between locules, but the seeds are attached to a gelatinous part of the placenta.  Sometimes the outside of a tomato is deeply lobed, cutting in between the locules.

Cherry tomatoes have two locules

Cherry tomatoes have two locules. Click for big.

I usually do not bother peeling tomatoes for ratatouille, but peeling does prevent little rolled up bits of tomato skin appearing in the final dish.  Chop the tomatoes into large cubes.  Turn the heat to medium under the pot containing the other vegetables.  When you hear the vegetables sizzle, add the tomato chunks along with any juice they have given up to the cutting board.  Gently push the vegetables away from the sides of the pot to let the tomatoes reach the bottom.  Once the tomatoes have started to break down and there is ample liquid in the pot, just barely stir the vegetables so that the zucchini and eggplant are submerged.  Turn the pot to low and let everything simmer gently.

6. Move the vegetables around occasionally to prevent burning and to speed evaporation.  The ratatouille is done when the eggplant is tender throughout and the liquid from the tomatoes has been reduced to your liking.  Add salt and black pepper to taste.

Ratatouille is usually served as a hot side or main dish, but it adapts well to other uses and can be served room temperature or cold.  Below are some alternatives.

  • Filling: Ratatouille is a terrific stuffing for crêpes.  You can fill and eat the crêpes right away or make a bunch and bake them in a dish.  A bit of sour cream or crème fraîche is good on top.  Ratatouille is also good stuffed inside baked vegetables or an omelette.
  • Bruschetta: Appetizers are a great way to use cold ratatouille.  Lightly toast slices of baguette or batard and smear them with a bit of soft fresh goat cheese.  Top with cold ratatouille that has been drained well with a slotted spoon.
  • Brown bag lunch:  Ratatouille is really flavorful at room temperature, so it’s perfect to pack for lunch.  Pre-cooked buttery white beans, such as gigante or white northerns, complement both the flavor and the color of the dish.  Garbanzo beans also work well, but kidney beans are too soft and strong tasting.  Chewy brown rice is another nice addition.

Angiosperm Phylogeny Group: http://www.mobot.org/MOBOT/Research/APweb/orders/solanalesweb.htm#Solanaceae

Daunay, M-C and Janick, J. 2007. History and iconography of eggplant. Chronica Horticulturae 47:16-22. 

Knapp, S. 2002. Tobacco to tomatoes: a phylogenetic perspective on fruit diversity in the Solanaceae. Journal of Experimental Botany 53: 2001-2022.

Olmstead, R. G. 2013. Phylogeny and biogeography in Solanaceae, Verbenaceae and Bignoniaceae: a comparison of continental and intercontinental diversification patterns. Botanical Journal of the Linnean Society 171: 80-102.

Madison, D. 2013. Vegetable Literacy. Ten Speed Press.

Poczai, P., and J. Hyvönen. 2011. On the origin of Solanum nigrum: can networks help? Molecular Biology Reports 38: 1171-1185.

Rupp, R. 2011. How carrots won the Trojan War: curious (but true) stories of common vegetables. Storey Publishing.

Stewart, A. 2009. Wicked Plants: The weed that killed Lincoln’s mother and other botanical atrocities.  Algonquin Books of Chapel Hill, Chapel Hill, NC.

Van Wyk, B.-E. 2005. Food plants of the world: an illustrated guide. Timber Press.

George Wong’s great account of the Irish potato famine:  http://www.botany.hawaii.edu/faculty/wong/BOT135/LECT06.HTM