Bamboo shoots: the facts about bracts, part 3

Bamboo shoots invade the lawn. The biggest two are ready for harvest (photo by David Inouye)

Jeanne continues Bract Month here at the Botanist in the Kitchen by describing the morphology of an interesting and delicious springtime specialty:  fresh bamboo shoots.

I had the distinct pleasure a few weeks ago of trying a temperate springtime speciality:  fresh bamboo shoots.  The friend who shared them with me has a backyard bamboo thicket and harvests the young shoots when they pop up as incursions into the lawn.  Globally, he is in good company, as the fresh shoots show up in springtime (or otherwise seasonally appropriate) markets in most of Asia, which is the native range of most of the 1400+ bamboo species.  Probably like many modern Americans, before steaming the fresh shoots and putting them in a lovely spring chopped salad last week (we used a lemon-garlic-dill vinaigrette), I had only eaten bamboo shoots as neat, thin, rectangular or julienne slices of canned bamboo shoots in various Thai curries at and Chinese soups in restaurants.

Peeled bamboo shoot, showing tender immature leaves and apical meristem tissue (photo by David Inouye)

The fresh shoots are a big improvement over the canned and have the texture of asparagus and a flavor like a mild, tangy corn.  These similarities might arise from shared evolutionary history between bamboo and both vegetables.  Like corn, bamboo is a grass (family Poaceae; see our post on the evolution of gluten within the grasses for a phylogenetic context of bamboo within the grasses).  Like asparagus, bamboo is a perennial monocot (see our food plant tree of life for a refresher on the monocot clade), and its shoot is a new young stem developing from underground stems called rhizomes that spread out from a parent plant as a form of asexual reproduction.  As Katherine described for asparagus, these monocot shoots arising from rhizomes are as thick as they ever will be, dictating the diameter of the bamboo pole that would develop if the shoot were left to grow.  And, like asparagus, timing is of the essence, as the shoots must be harvested before wood develops.

Beautiful striped, translucent bract unfurling from a cut bamboo shoot (photo by David Inouye)

Once harvested, a bit of time and effort is required to turn them from lawn ornament to foodstuff, as the tender edible portions of the shoot are covered in coarse, inedible, densely overlapping bracts, the specialized modified leaves that are particularly conspicuous in artichokes and asparagus.  Each bract subtends and protects what will eventually be a fully expanded leaf or branch, if the plant were allowed to elongate.  The edible portion of the shoot consists of developing leaf primordia and the large apical meristem (the stem-cell-like tissue from which all new parts of the growing plant originate that you learned about in the Brassica oleracea post).

Bamboo rhizome with fine roots and a shoot that had poked up into the lawn (photo by David Inouye)

Rhizomatous spreading is responsible for producing dense thickets of bamboo.  Many species of bamboos native to Asia have become naturalized or invasive in North America.  At the same time, the ranges of the three species of bamboo native to the United States (Arundinaria spp.), are dramatically reduced from their historic (pre-Europeans) distribution.  Arundinaria shoots were an important Native American food source.

Bamboo canes as rebar in concrete, Chandrapur, India (photo by Forrest Fleischman)

I don’t know the particular species that grows in my friend’s yard, but based on the fairly large diameter and height of the stand, it is probably an Asian Phyllostachys, one of the timber bamboos, an important construction material throughout much of the world (and in this impressive-but-prohibitively-expensive bike that I covet).  Bamboo spreads so aggressively that it is even banned now in some places.  The best way to stop bamboo spread is the simplest and most delicious:  harvest the shoots.  If you do, cook them before you eat them.  Our old toxic defense compound friends cyanogenic glucosides that seem to just pop up with extraordinary frequency in angiosperms (we’ve already talked about them in the Rosaceae and Dipsacales) are present in bamboo shoots in somewhat worrisome concentrations but can be disarmed by even light cooking (like steaming).

Preparing asparagus: the facts about bracts, part 2

If artichokes are big balls of spiny bracts, then asparagus spears are telescoped rods with membranous scales.  In this follow up post, Katherine takes on asparagus, both the tender and the tough, and explains why peeling can’t rescue a woody spear.

Asparagus is a hopeful spring vegetable.  Asparagus aspires, breathes in the warming spring air, and optimistically pokes its nose up from the ground.  Its tips are clusters of tiny developing branches, still packed tightly like an unexpanded telescope, containing all the potential of a season’s worth of growth.  Except that we whack them and eat them  before they can realize their audacious plant dreams.  There’s no need to feel entirely bad about this, though.  The spears stay alive for a while, stubbornly growing tougher until they are cooked or digested.

Preparing asparagus
Preparing asparagus always raises questions.  What’s the difference between thick and thin spears?  Should the triangular leaves be removed?  And should the whole thing be peeled?  Much has been written about spear size, so I’ll just say briefly that spears come up from the ground as thick or thin as they are going to be.  They do not grow thicker as the spears age; they grow thicker as the perennial underground parts grow.  Basically, the thicker and more robust the underground stem is over the winter, the thicker the crop of upright stems will be in the spring.

As for the little triangular leaves, I always pare them away, even though nobody else I know does this voluntarily.  They bug me, so I do it.  Many chefs recommend peeling the entire stalk, especially when the spears are fat.   (If you look at Mark Bittman’s helpful recommendations ignore the inaccurate part where he calls the tip a flower bud.  More on that particular tip later.)

Asparagus tip showing scale leaves and baby cladode

If your spears are stringy, the skin is not the problem
While it is true that a chef can eliminate some toughness by peeling an asparagus spear, the skin is only a small part of the problem.  That’s because asparagus is a monocot – it belongs to the same phylogenetic group as lilies, grasses, bananas, onions, and orchids.  One diagnostic feature of this group is the arrangement of its vascular bundles, or veins.  They are scattered throughout the stem instead of being arranged in a ring just under the skin.  This is very easy to see in an asparagus spear whose end has dried out.

Bottom of an asparagus stem, showing the vascular bundles scattered throughout

The veins stay stiff while the rest of the tissue shrinks back, leaving the tips of the bundles poking out a little bit.  The same thing happens to corn cobs and the stem end of a banana.

Those vascular bundles contain both soft living cells (the sugar-conducting phloem) and cells with thick stiff walls (the water conducting xylem vessels).

Asparagus vascular bundle at 100X magification

When a spear is very young, the vessels are not numerous and their walls are not very thick, but during normal development the vessel walls accumulate more and more tough lignin.  Lignin is a major component of wood, and we cannot digest it.  Chewing it is not so easy either.  Asparagus strands stuck in your teeth are not dignified, only lignified.  The thickness of the xylem walls and their lignin content is greatest at the base of the spear, so it is worth snapping the ends of the asparagus off and using them to make stock for asparagus soup or risotto.

Just under the delicate skin of the asparagus spear there is a layer of thick-walled cells called sclerenchyma, and these also multiply and take on lignin over time.  Peeling the asparagus does remove this layer and may reduce toughness a bit, even if the stringy vascular bundles are left behind.  The sclerenchyma sheath is there to support the much taller and more top-heavy shoot that our optimistic little spear was hoping to grow up to be.

Another challenge for us in the kitchen is that these stubborn asparagus stems don’t stop developing after they are cut.  The enzymes that turn sugar into carbon-rich lignin keep working, and tender spears straight from your local farmer can turn tough if you wait too long to kill them.  As it turns out, you can dramatically reduce the activity of these enzymes by putting spears in the refrigerator (Zurera et al. 2000) or depriving them of oxygen (Waldron and Selvendran 1990).  Refrigeration seems to work well enough that there’s no need to buy a fancy vacuum pump just to keep your asparagus fresh at home.  It is important, though, to avoid buying asparagus that may have been on display for several days and not kept cold.

The parts of an asparagus spear
Like many domesticated edible plants, asparagus spears can be hard to map onto our idea of what a plant looks like.  The spears are essentially stems whose leaves have been reduced to small scales by evolution and whose branches and flowers have been arrested by an early harvest.

In the post on artichokes, I described bracts as leaves that had been reduced in size, thickened, colored, or otherwise modified for another function. Bracts, however, are associated specifically with flowers or flowering branches and not with regular vegetative branches.  The tip of the asparagus is not a set of flower buds; consequently the thin little triangular leaves of asparagus are not bracts, but rather “scale leaves” that protect developing branch buds.  The tip of a growing asparagus spear is the part of the stem that will become branchy (if it is not harvested).  All those immature branches subtended by scale leaves are bunched together at the top like a new telescope, waiting to be extended.  I do not know whether all of the branches are in place at this stage, ready to spring forth, or whether still more nodes would be produced.  Maybe someone out there knows?

Lacking proper photosynthetic leaves, asparagus makes sugar with its stems.  In some species (e.g., asparagus ferns, which are not ferns but are asparagus), the stems have become flattened and look like leaves.  Leaf-like stems in general are called cladodes.  It was recently shown by Nakayama et al. (2012) that the African asparagus fern pulls off this trick by expressing leaf-identity genes in its stems that give them a top and a bottom side.  Garden asparagus (A. officinalis) has very thin cylindrical cladodes that look less leafy, although they are often described as fern-like (meaning leafy).  They also express leaf genes, but instead of gaining a top and a bottom side, the spatial pattern of gene activity makes them “bottom” all the way around and thus cylindrical.

Asparagus fern with flattened branches called cladodes.  Click to enlarge.

You can see immature cladodes in your kitchen among the closely spaced scales at the tip of an asparagus spear.  The leaf scales that make up the lowermost part of the fat tip only partially hide what would become a large and very feathery branched structure.  Those branches would eventually bear flowers, but because garden asparagus is dioecious, they would be either pollen-bearing (“male”) or fruit-bearing (“female”) flowers.

Illustration of garden asparagus. From Wikicommons; image is in the public domain.

Sweet pee and 23 and me
Although I’m taking us out of the kitchen here, it’s hard to resist mentioning what asparagus does to the smell of your pee.  A byproduct of asparagus digestion is methanethiol, a sulfurous gas.  Some people like the smell, some hate it, and some claim not to smell it at all.  A few years ago, some geneticists at 23andMe, a personalized genomics company, looked for a genetic signal of asparagus anosmia, or the inability to smell the methanethiol in pee.  The researchers found a variable region of DNA on human chromosome 1 that contains several genes for olfactory receptors and is significantly associated with the ability to smell asparagus pee.

In the end, I have to think that this lingering essence of asparagus is yet another example of its tenacious spirit.  Hope springs eternal.  And like every tender hope, it takes a tough core to make it stand up.

References

UPDATE: New York Times writer Elaine Sciolino just published a piece on white asparagus and the French who love it.   I did not talk about white asparagus here because it is not popular.  Indeed, I see it most often in Paris where it has (oddly) been some of the toughest, least edible asparagus I have ever had.  I’m normally there in July or August, though, when it is not in season and probably coming from a long distance.  On the other hand, the thinner-than-a-pencil green spears found in most Paris markets in the summer are amazing.

Eriksson et al. (2010) Web-Based, Participant-Driven Studies Yield Novel Genetic Associations for Common Traits. PLoS Genet 6(6): e1000993. doi:10.1371/journal.pgen.1000993

Nakayama, Yamaguchi, and Tsukaya (2012) Acquisition and Diversification of Cladodes: Leaf-Like Organs in the Genus Asparagus. Plant Cell 24(3): 929–940. doi: 10.1105/tpc.111.092924

Waldron and Selvendran (1990) Effects of maturation and storage on asparagus cell-wall composition.  Physiologia Plantarum 80: 576-583.

Zurera et al. (2000) Cytological and compositional evaluation of white asparagus spears as a function of variety, thickness, portion and storage conditions. J. Science Food Agriculture 80: 335-340.

How to make an artichoke: the facts about bracts, part 1

Inspired by spring and the appearance of both artichokes and asparagus, Katherine explains artichoke morphology in the first of two posts about bracts and scales.

Artichokes don’t exactly look like food, and their name in English is homely and offputting.  The scientific name is no better.  Cynara cardunculus variety scolymus rolls off the tongue like a giant ball of tough spiny bracts.  I’m not ready to call it an onomatopoeia, even though artichokes are giant balls of tough spiny bracts.  And the word “bract,” on its own, is just flat-out ugly.  But artichoke bracts have delicious meaty bases, and they protect the tender inner part of the bud which we call the heart, so I am a C. cardunculus var. scolymus bract fan.

Bracts are basically leaves that have been modified in some way, often made tougher, sometimes thicker, occasionally thinner and colorful.  They subtend (stick out below) a flower or cluster of flowers.  In most species they quietly and dutifully protect flower buds, but sometimes they join the flowers as part of the overall display.  The hot pink parts of bougainvillea “flowers” and the bright red “petals” of poinsettias are showy bracts.  Artichokes have two kinds of bracts: involucral bracts (also known as phyllaries) are the giant green scales making up the bulk of the artichoke; receptacular bracts, or chaff, are tiny, but they contribute to the irritating character of the choke.  We delve into more bract details in the section on artichoke evolution.

What is an artichoke?

Artichokes are in the sunflower family, the Asteraceae, along with dandelions and daisies and lettuce and chicories and thistles and all the other composites.  The edible part of an artichoke is a gigantic “flower” bud that develops on a large spiny Mediterranean thistle.  I qualify “flower” because the bud contains more than just a single flower.  If allowed to live out its natural life, the bud would open to reveal hundreds and hundreds of tiny long and thin rose-lavender colored tubular flowers.  Some nice images can be found here and here.  All of those flowers are clustered into a flat-topped composite head where together they mount an awesome display that attracts bees collecting both nectar and pollen.  Numerous ecological studies on many different species have shown that extra-large floral displays generally attract more pollinators than small ones do.  Some flowering plant species make single large and showy flowers, but many many species group their smaller flowers into inflorescences.  Inflorescences in the Asteraceae are particularly well-organized, and some members of the family go so far as to mimic single large flowers.  For example, the heads of sunflowers and daisies have an outer ring of flowers with extra-long tongue-shaped tubes of petals that look very much like the whorl of petals around a single flower.  The rest of the head is covered with short-petaled flowers that function primarily in reproduction.  By contrast, artichokes and other thistles have one kind of flower throughout the head, and all flowers serve to attract and reproduce.  In the Asteraceae, the head is properly called a capitulum.

The sunflower family is one of the largest families of flowering plants, with over 20,000 species.  Within the family, artichokes are closely related to cardoons – actually varieties of the same species – although cardoons are cultivated for their “edible” leaf midribs.  (Anyone interested in cooking cardoons should consult Deborah Madison’s excellent new cookbook, Vegetable Literacy.  It makes me perversely happy that she seems as skeptical as I am that cardoons are worth the trouble to prepare.)

How to make an artichoke if you are Evolution

One way to understand the structure of an artichoke is to imagine how it evolved from simpler and more familiar structures – specifically, the kind of inflorescences found in the families most closely related to the sunflower family.  The likely evolutionary transitions have recently been described by Pozner et al., based on careful observation of developmental morphology.  Their paper includes some very useful diagrams, and interested readers with academic library access should definitely track down a copy using the link below* .  My description here is more conceptual and omits some details but is essentially what they report.

If you were Evolution setting out to make a composite head, you would start with the raw ingredients available to you, which would include a flower-covered stalk, or an inflorescence.  Imagine that it is the kind of inflorescence found on some orchids (the kind a co-worker gives you for your birthday) or a larkspur or even a weedy mustard plant.  On these inflorescences, the oldest flowers are at the bottom, and new flowers continue to develop at the top.  The flowers are arranged around the stem like steps in a spiral staircase, and each flower is subtended by a small leaf or bract.

Now, if your flowering stalk has any little side branches with flowers, get rid of them.  Next, take Evolution’s handy opposable thumb and forefinger and grasp the stalk just under the lowermost flower.  Push all the flowers up towards the top of the stem so that they are clustered closely together.  Thicken up the flower-covered part of the stem so that it is fleshy, widest at the bottom, and tapering towards the top.  You now have a fat cone covered in flowers, which may even look a bit like the center of a black-eyed Susan.  Keep expanding the lower part of the cone so that the inflorescence becomes wider and flatter and finally takes on the shape of a shallow dish.  We can now call the “dish” what it is: a receptacle.  The receptacle, along with a very thick stem, provides mechanical support for the inflorescence, serves as a site for nutrient storage and transfer, and will also eventually be delicious for humans to eat with butter.  Thanks, Evolution.

But all of those tiny flowers will need to be protected from bumps, desiccation, and hungry animals until they are ready to open, and bracts are the perfect solution.  As explained above, artichoke bracts are modified leaves, made tough and thick and tipped with a sharp curved spine.  As Evolution, you will want to surround the receptacle and its flowers with several rows of tightly overlapping bracts, which botanists will call an involucre (“IN-VO-luker”).  The bracts will stay closed around the receptacle until the flowers are ready to be presented to pollinators.  When they spread to allow the flowers to appear, they will still serve as a defense against herbivores.  Because the bracts are derived from leaves, they are arranged on the stem in a Fibonacci spiral * , and this pattern becomes even more apparent when they are closely packed at the top of the stem.

Stiff hairy pappus, which replaces the sepals in a composite flower. The pappus becomes a parachute that helps disperse the fruit.

If the center of the artichoke – the choke part – is nothing but flower buds, it shouldn’t be irritating, and yet it is.  A second set of bracts, the receptacular (chaffy) bracts, are partly to blame.  Recall that flowers on an inflorescence are generally associated with bracts, and artichoke flowers follow this rule as well.  Their bracts are small but particularly stiff and bristly.  The flower buds themselves are also part of the problem.  The petals of the flowers are soft, but the sepals have been modified into a collection of long gag-inducing hairs called a pappus.  Thanks, Evolution.  This pappus is easy to see on a puffy white mature dandelion head.  Long after the petals have fallen, the pappus remains attached to the ovary and serves as a parachute to carry the fruit long distances through the air.

How to make an artichoke if you are in the kitchen

Another way to understand the structure of an artichoke is to prepare, cook, and eat one.  There are more elaborate ways to cook an artichoke, but here I’ll simply describe how to clean and boil one.

1. Examine the whole artichoke and notice that the involucral bracts are arranged in a spiral around it.  Carefully note the sharp spines at the tips of each bract.

2. Lick your fingers to taste the bitter compound cynarin.  The bitterness will stay on your hands, knife, and cutting board, so be sure to wash all of these before preparing other food.

Artichoke stem in cross section

3. Cut the stem from the bottom of the artichoke to make a nice flat base.  Notice the pithy inner core surrounded by a ring of tissue containing many vascular bundles that feed the bracts and flowers.

4. Remove some of the smaller lower bracts, which are not as closely associated with the receptacle and don’t have as much delicious tissue at the base.

5. Use scissors or a knife to trim the spines from the tops of the bracts.  Notice how tough the bracts are.

6. Saw the upper third from the top of the artichoke.  Doing so gives you a good view of the spiraled layers of the inner bracts.

None other than the California Artichoke Advisory Board erroneously calls the bracts “petals.”  Although the inner bracts may look like the unfurling petals of a rose, this is an alarming mistake.  I would not take advice from this organization on anything.

7. Gently spread the bracts under running water to flush out any hidden creatures or their frass (poop).  If you find frass, look carefully for the source in case it has not abandoned your artichoke.

8. Place the artichoke in a steamer basket over simmering water for about 40 minutes, until the bracts are soft and pull easily away from the receptacle.

9. Eat the artichoke by pulling off the bracts, dipping their fleshy bases into browned sage butter or salted oil or aioli, and pulling the dipped flesh through your teeth.

Although artichokes are notoriously ill-suited to wine, many experts recommend pairing them with an acidic dry white.  I like to sip on Navarro gewürztraminer because of its distinct citrus notes.

10.  Once you get down to the smallest inner bracts, they may come off in a cone.  Dip the base of the cone and eat this.

Raw choke base showing Fibonacci spirals

11. Examine the choke.  Run your finger over the hairs to appreciate why you wouldn’t want to eat them.  Remove the choke by drawing a table knife gently across the choke.  This will pull the flower buds and receptacular bracts away from the receptacle leaving (in spots) the impressions left behind by the ovaries.  Notice that the ovary scars form a Fibonacci spiral just as the bracts did.

12.  Trim any dark green, potentially bitter tissue away from the bottom of the artichoke and you will be left with only the receptacle.  Drag this through your butter, oil, or aioli, and enjoy the best part of the artichoke.

References

Pozner, Zanotti, and Johnson (2012) Evolutionary origin of the Asteraceae capitulum: Insights from Calyceraceae. American Journal of Botany 99:1-13.
http://www.amjbot.org/content/99/1/1.full

From the abstract: “Conclusions: Widely understood as a condensed raceme, the Asteraceae capitulum is the evolutionary result of a very reduced, condensed thyrsoid. Starting from that point, evolution worked separately only on the racemose developmental control/pattern within Asteraceae and mainly on the cymose developmental control/pattern within Calyceraceae, producing head-like inflorescences in both groups but with very different diversification potential. We also discuss possible remnants of the ancestral cephalioid structure in some Asteraceae.” Back

The fabulous Vi Hart explains the Fibonacci spiral http://www.youtube.com/watch?v=ahXIMUkSXX0 Back

Greens: why we eat the leaves that we do

Jeanne reveals which branches of the evolutionary tree of plants bear edible leaves and speculates about why that is.

Giant coconut palm (Cocos nucifera) fronds dwarf me

Most of the 300,000 + plant species have leaves, and the function of all of them is to perform photosynthesis.  They are the ultimate source for all of the oxygen and food for the rest of the food chain and help regulate the global carbon and water cycles.  They are also nutrition superstars.  To figure out why greens are good for you and whether all leaves are equal in this regard, we need to take quick look at global leaf structural variability and broad evolutionary patterns in the species that make their way onto our tables.

Leaves of sagebrush (Artemisia tridentata) are smaller than my fingernails

Across the global flora leaves are extraordinarily structurally variable.  They vary in size from a tiny duckweed leaf to a huge palm frond, and in thickness from a thin lettuce leaf to a thick leaf of a succulent aloe or pine needle.  We don’t fully understand yet why leaves are so structurally variable across species. My colleagues and I recently published a paper demonstrating that across the global flora leaf nutrient concentrations and physiological rates of maximum photosynthetic carbon gain and respiratory carbon loss change in proportion to leaf area more strongly than they change in proportion to leaf mass.  We demonstrate the consequences of this area-proportionality of leaf traits for how we measure, analyze and use these leaf traits in models of the global carbon cycle.

Microscope cross section of a privet leaf. Most photosynthesis is performed by the columnar cells of the palisade parenchyma, underneath the upper epidermis. Photo source unknown.

Of relevance here, however, is that the high area-proportionality of leaf traits simply means that the parts of the leaf that are most beneficial for humans (digestible protein, vitamins, minerals, carbohydrates, digestible fiber) are likely proportional in many species to the leaf’s photosynthetic apparatus, which is clustered in a relatively thin band of cells beneath the epidermis (see image).  Adding thickness to a leaf doesn’t necessarily add nutritious tissue and might instead add mostly fibrous, indigestible structural material (see agave photo below).  This does not mean that the thinner the leaf, the more nutritious it is; this is just a general tendency across all plants.  Scaled up to the level of the whole plant, though, we see that leaves have the highest ratio of nutritious material (digestible protein, vitamins and minerals, fiber) to indigestible material (for most species).  Some fruits probably come close, but fruit is rare, and leaves are abundant.

Blue agave (Agave tequilana) hearts ready to be turned into agave syrup and tequila, showing cross section of the base of the very thick (high LMA) leaves, which have been hacked off with a machete. The dark bluish-green outer leaf layer is where all the photosynthesis takes place and is where the nutrition in the leaf is primarily located, from the perspective of an herbivore. The white inner part of the leaves consists of structural material and storage. The inner fibers of the leaves are used to make rope! Photo source unknown.

One of the key variables modulating area- vs mass-proportionality of leaf traits is leaf mass per unit leaf area (LMA), which increases with leaf thickness and density.  Lettuce leaves have very low LMA, pine needles and holly leaves have high LMA.  Although there is very little data on the subject, the leaves that we eat tend to fall on the low end of the global LMA spectrum, shown here for a global dataset of more than 2500 wild species.

Histogram of LMA (g*m^-2) for over 2500 wild species (data from Wright et al. 2004). “Frequency” is number of species in the data with a given LMA.  Most species have relatively low LMA.  Thin leaves have low LMA (left side), thick and/or very dense leaves have high LMA (right side). LMA varies by 3 orders of magnitude across the global floral.

Leaf tenderness—reducing LMA–is certainly a trait that has undergone selection in cultivated varieties.  In species with varietals that have undergone selection on different structures, the varieties in which selection has been directed at the leaves invariably have more tender, less bitter, and larger leaves than do varieties in which selection has been directed at different structures, such as the roots.  For example, chard has larger and more tender greens than do beets (both Beta vulgaris, Caryophyllales); celery grown for stalks and leaves is more tender than the aboveground portion of the celery varietal grown for its root (both Apium graveolens, Apiales); and Siberian kale varieties have more tender leaves than do rutabaga varieties (both Brassica napus, Brassicales).

small, tough stems and leaves on celery root

Kale and collards, grown for their leaves, have leaves that are more tender than those of conspecific Brassica oleracea grown primarily for other structures:  kohlrabi, broccoli, cauliflower, Brussels sprouts (and for those of you keeping track at home, in the B. oleracea, Brassica, and Brassicaceae posts, I erroneously lumped all kales into Brassica oleracea—some of them are B. napus, including my favorite variety: Red Russian kale).  Don’t dare throw out your beet, celeriac, rutabaga or B. oleracea greens, though, if you’re lucky enough to get them—they need a bit more cooking or tenderizing, but they’re still delicious and nutritious.  At least put your celery root greens in stock.  LMA also varies within a varietal based on growing conditions.  Leaves grown in sun have higher LMA than leaves grown in shade.  The impact of growing conditions within species, however, will have to wait for another time.

LMA is hardly the only determinant of which species make it onto our plates.  Otherwise, we’d have our pick of hundreds of species on the low-LMA end of the global curve.  Instead, we can put most of our greens species on a fairly legible phylogenetic tree in need of only a few disclaimer paragraphs of omissions.

The greens tree: phylogenetic relationships among species whose leaves we eat. Taxon branches are shaded according to taxonomic order.  Click here for a refresher on reading trees and on plant evolutionary history.

Like many phylogenetic trees of more than a handful of species, this tree of our “greens species” commits sins of both commission and omission in the interest of producing a readable image. Omission because the species on the tree do not comprise an exhaustive list of species whose leaves people around the globe regularly consume. Commission because I included a few unusual items, such as sweet potato leaves (Ipomoea, Solanales), and pea shoots (Fabales), and a few taxa are single representatives of a large group.  Sister to sweet potato on the tree, for example, is the entire family Lamiaceae, the mints.

oregano, a mint

We mostly use leaves from the mints as herbs and spices:  peppermint and spearmint (Mentha sp.), basil (Ocimum sp.), thyme (Thymus sp.), oregano and marjoram (Origanum sp.), rosemary (Rosmarinus sp.), lemon balm (Melissa sp.), lavender (Lavandula sp.), shiso (Perilla frutescens), bee balm (Monarda sp.), and mountain mint (Pycnanthemum sp.).  Also, scallions are the only representatives on the tree from the genus Allium in the monocot order Asparagles, which includes many edible species, the leaves of some of which are regularly employed:  garlic, leeks, ramps, and onions.

I included some of the more exotic greens in the greens tree both to avoid rousing the ire of their champions and because I was a bit shocked that some large, charismatic orders that feature prominently in the diet at large are reduced to single entries on the greens tree or are gone altogether, meaning we primarily use roots, stems, fruits or seeds from these clades, not leaves.  Granted, this “greens tree” is mostly a tree of “greens obtained from Whole Foods and summer farmer’s markets in the continental United States.”

chickweed in the spinach

I’ve included a few weedy, wild things that I pick regularly and that one can actually occasionally pay money for at the farmer’s market:  nettle (Urtica dioica, Rosales), chickweed (Stellaria media, Caryophyllales), miner’s lettuce (Claytonia perfoliata, Caryophyllales), purslane (Portulaca oleracea, Caryophyllales), and dandelion (Taraxacum officinale, Asterales).  Largely, however, I’ve omitted species whose leaves are technically edible, at least in small doses at some point in their life cycle, but that are reduced to curiosities at the modern American table, not staples.  Therefore, please hold your fire if you’re shaking your fist at the tree because it omits the young beech (Fagus grandifolia, Fagales), sassafras (Sassafras albidum, Laureles), or violet (Viola odorata, Malpighiales) leaves you like to nibble on when you hike east of the Mississippi. Fern fiddlehead (assorted Polypodiales) enthusiasts, too, forgive me.  We use some leaves to prepare other food:  banana leaves and corn husks to wrap tamales, and palm fronds and fig leaves to wrap fish for baking or grilling.  I salute these leaves, but they’re not on the tree.  Some are gone simply because I didn’t have room, such as many of the crucifer greens, and asterids chrysanthemum, borage and comfrey (although those last two haven’t been placed to order yet and would therefore have been difficult to place within the asterids anyway).

Camelia sinensis–tea–in flower

I left off plants whose leaves are regularly consumed as infusions (“tea”), including, for example, the actual tea plant (Camelia sinensis, Ericales), yerba maté (Ilex paraguariensis, Aquifoliales), raspberry leaf (Rubus sp., Rosales), yerba santa (Piper sp., Piperales), blueberry leaf (Vaccinium sp., Ericales), conifer needles (Pinales), and oatstraw (Avena sp., Poales), each of which would add another order to our tree.  As with raspberry and blueberry leaves, I did not include some plants whose leaves we occasionally use but that are known primarily for the edibility of other structures, such as winter squash, peach (Prunus persica, Rosales), wheat (Triticum sp., Poales), citrus (e.g. kaffir lime and curry leaves, Rutaceae, Sapindales) or grape (Vitis sp., Vitales).

Enjoy sorrel leaves but beware the leaf blades of closely related rhubarb

I also did not include rhubarb (Rheum sp., Caryophyllales) as sister species to the sorrel in the greens tree as a public service.  The part of rhubarb that we eat, the stalk, is the petiole, which connects the leaf blade to the stem.  The rhubarb stalk is structurally similar to that of celery, another petiole.  Celery stalks, however, support delicious edible leaf blades, while rhubarb stalks support large, quite poisonous leaf blades, with incredibly high concentrations of calcium oxalate, which gives both rhubarb petioles and sorrel leaves their tang.

We technically eat the highly-reduced, vestigial, scale-like leaves of young asparagus shoots (Asparagus officinalis, Asparagales), but given that they are the plant equivalent of a human tailbone—not really useful but hard to evolutionarily get rid of—they don’t make the tree.  Same goes for cactus (Caryophyllales) paddles.  The cactus spines are technically the leaves, and the stem does all the photosynthesis and water storage.  So no nopales on our tree.

The greens tree is not phylogenetically random. It has lots of crucifers, like this Lacinato kale

Gone from the tree is the entire Solanaceae family—the nightshades—tomatoes, potatoes, eggplant, peppers, tomatillos, ground cherries, goji berries.  Their leaves are actually quite toxic.  Tobacco is in the Solanaceae, but I’m not about to include it in our G-rated greens tree.  The only representative on our tree of the order in which they nightshades appear, the Solanales, is sweet potato.  Gone too are the grass family (Poaceae) and all Fabaceae—the legumes—save pea shoots, and Rosaceae—the rose family—stone fruits, strawberries, rapsberries, blackberries, apples, pears, loquat.   Only nettle (family Urticaceae) is left in the order Rosales.  Gone are the nuts in the Fagales, the spices in the Zingiberales, the blueberries and cranberries in the Ericales, the coconut and date palms (Arecales).

cardoon (Cynara cardunculus), one species swelling the ranks of the Asteraceae greens

So given this level of culling, how well represented is the global flora by the leaves that we consume?  It depends on the taxonomic level of analysis.  If we look at species, then the greens on the greens tree plus the mentioned omissions together obviously represents a tiny sliver of global plant species diversity.  Even if we multiply the number of species mentioned here by 40, to account for some or all of the global greens consumed that are unknown to me, we still would only account for 1% of global plant species diversity.  At the family level, we see that the greens tree has 15 families, but that most of the greens regularly consumed in the Western world are from only five of the 415+ families of seed plants currently recognized: Amaranthaceae (goosefoot family), Apiaceae (the carrot family), Asteraceae (the sunflower family), Lamiaceae (the mints) and Brassicaceae (the mustard family).

Parsley (Petroselinum crispum, Apiaceae)

Therefore, the greens tree is far from a random sample of the branches of the phylogenetic tree of the global flora–a few clades dominate the greens tree.  If we restrict our analysis to the 76 orders of land plants currently recognized, 35 orders adorn our tree of the orders with edibles (below), 11 of which are highlighted in color on the greens species tree, and an additional 10ish could have been included (some of the omissions listed above, in grey on the orders tree).

Plant taxonomic orders with edible species. Orders from the greens tree are shaded with the same colors as in the greens tree. Orders containing most arguably illegitimate omissions from the greens tree are shaded grey.

So, between about a quarter and a third of land plant orders include species whose leaves appear in some form in, say, the annual diet of an exceptionally adventuresome American vegetable enthusiast.

Whether a quarter or a third sound high or low is up to you, but the upper limit is surely dictated in part by LMA, in part by geography, and part by the other stuff packed into and onto leaves:  defense traits.  Do those species that make it into our diets as primary ingredients in salads, soups, frittatas, stir-frys and sauteés have low(ish) LMA? Surely. LMA is largely irrelevant, however, for those species that we use as teas or ground spices or juice (e.g. wheatgrass).

Some, perhaps most, leaves in the Western diet are due to geographic chance.  In Guns, Germs and Steel Jared Diamond makes the case that certain plants on the Eurasian continent, like wheat, were particularly easy to domesticate and could provide enough nourishment to fuel cities and armies, contributing the Europe’s colonization effectiveness.  Many greens in our diet are Mediterranean and likely just tagged along for the ride.  I speculate that there is little overlap between the greens that I eat and the greens consumed by groups of people whose diets are relatively unaffected by European contact.  I would love to know where their leaves fit on the greens tree.  Then we could make inferences about the relative strengths of the contributions of both human culture and plant biology to our greens species selections.

Lettuce (Lactuca sativa) has low LMA and milky latex defense compounds

All leaves, however, are defended, either by physical or chemical weaponry that renders the leaves of 67-75% entire orders of the global flora totally inedible.  Physical weaponry includes thick, brittle epidermises (think holly, Ilex, sp., one of which is yerba maté), fuzzy or prickly hairs, chemical-rich hairs (nettle is disarmed by drying or cooking), or high silica crystal concentrations, as in leaves of grasses (Poaceae), which likely contributed to the evolution of large, heavy, well-enameled molars in grazing ungulates.

Once the spines are removed by cutting off the leaf perimeter with scissors, weedy milk thistle (Silybum marianum, Asteraceae) leaves make an excellent salad.

Chemical weapons, called secondary compounds, are toxic or otherwise deterrent compounds in or on the leaves and are as diverse as they are abundant across the global flora. They tend to be somewhat phylogenetically conserved, at least at the level of taxonomic family or order, and so contribute to some similarity of flavor among closely-related species.

Try as an exercise to eat leaves within families serially for a few days and then switch families to develop a gestalt for the differences among clades (we’ll fill you in on the biological details in good time).  Start with your Amaranthaceae: beets sauteéd with their leaves in butter one night, chard the next night, spinach or amaranth or lambsquarters greens the third.

Arugula

Switch to the crucifers and try creamed kale (recipe below from chef friend of the blog Michelle Fuerst), cabbage slaw, and arugula salad.  Switch to a few days of meals with dishes dominated by the Apiaceae:  parsley in tabouleh, a cilantro-heavy salsa, a celery-fennel salad, a generous helping of chopped dill mixed into Greek yogurt on top of roast carrots and parsnips (also Apiaceae).  Hit up your mints:  Potatoes roasted with rosemary, butternut squash roasted with thyme, a quinoa salad seasoned with mint, beans flavored with oregano or marjoram.Dive into the Asteraceae:  try a few different lettuces, radicchio and endives in salad, dip some steamed cardoon leaves in a bagna cauda sauce, steep tarragon in some butter and dump it on…anything.  Get more ideas for cooking within plant families  in Deborah Madison’s amazing new book, Vegetable Literacy, a cookbook dedicated to 12 plant families.  The book’s associated blog has excellent ideas on preparing greens.

Creamed Kale with Leeks (recipe by Michelle Fuerst)

2 medium leeks (about 3/4 pound)

2 bunches of black kale (about 1 pound)

2 tablespoons of butter

1/2 cup of cream

Slice leeks in half and rinse under water to remove  the dirt between the layers. Trim the ends of the leeks to remove the root end. Cut the leeks into a small dice, about 1/2 inch wide.  Strip the kale off of the stems and plunge leaves in a large bowl of water. Discard the stems. Remove leaves from water, shake off excess water, but don’t dry. Rough chop the kale into bite size pieces.  In a medium saute pan, over medium heat, add butter. When melted, stir in leeks and coat with butter. Season with salt and cover pan. Cook for about 5-8 minutes until leeks are softened.  In two batches, add the kale. After the first batch begins to wilt, add the second and continue to cook until all the kale is wilted, about 3-4 minutes.  Add cream and cook until cream is reduced and kale is tender, about another 8-10 minutes. Taste and adjust seasoning with salt and black pepper.

Optional:  instead of leeks, use red onion, and when you add the cream, add half a cup of sliced dried cranberries and a tablespoon or grated orange zest.

Cucurbita squash diversity

Jeanne introduces the diversity of some American natives, the squashes in the genus Cucurbita.

Spring is officially here, and I have squash on my mind.  We’ve ordered zucchini seeds for the upcoming summer garden but still have acorn squash from the fall sitting in the pantry (both are varieties of Cucurbita pepo). Our winter vegetable CSA box recently bequeathed to us the tastiest winter squash I’ve ever eaten, a Seminole pumpkin, which is a different variety of the same species (Cucurbita moschata) as the butternut squash sitting on the counter, destined for dinner.  Now between last year’s hard winter squashes and the tender summer squashes to come seems a good time to remind ourselves of the origins and diversity of squashes in the genus Cucurbita.

Seminole pumpkin

Four hundred years ago, stored C. moschata would certainly have been a springtime diet staple of the Native American tribes in what is now the southeastern United States, including the Seminole.  The squash that bears their name grows as a sprawling plant with the huge palmate leaves and curling tendrils typical of cucurbits, members of the gourd family (Cucurbitaceae), which also includes cucumbers, melons and inedible gourds and natural loofah sponge. Like many other cucurbits, Seminole squash plants will spread over the ground, and more than most other cucurbits, it can grow as a vine (some Cucurbita varieties resemble a bush more than a vine and do not climb).  Accounts of early Spanish explorers to Florida record their surprise at seeing Seminole squash fruits hanging from trees over rivers or decorating dead oaks that the Native Americans used as trellises to farm the squashes.  Native Americans traditionally sliced and sun-dried strips of the squash for transport and storage.  The large, nutritious seeds, however, were as important a food source as the sweet flesh.

C. moschata vine

Seminole pumpkin vines have a reputation as particularly resistant to the abundant pests in its swampy native environment, and the hard outer shell on this grapefruit-sized squash helps keep the fruit impervious, for up to months.  Pathogens and insects still affect it, but if left to its own devices in a swamp, it would fare better than the average butternut from the seed catalog.  In the absence of a hard winter freeze, roots can develop from nodes in contact with soil, continuously producing new plants and fruit.  Some authors attribute the climbing tendencies, ability to self-perpetuate through adventitious roots and self-seeding, and pest-hardiness of the Seminole variety of C. moschata to occasional hybridization with other wild squashes native to the Florida swamps, such as C. okeechobeensis (Morton 1975).

A vining cucurbit (snake gourd–not sure of the species)

Hardiness of the plants is usurprising given the means by which the cucurbit lineages arrived in the Neotropics.  The family Cucurbitaceae arose near modern-day India in the late Cretaceous.  The leading hypothesis for how cucurbits migrated from the Asian subcontinent to every other continent except Antarctica is transoceanic long-distance dispersal.  That is, the hardy squashes floated across the ocean, or the seeds were carried in the guts of birds that flew them from A to B.  Ancestral cucurbits made the journey from Africa to South America approximately five times, where their descendants radiated into around 350 modern species.  The genus Cucurbita split from its sister clade (Peponposis) between 9 and 23 million years ago in Central or South America.  Between then and the onset of American agriculture ancestral Cucurbita expanded from Central to North America around seven times (Schaefer et al. 2009).

Long Island cheese pumpkin variety of C. moschata on the left, various C. pepo on the right

Those northern radiations of the genus resulted in three highly variable species that constitute the vast majority of pumpkins and other squash consumed in the world:  Cucurbita pepo, C. moschata, and C. maxima.  Two additional Cucurbita species, C. argyrosperma (cushaw) and C. ficifolia (fig-leaf gourd), are cultivated in a few areas.  C. pepo includes most varieties of the round, orange squashes called “pumpkins” sold for Halloween Jack-O-Lanterns, although all Cucurbita species have some varieties with names including “pumpkin,” such as the amazing variety of C. maxima called “Amish pie pumpkin” and the “Long Island Cheese Pumpkin” variety of C. moschata.  C. pepo also includes zucchini, yellow summer squash, pattypan, delicata, spaghetti squash, and acorn squash. C. maxima includes the Hubbard, turban, and kabocha squashes.  C. moschata are the butternuts.

Amish pie pumpkin (C. maxima)

All Cucurbita species are native to the Americas, a dozen or so species with scattered wild distributions, mostly in Mexico.  Domestication of at least five Cucurbita species predated European exploration of the Americas by several thousand years.  C. pepo may have been domesticated in the Americas ten thousand years ago, around the same time that wheat was domesticated in the Mediterranean.

Kabocha (C. maxima)

Remains of C. moschata have been identified from abandoned Native American settlements from the southwestern United States (from 1500-2000 year BCE) and Guatemala (from around 900 AD).  The species likely arose in Central America and from there spread south, where its seeds have been found in centuries-old Peruvian tombs, and north, with corn and beans. European explorers introduced American Cucurbita to the rest of the world starting in the early 16^th century.  Climatic preference differences among Cucurbita species influences their modern distribution around the globe’s agricultural regions.  C. moschata prefers hotter, more humid conditions than its congeners.

summer squash and zucchini (C. pepo)

Like cultivars of Brassica oleracea, distinct named cultivars of these Cucurbita squash species have been developed through selective breeding over centuries, and most are totally interfertile within a species, and the species do occasionally hybridize.  Some cultivars, especially the zucchini, yellow summer squash and vegetable marrow cultivars of C. pepo, have been developed for their immature fruit, meant to be picked when the skin is still soft, inner flesh still firm, and seeds small and tender, before maturation of the hard outer shell and hollow interior full of stringy endocarp.

Compost pile volunteer C. pepo (presumably) vine, probably produced by the hybridization of two commercial varieties grown by the farmer from whom we purchased a squash the previous summer.

Most of the dizzying number of squash varieties, however, have been developed under selection for various characteristics of the mature fruit, including texture, flavor and quantity of the firm flesh; shell color; fruit shape; and long-term storage stability.  At least one variety in Mexico was developed primarily for the mature seeds—pepitas—a staple of Oaxacan cuisine (if you’ve never put toasted hulled pumpkin seeds on your salad, do it, especially if you’ve soaked the raw seeds in a bit of chipotle-maple marinade before toasting them).

Much is known of the genetic basis of the tremendous variation among Cucurbita species in fruit shape (long like a zucchini, crookneck-shaped, butternut-shaped, spherical-ish, disk-shaped, deeply ribbed like acorn squash); shell lignification (becoming hard and woody instead of staying mostly soft, like in zucchini); color (white, green, mottled, striped, orange, yellow, bluish); size (think mini pumpkin vs prize-winning 1800-pound monsters); beta carotene content (carotenes—Vitamin A—make the fruit of most winter squashes yellow to orange); and many other complex traits (Paris and Brown 2005).

C. pepo

A gene unique to C. pepo that I think is especially interesting is called “sp.”  When a hapless C. pepo inherits two copies of the recessive allele (gene variant) of the sp gene from its parents, the fruit flesh will have “spaghetti” texture, breaking into long strands when cooked (Paris and Brown 2005).  Hence, the name for “spaghetti squash” or “vegetable spaghetti.”  As cultivars of C. pepo, pumpkins, acorn squash, and over-ripe zucchini can also be stringy.  If you cook up your Jack-O-Lantern pumpkins after Halloween, you might have to put the flesh through the blender to puree the strings.  The anatomical basis of spaghetti texture is not well known. My literature search on the topic only turned up one conference paper (Ishii et al. 2011) that demonstrated that the strands disassociate because of the breakdown during cooking of pectins that help glue cells together.

Spaghetti squash (C. pepo)

The strands, however, are hundreds of cells thick, so the mechanism causing particularly soluble pectins at regular intervals, and how it could be related to the product of gene sp, is unknown.  We also know that the fruit becomes more stringy as the fruit ages and matures, so strand formation is probably also related to fruit maturation. What I do know is that I am one of many squash enthusiasts who strongly prefers the flavor and firm texture of C. maxima or C. moschata over that of C. pepo.  It’s no mystery why most canned pumpkin is C. maxima.  It makes better pies. I think any recipe that calls for pumpkin, be it soup, pie, or Moroccan tagine, would be best prepared using either a C. maxima or a C. moschata instead of a C. pepo, although delicata squash seems to be the most flavorful and least stringy of the C. pepo group, but it would take a lot of delicata to produce as much pulp as an average butternut.

Zucchini flower

If you grow squash plants for the fruit, go easy on the flowers.  The edible Cucurbita species have separate male and female flowers, both of which are large, the size of tiger lilies.  Most gardeners do not need all of their male flowers to ensure pollination of the female flowers (by bees and necessary for fruit set).  Extra male flowers can be picked as soon as they’re open, stuffed, battered, and fried as delicious fritters.  If you’re particularly enthusiastic about eating squash blossoms, you can make proportionately more male flowers by damaging leaves prior to flower development.  In plants that produce unisexual flowers, like squash, leaf herbivory has been shown to bias the sex ratio of flowers on a particular plant to favor male flowers, which require fewer resources to produce than female flowers (Krupnick et al. 2000).  Of course, you might just end up with fewer flowers overall because leaf damage depletes the plant’s energy and nutrient reserves.  C. ficifolia is sometimes cultivated for salad greens as well as fruit and seeds, but if maximizing fruitset is your goal, I’d leave the leaves alone.  I suppose the big leaves of other Cucurbita species are edible, but they might be bitter, due to the high concentrations of exceedingly bitter cucurbitacin defense compounds.  If you are familiar with bitter melon (Momordica charantia), a south Asian cucurbit, then you are familiar with cucurbitacins.  Sometimes you can taste cucurbitacin bitterness in the rind of cucumber or slightly immature hard winter squashes.

Bitter melon

Tonight’s butternut, though, looks plenty ripe.  It will get cubed, tossed in melted butter and olive oil with some salt and roasted.  Ever since we ate the Seminole squash, roasting C. moschata makes me think of vines dripping over steamy Florida swamps. Maybe you too will now see American history in your squash.  Or maybe I just need a tropical vacation.

References

Fritz, G.J. 1994. Precolumbian Cucurbita argyrosperma spp. argyrosperma (Cucurbitaceae) in the Eastern Woodlands of North America. Economic Botany 48: 280-292.

Ishii, K., A. Teramoto, H. Kuwada, Y. Jibu, M. Tabuchi, Y. Kimura, and M. Fuchigami. 2011. Relationship between pectic substances and strand separation of cooked spaghetti squash. http://www.icef11.org/content/papers/aft/AFT594.pdf

Morton, J.F. 1975. The sturdy Seminole pumpkin provides much food with little effort. Proc. Florida State Hort. Soc. 88:137-142.

Paris, HS, N Yonash, V Portnoy, N Mozees-Daube, G Tzuri and N Katzir. 2003. Assessment of genetic relationships in Cucurbita pepo (Cucurbitaceae) using DNA markers. Theoretical and Applied Genetics 106: 971-978.

Paris, H. S., and R. N. Brown. 2005. The genes of pumpkin and squash. HortScience 40: 1620-1630.

Schaefer, H., C. Heibl, and S. S. Renner. 2009. Gourds afloat: a dated phylogeny reveals an Asian origin of the gourd family (Cucurbitaceae) and numerous oversea dispersal events. Proceedings of the Royal Society B 276: 843-851.

Maple syrup mechanics: xylem, sap flow, and sugar content

It’s maple syrup making time in the Northeast.  Jeanne explains the mechanics of sap flow, collecting sap for syrup making, and why maples are special in this regard.  

Proctor maple research field station, Underhill, VT

I had the great pleasure last weekend to visit the University of Vermont’s Proctor Maple Research Center in Underhill, VT, where the sugar maple (Acer saccharum, Sapindaceae) sap is flowing.  Sugar maple trees all around the northern hardwood forests in the Midwestern and Northeastern United States and southeastern Canada can now be “tapped,” fitted with a hollow tube in the sapwood, out of which sap flows and is collected and boiled down to maple syrup.

Tapping sugar maple to collect sap, Proctor research station, Underhill, VT.

Maple syrup might be the oldest agricultural product in North America. Early 17^th-century written records from Europeans exploring North America describe Native American use of sugar maple sap.  We of course can’t know how Native Americans discovered sugar maple sap, but it may have been by sampling a “sapsicle,” icicles made from frozen maple sap that forms at the end of a broken twig.  The evaporation of water during ice crystallization partially concentrates the sugar in the sap, making the sapsicle particularly sweet.  To understand how that sap got to the end of the twig in the dead of winter and why it’s so sweet, we need some basics about plant vasculature and carbohydrate storage and must figure out what makes maples so special.

Sugar maple canopy leaf sampling

On a hot day in summer, a big sugar maple tree with its crown at the top of the forest canopy might move around 200 liters of water from the roots to the evaporating surfaces of leaves, 30 meters above the forest floor.  Mineral nutrients from the soil, along with sugar and hormones and other physiological constituents manufactured or stored in the roots or stems sometimes also rise dissolved in the water and collectively form the sap.  The sap ascends from root to leaf in the xylem.

Xylem out of xylem: vessel structure sculpture, Kew Botanic Gardens, UK

Xylem consists of tracheary elements through which sap flows and the living and structural tissues (parenchyma and fibers) that surround and support them. Tracheary elements—made of vessels in angiosperms and tracheids in gymnosperms and more evolutionarily basal vascular plants—form a network of interwoven pipes, made of dead, hollow cells, joined end-to-end. Phloem is another set of pipes made of living cells that moves solutions within the plant with the aid of metabolized energy.  The xylem and the phloem together comprise the vascular system in most land plants and evolved not long after plants colonized land.

Tracheary elements sculptures, Kew Botanic Garden, UK

Sap in the xylem moves passively, in response to physical forces, pressure and osmotic gradients.  Three primary processes can cause xylem sap to flow:  transpiration, root pressure and stem pressure.  In order to turn carbon dioxide into sugar through photosynthesis, the leaf must open its stomata (small pores on the leaf surface) so that carbon dioxide can diffuse into the leaf.  In doing so, however, the leaf must inevitably lose water by evaporation out of the stomata.  This water loss is called transpiration and creates tension, or negative pressure, in the water in the xylem.  Water moves up through the vessels to replace water lost through transpiration, in response to this pressure gradient. Transpiration during photosynthesis therefore pulls water up through the plant from the soil.  When photosynthesis is done for the day, tension relaxes on the column of water in the xylem, and vessel pressure approximately equilibriates with that of soil water at the root surface.

Positive root pressure causes early spring sap flow in paper birch (Betula papyrifera)

Sap might also rise in the absence of photosynthesis because of root pressure.  Sometimes a pressure gradient develops between the soil water and roots, usually because of high concentrations of minerals and organic compounds in the roots when photosynthesis is inactive.  Water then flows into the roots and produces positive root pressure, pushing sap up through the xylem.  Root pressure-generated sap flow in trees increases as the soil warms in the spring and ceases only when leaves develop enough to begin transpiring, thus eliminating day-time root pressure.  Positive root pressure may still develop at night during the growing season and is partially responsible for dew production.

Pressure gauge attached to tap in sugar maple used in vacuum-assisted sap collection, Proctor research station, Underhill, VT.

Maple sap flow during the leafless season is physiologically unique in that it is largely independent of root pressure and only occurs on occasions between October and April when warm days follow freezing nights.  Maple winter sap flow is caused by pressure in the stem generated by alternating daily cycles of night freezes and warm days.  Cool evening temperatures generate negative pressure from the dissolution of gases in the xylem, which were seeded in from adjacent parenchyma and intercellular spaces.  The negative pressure replicates the effect of transpiration, which draws still-liquid water from the soil into the roots.  As the night freeze deepens, water freezes along the inner walls of the hollow fiber cells adjacent to the xylem and in intercellular spaces.

Cross section of xyelm in a sugar maple twig (2-years-old), 10x magnification. The large holes are vessels; the dark radial bands are rays

Eventually vaporized water on the surfaces of all cells freezes. The ice formation compresses and traps gases in the stem.  The heat of the day melts the ice and causes expansion of the compressed gases, which generates positive pressure in the stem that pushes the sap up the stem and out the nearest exit, if one exists, such as a maple producer’s spile.  Applying vacuum pressure to the tap allows a maple producer to collect up to three times the normal amount of sap, and doing so has been the industry standard since the late sixties.

Tubes run from the vacuum device to the tapped sugar maples and carry sap to the collection location, Proctor research station, Underhill, VT.

Because it depends on weather (and water availability) instead of spring leaf flush, maple winter sap flow is intermittent and highly variable from year to year.  In any given season, a maple producer may have the opportunity to collect sap on one or ten or more daily “runs” of sap.  The process occurs in all maples and sycamore (genera Acer and Platanus) and some other species, including butternut and walnut (Juglans spp., Juglandaceae) that spend the winter with water-filled vessels with air-filled fiber cells adjacent to the vessels.  Most other species in the northern woods (including willow (Salix), aspen (Populus), elm (Ulmus), ash (Fraxinus), and oak (Quercus)) do not exude sap and spend the winter with gas-filled vessels and water-filled adjacent fibers.  Some tree species (see below), most notably the birches (Betula), will exude sap from a spile in the later spring in response to positive root pressure.

Walnut (Juglans) has maple-like late winter sap flow

The sugar in the sap stream is from carbohydrates stored in the living ray parenchyma and fiber cells adjacent to the vessels in the xylem and is mobilized in late winter and exuded into the xylem sapstream to fuel flowering and leaf expansion (maples flower before they leaf out).  Most of the carbohydrates in the rays are actually stored as starch grains, and an enzyme released into the xylem throughout late winter converts the starch to sucrose and mobilizes it in the xylem.  Sap sugar content also varies within a sap flow season, peaking in the middle. Therefore, there is quite an art to the timing of the tree tap.  Producers should tap right before the peak sugar content sap flows.  If they tap too early, the tap site might dry out, and they will collect mostly lower sugar content sap.

Maple syrup is mostly sucrose, but the maple xylem sap stream also contains glucose, inorganic salts, protein precursors (peptides and amino acids), some enzymes, and a few mystery organic compounds.  The distinctive maple flavor is still something of a mystery but is variously attributable to the organic constituents of the sap and is amplified by heating.  That is why maple syrup tastes differently from syrup from other trees, such as birch, or from syrup made from other plants, like sugarcane or sorghum (molasses is made from the combined xylem and phloem sap from both species).

Riverside yellow birch (Betula alleghaniensis) in the sun are good candidates for sap collection

Sugar maple is not the only tree species from which xylem sap is collected for consumption.  Many species of birch trees (Betula spp., Fagaceae) are tapped for sap throughout the northern hemisphere.  Sometimes the sap is boiled down for syrup, and there’s a growing birch syrup industry in Alaska, Russia and Scandinavia.  Often, however, the birch sap is drunk or bottled as is, as a refreshing tonic.  In North America sap is also sometimes collected from species in the Juglandaceae family (hickories and walnuts) and basswoods (Tilia spp: Tiliaceae).  I suppose one might be able to extract sap from conifers during periods of root pressure sap flow, but this might be practically unfeasible because the collected sap might be inevitably contaminated with resin, the sticky pitch that visibly oozes from conifer wounds (which is not sap, even though it is sometimes called that).  Throughout the tropics sap is collected from the crowns of palms for miel de palma (“palm honey”) and palm sugar.  Maple species other than sugar maple also apparently make delicious maple syrup but have lower sap sugar contents, so one needs more sap to make syrup.

I’ve read that some people have made early spring syrup from basswood (Tilia americana).

This list begs the question of whether any sufficiently large tree can be tapped for sap.  The short answer is no.  Whether a tree is worth the tap depends on geography and how much work you’re willing to invest and what flavor of syrup you’re willing to settle for.  Sap can be collected from any tree in which sap flows from positive root pressure in the spring or from any maple (including sycamores) or walnut/butternut (Juglans) growing in a place with abundant soil water that experiences daily freeze-thaw cycles in late winter.  The list of candidate trees is therefore winnowed down by first by location. The remaining questions to ask of your tree are: (1) will enough sap volume with (2) a high enough sugar content be collected to make it worthwhile to boil it down for syrup; and (3) what will the flavor of the syrup be when it’s done?

Measuring volume production by sugar maple trees, Proctor research station, Underhill, VT.

The volume question depends on the size of the tree, the diameter of the xylem storing water in the trunk (minimum 12” in diameter for a sugar maple, around a 40-year old tree), and more critically, on the water availability to the tree and the weather.  For sugar maples, the larger the temperature difference between the night freeze and the daily thaw and greater the number of days in a season with big freeze-thaw cycles, the greater will be the volume of sap collected.

The sugar content question is economically critical.  To make syrup, sap is boiled down until it is about 2/3 sugar.  This means that if the sap has 2% sugar, as is typical for sugar maples, it takes 40-43 gallons of sap to make one gallon of syrup. Birch sap usually hovers around 1% sugar, so it takes over 80 gallons of sap to make one gallon of syrup.  Some syrup producers run their sap through a reverse osmosis machine to remove some of the water from the sap to concentrate the solution before the boiling begins, so that less time and fuel will be necessary to reach the syrup stage.  The higher the sugar content at the beginning of boiling, the lighter and more delicate will be the resulting syrup when it hits 66 or 67%, because the sugar will be less caramelized.

Dark Grade B maple syrup

That is the difference between Grade A maple syrup and Grade B.  More boiling, more heat, was necessary to produce Grade B because the sugar in the starting solution was more dilute.  All the other stuff in sap, though, that contributes to the maple flavor, may or may not be more dilute in the sap destined for Grade B syrup, so Grade B might have a more intense maple flavor than the lighter Grade A.  I actually always buy Grade B for this reason.

Northern hardwood forest in Wisconsin with shaded sugar maple saplings in understory

The sugar content depends on the species of tree and how much sugar it was able to store in its trunk over the winter.  Shade tolerant species, such as syrup star sugar maple, tend to store more sugar in their trunks than do shade-intolerant species, such as aspen.  If a sugar maple canopy sees enough sun and has enough water and nutrients in a summer to photosynthesize more sugar than it needs for its immediate, perhaps pre-programmed determinate, amount of growth and maintenance, it stores the extra sugar in the ray parenchyma cells in its sapwood, adjacent to the xylem.  A shade-tolerant tree seems to always expect to have all or part of its future canopy shaded, either by itself or a neighbor, so it saves sugar to survive the lean times.

Quaking aspen (Populus tremuloides) incursion into a sunny abandoned field

Shade-intolerant, species, however, have more of a “can’t take it with you when you go” philosophy, and quickly burn through much of their photosynthate by growing as much as they possibly can during the season.  They’ll store enough sugar in their roots and stems to support respiration over the dormant season, provide frost hardiness (sugar is antifreeze), and to fuel budburst and leaf expansion or maybe even reproduction in the following year, and they might even hedge their bets by storing enough to fuel healing after injury or pest attack.  The end result, though, is less stored sugar, so less sweet sap. So, even if sap can be collected from relatively shade-intolerant trees, which is sometimes the case, as with paper birch (Betula papyrifera), the sap will have a low sugar concentration relative to sugar maple sap, so it will take much longer to boil it down to make any kind of a syrup out of it.  The sap of even a shade tolerant tree, however, might be too dilute to merit syrup making if the stored sugar doesn’t make it into the xylem sap when the sap could be profitably collected before budbreak.  Such apparently is the case with American beech (Fagus).

Red maple (Acer rubrum) also makes good syrup but flowers earlier than sugar maple

Budbreak, the opening of the flower or leaf buds on the aerial shoots in the spring, requires the addition to the xylem sap of additional hormones, amino acids, defense compounds, and other physiological constituents that are either stored or largely manufactured in the roots and sometimes stems.  According to most palates, these budbreak precursors in sap negatively affect the flavor of syrup produced from all tree species.  Consequently, even if sap is still flowing, sap collection abruptly ends at budbreak for anyone wishing to avoid “buddy” syrup.  Also, if you’ve got your eye on roadside maples for tapping and are particular about your syrup flavor, beware de-icing salt.  Some studies  have demonstrated that road salt in winter not only hurts the trees, because salt is toxic to most plants, but also imparts a bitter, salty flavor to the syrup made from sap of roadside trees subjected to salt.

Sugar content of sugar maple sap is highly variable across trees, as a consequence of genetic variation among individuals, soil fertility and water availability of the growing site, and previous year’s photosynthetic success. Sugar maple sap sugar content varies from less than 1 to over 10% but usually averages around 2 or 3%. More water and soil fertility and sun exposure is better for sugar yield.

Sugar maple branch growing into a sunny forest gap

Interestingly, then, if you wanted to plant a plantation of sugar maples, you’d have to space your trees far more widely for optimal syrup making than for optimal harvest of the highly-prized sugar maple timber.  Wider spacing allows the branches to sprawl out and take advantage of the most sun per leaf, which maximizes seasonal sugar storage, whereas close spacing creates competition for light and causes the trees to invest in tall, straight trunks in their race to the top of the canopy, at the expense of stored sugar.

Herbivore damage on sugar maple leaves

Drought stress or pests or diseases, especially defoliating ones, can also decrease seasonal sugar storage.  A bad summer for the trees will lead to a bad spring for the sap collector.  As long as there is water in the soil, there may be enough sap volume to collect, but the sugar reserves in it will be low.  Research is ongoing into the genetic variation underlying sap sugar content and involves an effort to identify particularly “sweet” trees and propagate their offspring.  Particularly high-sugar-yielding trees might have more and larger xylem rays than low yielding trees.

The hole in the sugar maple trunk from the tap heals in about 6 weeks.  While the tap is in place and after it is removed, diseases could enter the tree through the hole.  While the amount of sap removed by the syrup producer, accounting for an average 10% of stored sugar, will not mortally harm the tree, it does reduce its vigor a little bit and may make the tree slightly more susceptible to diseases during the healing period.  Therefore, the conscientious syrup producer should seal the tap wound with an appropriate compound.

Young sugar maple stand, Proctor research field station, Underhill, VT

Sugar maple trees like it cold, and repeated freezing temperatures are necessary for a good spring sap flow.   The current range of sugar maples is centered in the northern part of the Midwestern and Northeastern United States, with fingers of the tree’s range extending south to high elevation (coldish) sites. The range of sugar maple in North America is projected to shrink and move north, almost entirely out of the United States, over the next century in response to global climate change.  The last few warm winters in the Mid-Atlantic and Northeastern U.S. have not been kind to maple sugar producers.  So, if you love your maple syrup—not to mention the northern hardwood forests where sugar maples live—be sure to educate yourself about climate change and climate policy.  

References

Pallardy, S. G. 2008. Physiology of Woody Plants. Third Edition. Academic Press.

Mohan, J. E., R.M. Cox, and L. R. Iverson. 2009. Composition and carbon dynamics of forests in northeastern North America in a future, warmer world. Canadian Journal of Forest Research 39: 213-230. 

Valladares, F., and U. Niinemets. 2008. Shade tolerance, a key plant feature of complex nature and consequences. Annual Reviews of Ecology, Evolution and Systematics 39: 237-57.   

A very close look at potato leek soup

To understand how potatoes behave in the stock pot, Katherine puts a favorite soup under the microscope – literally.

Potato leek soup is the perfect soup. It is heaven pulled from the ground in all its humble grassy beauty. Potato leek soup is good-looking, simple, and flexible. It can be made vegan and provides nutrients and fiber with few calories. It is cheap, scales up for a crowd, and freezes well. Plus you have to love a soup with more names than ingredients. As a comforting wintertime staple, we call it what it is – potato leek soup. In tiny cups, sprinkled with chopped parsley and freshly ground black pepper, it becomes potage Parmentier, a rich tasting but delicate entrée to an elegant dinner party * . Chilled, with fresh cream, it is Vichyssoise, the cool, light partner of a good baguette and a glass of Pouilly-Fumé on the patio in summer. And my mother-in-law has demonstrated many times that when the holidays overwhelm your fridge, you can store a huge pot of potato leek soup on the porch overnight – as long as you put a brick on the lid to keep the raccoons out.

This amazing soup is the just about easiest thing in the world to make. Julia Child’s version is probably the most widely used, and the one I like: simmer equal parts cubed potato and sliced leek in water until they are tender. Add salt to taste and puree. A bit of cream is optional. A dusting of chopped parsley and freshly ground black pepper is divine. I like to err on the side of more potatoes than leeks, but the soup is robust to variations in proportion.

But is it really so easy? If you trust the internet more than you trust your favorite dog-eared chocolate-spattered cookbook with the broken spine and decades of marginalia (silly you), you may worry that without the right kind of potato and extremely careful handling, your soup will end up gluey. Is any wallpaper not pre-pasted these days? Doubtful, but everyone seems to describe gluey potato soup or mashed potatoes as “wallpaper paste.” I will say right up front that gluey soup has never happened to me, but given all the stress over this utterly simple soup, it seemed worth investigating.

Types of potatoes and types of starch
The many varieties of potato can differ a lot in flavor and texture, so which should we use in our soup? Julia Child uncharacteristically abandons us here in Mastering the Art of French Cooking, although in a later book (The Way to Cook), she does suggest using ordinary scaly brown baking potatoes. Mark Bittman explicitly says that “any kind” will do, whereas Alton Brown calls for Yukon gold, or a similar round and moist variety. America’s Test Kitchen calls for russet baking potatoes but does not explain why.

Potato varieties vary in both the amount and the combination of starches they contain, which has a big effect on their texture. Ordinary baking potatoes, such as the ubiquispud Russet Burbank, contain a lot of linear starch molecules that are hard to break apart. This makes them fluffy and dry and good for baking and making French fries. Fingerlings, red potatoes, and other “waxy” varieties contain less starch overall and a higher proportion is the branched sticky form. They absorb less water and stay firm when cooked, making them good roasted and in potato salad. Yukon golds are sometimes called all-purpose potatoes because they perform fairly well under both conditions.

Table 1

Starch is the main molecule plants use for storing energy within or between growing seasons. Starch is found throughout a plant body, even in green leaves, but it is most highly concentrated in seeds and specialized storage organs like potato tubers. Plants build starch molecules by stringing together the smaller glucose (sugar) molecules they make through photosynthesis and packing them tightly and efficiently into granules. When a plant needs sugar to fuel metabolism, it can just break down some starch and transport the sugars to the active cells. For example, when a barley seed germinates, enzymes digest the starch into glucose and maltose to be used by the growing seedling or to be toasted and brewed into beer.

As mentioned above, starch comes in two forms, which normally co-occur: long coiled chains called amylose and shorter branched chains called amylopectin. The linear chains of amylose can be packed very efficiently into a small amount of space; but branchy amylopectin is about three to four times as abundant as amylose in most plants, and it is more readily converted back into sugars. The cook can easily see the differences between these starch types by comparing rice varieties: long grain rice has a relatively high proportion of amylose chains, and consequently it takes more water to cook and stays drier than sticky sushi-type rice, which contains almost all branchy amylopectin. (Sticky rice is sometimes called “glutinous,” although it contains no gluten proteins.)

Starch granules, stained with iodine and viewed at 100X magnification. Cell walls are hard to see here.

Starch molecules are packed into starch granules, which look a bit like freshwater pearls, layered and slightly irregular. Staining the granules with iodine turns them inky blue and makes the alternating layers of amylose and amylopectin more visible. In living plants, the granules are surrounded by membranes (forming amyloplasts) and float like jelly beans inside large thin-walled storage cells. When I stained raw starch grains from amylose-rich baking potatoes and amylose-poor fingerlings, I could not tell any difference between them, even at 400X magnification.

Russet starch granule

Waxy starch granule

Does starch type matter? Experimental attempts at gluey soup
Because I’d never made gluey soup, I was excited to try. Internet lore suggests that potato leek soup texture depends less on the potato variety than on the violence with which you handle your soup after it is cooked. Rough handling with a blender or food processor is said to damage the starch granules and let too much starch leak out into the liquid to gum things up. Authors suggest using a nice gentle potato ricer or a hand-cranked food mill, as one does for mashed potatoes and finicky gnocchi.

But I was interested comparing potato varieties, not handling technique. I thought that russets would make the thickest soup because of their high total starch content and their relatively greater proportion of amylose. As Hal McGee explains in his book On Food and Cooking, long amylose molecules are more effective thickeners than branchy amylopectins. But being thick is not the same as being gluey, and whether russet-based soup was also more vulnerable to becoming wallpaper paste was an open question.

Russets and waxy potatoes may differ slightly in starch ratios, but they can also vary dramatically in flavor. What’s the point of worrying over texture if your lovely soup doesn’t taste right? At many farmers markets, there are dozens of oddly shaped and colored waxy potato varieties whose complex flavors really sing as part of a roasted vegetable medley. By contrast, ordinary scruffy brown bakers can be hard to find. A farmer who does sell russets told me that nobody buys them, so only a few farmers bring them to our local market. Apparently people find them boring. Would snooty varieties make better tasting soup?

To find out, I bought several types of potato and treated them as violently as possible. I got Tater-tino on those spuds. The idea was to compare potatoes at the extreme, so I pushed them as far as I could towards being gluey. Then to see whether the starch granules did break down under this treatment, I put the purée under the microscope.

The (loosely defined) “experiment” tested three types of potato: russet, a waxy round red kind, and a yellow-fleshed variety I thought would be intermediate. To estimate relative total starch, I replicated the 11%-salt-water-floating technique used by America’s Test Kitchen.  This was fun, but it didn’t turn out as planned.  The allegedly dense russet floated, bobbing alongside the waxy red potato, while the intermediate yellow variety sank. I decided to cook all three but focus my comparison on the two floaters.

For each variety, I grated 3 oz of potato flesh into 9 oz of water (by weight). The russet immediately started to oxidize and turn pink. I brought the mixtures to a serious boil and held them there for 30 minutes, stirring until the potatoes were very well cooked. Then I put each batch separately into a mini food processor and liquified the living daylights out of it.

In theory, here’s what was happening inside the potato bits as they were boiled and then processed. When the water temperature reached about 135 – 150 ºF (55 – 65 ºC), the starch granules inside the cells started to gel. The hot water worked its way into the granules, disrupting the tight packing, and turning the small grains into big bags of water-logged starch networks. With continued heating, the granules started to leak starch molecules into the water where they could bind one another into a loose mesh, trapping water and any remaining granules. As it cooled, the mesh became stronger because there was less disruptive heat energy to keep the molecules from binding. Recall that because of their shape, amylose molecules contribute more to the thickening than amylopectin molecules do.

Now for the violent part. Blending the potatoes into a very fine purée was supposed to break open the already swollen and leaky starch granules, allowing the rest of the starch to pour out into the water and take part in the gelling. If rough handling really leads to a gluey texture, then all those liberated starch molecules must be to blame.

Table 2

The results are shown in Table 2.

First, I failed utterly to produce a gluey purée. The russet did form a very thick mixture, but it was not at all gummy or sticky. It was rich feeling and just a tiny bit grainy. The color verged on a very pale grey. The waxy soups were much thinner and greener in color. They were very smooth and slippery, and even almost slimy in a pleasant way. The snooty potatoes were a little snotty.

Second, even a long hot boil and sustained whipping did not damage all of the starch granules. Both the russet and the round red potato still had plenty of starch granules visible under the microscope. My rough measurements suggest that the granules had swollen to nearly 40 times their original volume. The food processor blades did manage to destroy essentially all of the thin-walled storage cells that make up a tuber, but tough fragments of some of the reinforced water-carrying xylem vessels were still floating around.

Waiter, there’s a vein in my soup (dark fragment in lower right)! Unstained russet purée at 100X magnification, showing three starch granules.

My most important observation was probably the dramatic difference in taste between the purées. Russets tasted like potatoes, earthy and rich, with a bit of umami. The waxy potatoes tasted grassy and herbaceous and even slightly sweet, with a bit of vanilla flavor. Salt didn’t help much. To be fair, I had stored all of the potatoes in the fridge for a couple of weeks, which does cause some starch (especially amylopectin) to be broken down into sugar. Nevertheless, it is hard to imagine ever getting a deep potato flavor from these waxy varieties. Pairing them with leeks would push the soup even farther in the green direction.

These observations suggest that russet potatoes make the best tasting potato leek soup, but that adding a few waxy potatoes might enhance the smoothness and brighten the color. I’m also not going to abandon my immersion blender in favor of a ricer, but I respect anyone willing to go to the extra trouble just to be on the safe side.

The Vichyssponge
Earlier I claimed that potato leek soup freezes well, and it absolutely does. It’s the thawing that can be a problem. In the photo below, that chunk of soup in my hand is at room temperature all the way through. It has become a squeezable starch sponge due to a process called retrogradation. At low temperatures, starch molecules (especially amylose) line up and bond together in a crystalline structure, which remains even when the soup is thawed. The water is expelled from between the molecules, leaving just the spongey part behind. McGee notes that starch retrogradation also causes bread to go stale and long-grain rice to get hard in the refrigerator. Once the soup (or the bread or rice) is reheated, the starch dissolves again, and it regains its lovely texture.

The Vichyssponge

It turns out, then, that contrary to internet opinion, it is possible to redeem your potato leek soup if somehow it becomes gluey. Ladle some into a freezer bag, freeze it flat, thaw it, and pour off the water. Remove the congealed starch from the bag, sprinkle one side of it with baking soda, and use it to clean your sink. Voilà, the ultimate compostable sponge.

*M. Parmentier himself was a bit of a wild card at dinner parties. He is known to have pulled outrageous stunts in polite society, such as serving Ben Franklin a potato at a fancy dinner to demonstrate that they were not poisonous. Back to top

For more information:
http://www.foodproductdesign.com/articles/1996/01/understanding-starch-functionality.aspx
McGee, H. (2004) On Food and Cooking: The Science and Lore of the Kitchen. Scribner, New York.

A look at leeks

FLUELLEN …and I do believe your majesty takes no scorn to wear the leek upon Saint Tavy’s day.
KING HENRY V I wear it for a memorable honour; For I am Welsh, you know, good countryman.
Shakespeare, Henry V, act 4, scene 7

Break out the daffodils and leeks!  This past Friday, March 1, was St. David’s Day, the feast day of the patron saint of Wales and a national holiday in that country.  As long as you are cleaning and slicing leeks, let’s take a quick close look at the vegetable, one of the national symbols of Wales.

Leeks, along with onions and garlic, are in the genus Allium in the family Amaryllidaceae.  Like many other monocots, they have long slender leaves striped with parallel veins.  Also common in monocots, and sometimes confusing, is the way their leaves develop.  The white part of a leek looks like a big solid cylinder composed of concentric leaf tubes, and essentially it is.  Each new leaf develops at the center of the cylinder in the form of a tube that surrounds the next little mound of tissue that will develop into a leaf.  This is easy to see if you slice the root end of the leek in half as close to the center as possible and examine the innermost leaves.  In mature leaves, the top part of the tube opens up into a flat green blade.

The pattern of leaves enclosing leaves at the base is evident in many monocot houseplants, such as peace lilies and orchids.  It is also easy to see in corn plants as well as onions and garlic.

A somewhat blurry image of contractile roots.

One of the most fun aspects of leek morphology, however, is their contractile roots.  Look closely at the roots, and you will see that many are ribbed horizontally like a pushed-up sleeve or a tubular accordion.  Such ribbed roots have already done their job, contracting to pull the plant down deeper into the soil.  No wonder those resourceful leeks were chosen as a national symbol.  As Fluellen says later on in the play, “Ay, leeks is good.”

If you have any leftover leeks, please save them for our post on potato leek soup.

Pomegranates and the art of herbivore attraction

Jeanne walks you through the botany you need to know to understand pomegranate fruit structure.  Jeanne’s definition of “need to know” is arguably a bit broad and includes a brief tour of the many different structures plants modify in order to entice herbivores, and at least one goddess, to disperse seeds. 

pomegranate fruit (persistent calyx and stamens visible)

Pomegranates (Punica granatum, family Lythraceae, rosid order Myrtales) were one of the earliest domesticated plant species.  According to ancient Greek mythology, they even predate the seasons.  The story goes that Hades, god of the underworld, kidnapped his beloved Persephone, daughter of Demeter, goddess of the harvest.  Demeter’s grief over Persephone’s disappearance caused the crops to wither and wreaked havoc with humanity.  The plight of the starving masses coerced Zeus to convince Hades to return Persephone to her mother.  Before she left the underworld, however, Hades tricked her into eating a pomegranate seed, which bound her to evermore spend part of the year with her happy mother, during which time plants flourished, and part of the year in the underworld, during which time plants go fallow. Thus, seasons arose.

Pomegranate seeds

We can hardly blame poor Persephone for finding pomegranate seeds irresistible.  They look like faceted jewels and have a refreshing, tangy sweetness and a satisfying crunch. We have an additional reason to be drawn to pomegranates: even if they can’t help us understand the seasons, deciphering the structures of the beautiful pomegranate fruit helps us understand the diversity of mechanisms plants use to entice animals, including humans, to disperse seeds.  The delicious, nutritious or fibrous attractive structure is payment for the animal’s labor. As you will see in this post, there is no single anatomical recipe for creating the colorful, fleshy and/or juicy reward for a seed-dispersing herbivore, mortal or otherwise.  Many of the myriad flower, fruit and seed structures are variously promoted to the role of what is colloquially thought of as “fruit.”

Pomegranate, halved

Botanically, the juicy, often ruby-red part of the pomegranate seed is a sarcotesta, a modification of the outer epidermal cell layers in the seed coat (the testa). Pomegranate fruits often split when they are quite ripe, exposing the bright red sarcotesta to hungry birds and, apparently, goddesses. There is a common misconception that the juicy part is an aril, not a sarcotesta. An aril is an outgrowth from a particular point on the seed, like one of its ends, or of the funiculus, which attaches the seed coat to the ovary wall in the fruit and is the plant equivalent of an umbilical cord. The fluffy fibers of cotton (Gossypium sp.; Malvaceae) are arils.  So are the delicious parts lychee (Litchi chinensis), longan (Dimocarpus longan), and rambutan (Nephelium lappaceum) fruits from the Sapindaceae. The red aril surrounding the seed of the gymnosperm conifer yew (Taxus) is technically edible–sweet but bland, I’m told–but everything else on the yew plant, especially the seed inside the aril, is deadly poisonous in minute amounts to most creatures except birds and deer. The genus name Taxus comes from either taxic or toxic, which in Greek means “that in which arrows are dipped.” I can’t fathom why the thing is a common landscaping plant. Maybe it’s those pretty arils. Speaking of pretty arils, the spice mace is a lacy aril around the nutmeg seed, forming a two-for-one spice from Myristica fragrans (Myristicaceae).

Arils, clockwise from upper left: fluffy cotton arils; Sapindaceae fruits with translucent fleshy arils: lychee (left, peeled and unpeeled), longan (center front, peeled), and rambutan (center back peeled, right unpeeled) (photo by Erin Kurten); red aril on yew seed (photo by Didier Decouens); red lacy mace aril around nutmeg seed within fruit (photo by Brocken Inaglory)

Some arils are particularly rich in lipids (fats) and are called caruncles or elaiosomes.  While birds and mammals are happy to avail themselves of these, they are particularly attractive to ants.  An ant drags the well-protected seed by its elaiosome down into its underground nest.

Ants drag bloodroot (Sanguinaria canadensis) seeds by their elaiosomes (photo by Alexander Wild, http://www.alexanderwild.com)

Colony ants eat the elaiosome and put the seed itself on their underground compost pile, along with dead ants and other organic waste generated by the colony.  In doing so, the ants have done the seed a great favor by planting it underground in a big pile of fertilizer and guarding it from other would-be seed eaters.  Some invasive plant species, such as leafy spurge (Euphorbiaceae) in the United States, employ elaiosomes, and their march across their new frontier is likely aided by ants.  This mutualism with ants, called myrmecochory, is so successful that it has been mimicked by walking stick insects (Diapheromera sp.; Phasmatodea).  Walking stick insects are so named because they resemble twigs or leaves on the plants that they eat.  Their eggs resemble plant seeds complete with a fatty mimic elaiosome, called a capitulum.   Just like a seed with an elaiosome, the ants haul the egg down into the nest, eat the capitulum, and throw the unharmed egg onto the compost pile.  When it hatches on the pile, the young walking stick looks and smells like an ant, and it thereby finds its way out of the nest unharmed and heads to the nearest suitable plant.

In angiosperms the herbivore-attracting structure is usually a fruit, in both the botanical and colloquial sense, as we explain below, but arils, sarcotesta and modifications of other floral structures are quite common. Arils and sarcotesta have independently evolved numerous times in most extant major clades of both angiosperms and gymnosperms.  Myrmecochory mediated by lipid-rich elaiosomes alone has independently evolved between 101 and 146 times during plant evolution and is present in 17% of angiosperm plant families (Lengyel et .al. 2010). Sarcotestal seeds pop up in a few gymnosperms and in 17 angiosperm families on far-flung branches of the plant phylogeny (Werker 1997).  Recall that the evolution of flowers defines the angiosperms, so gymnosperms don’t have flowers or fruits (see our exceedingly brief history of plant evolution for a refresher if necessary).  The name gymnosperm means “naked seed,” as the seeds are simply borne on cone scales or stalks and lack a protective fruit.  Therefore, a fleshy aril, as in Taxus, or sarcotesta, as in Gingko biloba, are among a gymnosperm’s limited options for producing a nutritious reward for an animal seed disperser.

Gingko biloba seed with fleshy sarcotesta (photo: Oregon State University Dept. Horticulture)

Gingko’s sarcotesta is full of foul-smelling butyric acid, which also forms when butter goes rancid, so woe to any person who walks on a sidewalk lined with female gingko trees when the seeds drop.  Many animals, however, don’t seem to mind the odor and are likely drawn by it. The sarcotesta of familiar angiosperm papaya (Carica papaya) seeds, though, adds a pleasant sweetness to a papaya seed vinaigrette.  The seeds themselves have a peppery mustard flavor, giving away the membership of the papaya family (Caricaceae) in the Brassicales order, named for the mustard family (Brassicaceae).

papaya seeds

To make the vinaigrette, cut open your ripe papaya, save out a few seeds to impress your dinner companions, and scoop out the rest of the seeds and throw them in the blender (or mortar and pestle) with lemon juice, salt, and olive oil (and an optional egg yolk and/or an anchovy filet for a papaya seed Caesar).  Exact proportions of ingredients depend on your preferences and the size of your papaya. The high frequency with which fleshy sarcotesta and arils pop up in the global flora demonstrates the evolutionary old age and relative lability of both animal dispersal of seeds and the structural devices plants use to attract said dispersers.

Strictly speaking, the anatomical definition of a fruit is the structure that develops from the flower’s carpel, or gynoecium, and contains the seeds, which develop from the ovules in the ovaries (see diagram of flower structure):

labeled generalized flower diagram (source: Andromeda Botanic Gardens)

The wall of the ovary stratifies into different tissue layers of the pericarp of the fruit.  Katherine described this stratification in stone fruits: the exocarp is the peel, the thick mesocarp is the sweet pulpy part, and the endocarp is the hard outer layer of the “stone” of the fruit, protecting the seed inside.  The endocarp of other fruits may be soft instead of stony, like much of the pulpy, stringy tissue surrounding seeds in squashes (Cucurbita sp.) or papaya (Carica papaya) (some of these strings are funiculus). Some plants bear what are called multiple fruits, in which the mature edible structure is constructed by the fusion of several fruits developing from multiple ovaries within a single flower (as in raspberry or blackberry) or from multiple individual flowers arrayed on an inflorescence (fig, mulberry or pineapple). If any part of the flower other than the ovary wall becomes all or part of the enticing structure, the result is called an accessory fruit.

The receptacle supporting the flower often becomes all or part of the sweet, attractive accessory fruit, but it has different ways of doing so across the global flora.  Often, the receptacle, along with tissue from the calyx (the whorl of sepals), gets roped into the animal attractant role simply because it protects the ovary, so as the ovary wall grows into a fruit, the receptacle around it must grow too, and become fused with the pericarp on the inside and something pretty to look at for an animal on the outside.  This is the case for plants whose flowers have an inferior ovary.  The use of ‘inferior” here is physically descriptive and is not a commentary on quality:  the ovary is physically located below the calyx whorl of sepals, buried within the receptacle (see diagram).  Contrast this with a “superior” ovary and intermediate cases.

Ovary position diagram: I: superior ovary; II: semi-inferior ovary; III: inferior ovary; receptacle (r), sepals (s), petals (p), gynoecium (g), and anthers (a) labeled (source: Wikipedia)

squash: the hard outer shell is receptacle fused to exocarp; the flesh that is cooked is mesocarp, and the stringy endocarp is scooped out

Many familiar plants develop charismatic fruits from inferior ovaries.  In bananas, squashes and melons, the receptacle tissue simply fuses to the exocarp and does not become sweet and juicy itself.  In contrast, the fleshy, red part of the wintergreen (Gaultheria procumbens; Ericaceae) “berry” is the calyx and receptacle surrounding the dry capsule fruit.  In many pome-type fruits, like apples and pears (Pyrus sp. and Malus sp., respectively; Rosaceae), the receptacle becomes fleshy, as does part of the pericarp underneath it.  The place where the two tasty tissue types fuse can sometimes be seen in a cut apple (see picture).  In pomes, the endocarp is the leathery “core.” The remnants of the calyx or even the anthers and stigmas often remain on these accessory fruits (see apple and pear picture) from inferior ovaries.

Apple development, from flower to fruit. Blue circle around sepal and persistent calyx on apple and pear; orange arrows pointing to juncture of receptacle and pericarp (diagram from Raymond Milewski, East Stroudsburg University)

The receptacle may become an attractive accessory fruit even if the flower has one or more superior ovaries.  Strawberry (Fragaria sp.; Rosaceae) flowers have multiple superior ovaries, each with its own stigma and style.  The red fleshy part of a ripe strawberry is the receptacle.

strawberries

As the receptacle grows and expands into the red strawberry, it pushes the ovaries apart from each other, and each ovary develops into a little fruit called an achene. These achenes are commonly perceived as “seeds” on the surface of the strawberry, but they are bonafide little fruits, each with a single seed inside it, fused to the achene wall.  Often the style is still attached to the achene, which you can see as a little thread.

cashew (photo by Forrest Fleishman)

Most Americans are familiar with cashew (Anacardium sp.) nuts, but few are familiar with the curious structure that bears them.  The juice from the “cashew apple” or marañón, is popular all over the tropics, where cashew trees are widely grown. The fleshy, pear-sized red or yellow marañón (see picture) also develops from the pedicel and receptacle.  The actual cashew fruit is a green, inedible drupe (like a stone fruit) that dangles from the end of the marañón and is shaped like the cashew nut inside it.  The green drupe tissue surrounding the cashew nut is utterly inedible due to its high content of anacardic acid, which is chemically similar to the better-known urushiol, the irritant oil from poison ivy, poison oak, and poison sumac, which are in the same family as cashew (Anacardiaceae), along with mango and pistachio.  The irritant must be removed chemically or with fire before the cashew nut can be eaten.

Now you have almost all the information you need to interpret pomegranate structure.

Pomegranate (P. granatum) flowers: pollination by hummingbird; closeup of numerous stamens; diagram of internal structure and prominent thick stamens that will remain on the fruit

The persistent calyx from the flower (see flower photos above) on the ripe fruit is particularly conspicuous in pomegranate, and sometimes inside it you can still see the numerous anther filaments and the style (see fruit photo below). Therefore you know that pomegranate fruit develops from an inferior ovary, and the tough, red outer rind is the receptacle fused to the pericarp. The arrangement of the sarcotestal seeds within the fruit, however, is a bit unique. To understand why, it is useful to look at the simpler wild pomegranate, P. protopunica, which is the only other species in the genus Punica and the putative ancestor of cultivated P. granatum. P. protopunica is wild only on the island of Socotra, part of the Republic of Yemen in the Indian Ocean. The earliest (Bronze Age) archaeological records of the genus are from the Arabian Peninsula, the closest mainland to Socotra. The fruits of P. protopunica are smaller, structurally simpler, and have less sweet seeds than the cultivated species. At some point during the domestication of P. granatum, the number of layers (whorls) of carpels (chambers of the ovary) went from one, as in P. protopunica and the vast majority of plants, to two or three.

Punica protopunica flower and fruit, P. granatum fruit (Picture modified from Richard Wettstein – Handbuch der Systematischen Botanik (1924) – Permission granted to use under GFDL by Kurt Stueber. Source: http://www.biolib.de)

The illustration above shows a cross section of P. protopunica flower and fruit (in the green box) and fruit of P. granatum with two layers of carpels.  Often, however, a cross section of the real P. granatum won’t look as perfectly grid-like as this illustration (see photo below), as the seeds in different regions of the fruit develop at different times, which twists the entire internal structure of the fruit.

Pomegranate halved longitudinally with digram of cross sections of upper and lower carpel whorls. Notice the persistent calyx and stamens. Diagrams from Watson and Dallwitz (1992).

The lower whorl has up to three carpels with axial placentation of the seeds, meaning the placenta tissue from the pericarp is in the middle of the fruit, whereas the upper whorl has up to 6 carpels with parietal placentation, meaning the ovules develop from the outer and lower parts of the carpels.  The white membrane that covers the tops of the seeds is the septum that separates the carpels.  The seeds are embedded in spongy white tissue from the pericarp called placenta, which is structurally very different from the placenta in mammals but serves a similar function.  The septa separate easily from the tops of the seeds, and the seeds are most easily and least messily removed from the pericarp in a bowl of water.  The seeds will sink, and the pericarp will rise.

As the daughter of the original botanist, presumably Persephone knew all of this, even if she didn’t know that being a diligent seed disperser would land her in the underworld.

Many special thanks to Missouri Botanical Garden (www.mobot.org) curators Shirley Graham and Roy Gereau for extraordinarily helpful conversations about pomegranate and Sapindaceae fruit structure, respectively.

References

Lengyel, S. et al. (2010). “Convergent evolution of seed dispersal by ants, and phylogeny and biogeography in flowering plants: a global survey”. Perspectives in Plant Ecology, Evolution and Systematics 12 (1): 43–55. doi:10.1016/j.ppees.2009.08.001.

Watson, L., and Dallwitz, M.J. 1992. The families of flowering plants: descriptions, illustrations, identification, and information retrieval. Version: 19th December 2012. http://delta-intkey.com.

Werker, E. (1997). Seed Anatomy. Borntraeger, Berlin and Stuttgart, Germany.

Welcome to 2013, the International Year of Quinoa

Can we call quinoa a grain? Why do people care?  Where did all these geese feet come from, and what does Ban Ki-moon have to do with it?  On long winter runs, Katherine’s mind wanders over such questions.

Quinoa seeds (Chenopodium quinoa)

In the final two months of 2012, questions about quinoa and its status as a “grain” came up three separate times within my earshot.  This was odd in itself, but it launched a cascade of coincidences.  On a run near the baylands, my mind was idling back over those conversations, when I noticed for the first time a little weed along the trail, looking much like one of quinoa’s relatives, a saltbush.  (The crushed specimen I carried home in my shoe laces keyed out as Atriplex semibaccata, Australian saltbush.)  There is also a gorgeous and much larger saltbush species along the trail, and yet another relative, an edible Salicornia species (“sea beans”) that fills the marshy areas next to the bay.  Along with quinoa, spinach, beets, and chard, all of these species belong to the (former) goosefoot family – the Chenopodiaceae – which is now considered a branch nested within the Amaranth family.  Quinoa is a central member of this old family, belonging in the namesake genus Chenopodium.

Click to enlarge. Relationships among edible species in the Amaranthaceae, part of the order Caryophyllales. Relationships and characters based on Judd et al. (2nd ed) and the Angiosperm Phylogeny Website ver. 12.

Just as I was feeling surrounded by goosefeet on the trail, I ran into a merry band of actual avian Canada geese, who clog up the trail this time of year and leave goose footprints in the mud.  This seemed like a convergence worthy of the Mayan calendar (also much discussed in December of 2012), but I dismissed it, since quinoa was domesticated by people who lived much farther south, in the Andean highlands.   It all came together that very afternoon, though, when co-author Jeanne reported that the United Nations General Assembly had declared 2013 to be the International Year of Quinoa.  Suddenly it made sense that various friends and family would be much more interested in quinoa than the possible end of the world.  The UN had planted the seeds of this superfood in their brains.

Is quinoa a grain?
There are at least three solid reasons for asking whether quinoa is a grain: worries about gluten; plans for an alternative pilaf; and genuine interest in what counts as a botanical grain and the seed morphology of quinoa.  First, there is no need to worry about gluten in quinoa.  As Jeanne has explained elsewhere, troublesome gluten is found only in a single tribe of the grass family, which is very distantly related to the Amaranth family.  Of course lacking gluten means that quinoa can’t be ground to make a yeast-leavened bread, but – to answer the second question – it is terrific in a pilaf, salad, or soup.  See Michelle’s recipe below for a delicious wintertime version of quinoa salad.

So from a dietary point of view, quinoa is not a “grain” to be avoided, although it functions as a grain in recipes.  What do botanists consider to be grains?  Even in botany, grain is a very general term for something small and round, such as a pollen or starch grain.  What most people mean by a botanical grain – wheat, rice, corn – is a special type of fruit, technically called a caryopsis.  Not that botanists go around talking about multi-caryopsis cereal or anything, as tempting as it may be.  A caryopsis is a one-seeded fruit whose wall is dry and thin and fused to the seed inside.  Usually the entire fruit functions much like a seed would.  It is dispersed and planted just as many other seeds would be.  As an example, imagine an unpopped kernel of popcorn.  It is an entire fruit by itself, although it’s hard to see it as more than a seed.  Compare it to a tomato, which is large, fleshy, and contains many obvious and discrete seeds.  A caryopsis is similar in some ways to an acorn, which is also a dry, thin-walled fruit containing a single seed.  The acorn fruit wall, however, is not fused to the seed inside.  In any case, the caryopsis is the fruit type typical of the grass family, and as discussed above, the grass family is only distantly related to the amaranth family.

What is a “grain” (caryopsis)?

What is the quinoa pseudograin then?  
What we buy in the store is simply the quinoa seed, removed from its fruit.  (Yes, the fruit is dry, with thin walls, and contains only a single seed, but it is not fused to that seed, so it is not a caryopsis.)  The seeds are usually washed and dried before packaging, to remove bitter saponin, a digestive irritant that deters herbivores.  Most recipes nevertheless recommend rinsing the seeds before cooking them.

Whether you rinse or not, it’s instructive to play with the seeds before cooking them.  The seed has an interesting shape, much like a slightly flattened cat curled up on a bed.  As with many seeds, the bulk of it consists of nutritive tissue that would normally serve to feed the embryo during germination and early seedling growth.  In an uncooked seed, that tissue is chalky and opaque looking, but it becomes almost translucent when cooked.  In the Amaranth family, the tissue is perisperm, instead of the usual endosperm, and it is derived from the mother plant only, not from both parents.  This is not a difference the cook can see or taste, however.

Wrapped around this tissue is the embryo itself, long and very thin and curved.  As noted on the phylogenetic tree above, a curved embryo characterizes the core group of the order Caryophyllales.  In cooked quinoa, the embryo often escapes its perisperm and little embryos can be seen scattered over other ingredients in the dish.  You can distinguish the baby leaves from the root-end of the embryo by pressing gently on it to separate the leaves (cotyledons).

Embryonic roots (radicles) can be seen elongating

If you save out a pinch of uncooked seeds, you can sprout them and watch the embryo turn green and stretch up towards the light.  Simply wet them and spread them out on a wet paper towel in the bottom of a glass dish.  Keep them covered loosely for a day or two, but allow them to see the sun once they start to grow.  The roots (radicles) will come out first, followed by the shoots, which will green up over a period of days.  Some of the seeds will fail to germinate and begin to rot, so it’s good to rinse the seedlings gently with clean water occasionally.  Sprouted quinoa can be cooked, and very green sprouts may be eaten like alfalfa sprouts.

Much has been written about quinoa’s history and its high nutritional value, so I won’t take the time to go into those aspects here (see instead here and here).  The UN General Assembly has eloquently summarized quinoa’s many merits in its International-Year-of-Quinoa declaration.  For our purposes, it’s enough to know that it is delicious, nutritious, versatile, easy to cook, and gluten-free.  Perhaps even more importantly, it is very often produced by fair-trade cooperatives of small-scale farmers in Bolivia and Peru.  (However, recent articles in Time and The Guardian and another in Mother Jones raised concerns about the social and environmental effects of sudden high demand for the crop).  In the US, the only real domestic source is high-elevation farms in Colorado.

Where did all these goosefoot species come from?
Why do these goosefoot relatives grow so abundantly in the marshy areas around the San Francisco Bay?  For the same reason that the ancestors of our domesticated beets evolved along the shores of the Mediterranean.  Many plants in the Amaranth family tolerate dry or very saline conditions remarkably well, and both conditions are common along sea shores and in baylands.  For many species in the family, being tolerant of water stress involves being succulent – fleshy and composed of large water-storing cells.  Saltbush and beets are not really succulent, but spinach sometimes leans in that direction.  And then there is Salicornia, a.k.a. sea beans, saltwort, samphire, or pickleweed.  One of several edible Salicornia (Greek: “salt horn”) species fills the low areas by the bay that are routinely flooded with salty water, and it is definitely succulent.  The plant looks like a collection of thin, fleshy green fingers, often with red tips.  The stem is fleshy and partially enclosed by fleshy leaves that are barely larger than scales.  Even the flowers are embedded in the fleshy stems and bracts.  Older stems contain a tough central strand of vascular tissue, but younger stems are tender, crunchy, and salty.  These are best collected in the summer.

Sea beans

A very nice description, and what looks to be a terrific recipe for sea beans can by found on  the site Hunter, Angler, Gardener, Cook.

For now, though, let’s ring in the International Year of Quinoa with a lovely winter salad.  Best wishes for a happy and healthy 2013 from the Botanist in the Kitchen.

Quinoa salad with mushrooms, butternut squash and watercress

Here is another salad that is flexible to what you have available. Feel free to use a combination of mushrooms or to change the squash or greens. Just remember, the grains contribute the nuttiness, the squash give the sweet caramelized flavor and the greens the spiciness. Delicata squash and arugula would be great alternatives. If using shiitakes, be sure to stem them.

(Note from KP: For a three-goosefoot version, use spinach for the greens and roasted beets in place of the roasted squash.  The spinach will be less spicy than watercress, and the beets less sweet than squash, and the entire dish will have a much earthier flavor.)

Serves 4

1 cup quinoa
1/2 pound maitake, shiitake, or oyster mushrooms, sliced
2 1/2 pound butternut squash, peeled, deseeded, and sliced 1/4 inch
thick
1/2 cup good olive oil
Pinch of chili flakes
1 large shallot, minced (about 3 tablespoons)
Juice of one lemon
2 ounce chunk of Parmesan for shards (cut with a vegetable peeler)
Small handful of mint, chopped
2 large handfuls of watercress (about 2 ounces)

Heat oven to 450° degrees.
Rinse quinoa under cold water to remove the bitter coating. In a medium pot, bring quinoa and 2 cups water to a boil. Cover, reduce heat to low, and simmer until quinoa is tender, about 15 minutes. With a fine mesh strainer, drain any excess water from quinoa. Return drained quinoa to the pot, cover and let rest for another fifteen minutes. Pour quinoa into a large bowl and drizzle two tablespoons of olive oil over it.

In another large bowl, toss squash with 2-3 tablespoons of olive oil, salt and chili flakes. Spread squash out, in a single layer, on two baking sheets and put in oven. After 10 minutes, rotate pans to make sure that squash is roasting evenly. Carefully move any dark pieces to the middle of the pan and any lighter pieces to the edge. Roast for 5-10 more minutes until squash is tender and caramelized. Remove from oven and set aside.

Heat a large sauté pan over medium high heat. When hot, add 2 tablespoons of oil and the mushrooms. Stirring occasionally, sauté mushrooms till golden and caramelized, about 5-6 minutes. Add shallots to the mushrooms and cook for 1-2 more minutes. Add mushroom mix to the bowl of quinoa.

Squeeze juice from 1/2 a lemon over the quinoa, and toss in Parmesan, mint and watercress. Taste salad and adjust with more lemon, oil and salt. Gently fold in squash slices and serve with fresh ground pepper.