Tag Archives: Jeanne L. D. Osnas

The Botanist in the Root Cellar

How much actual root is in “root vegetables”?

The wintertime pantry is a study in vegetable dormancy. Our shelves brim with structures plants use to store their own provisions. Each embryonic plant in a seed—the next generation of oats, quinoa, dry beans, walnuts—rests in the concentrated nutritive tissue gifted to it by its parent. The starchy flesh within the impervious shell of a winter squash is alive, as are apples, hopeful vessels of seed dispersal. Maple and birch syrup are stored energy made liquid and bottled. And then there are the so-called “root vegetables.” The term covers a surprisingly anatomically varied set of nutrient storage structures, only some of which are actual roots. Our familiar root vegetables represent only a sliver of global plant species diversity but nonetheless include the majority of contrivances herbaceous plants use in order to live to sprout another season: taproots, hypocotyls, stem tubers, root tubers, corms, and rhizomes. Raiding your root cellar for the ingredients for a roasted root vegetable medley, then, provides a great opportunity to turn your dinner prep into a botany lab. All you need is a knife and cutting board.

Roasted stacks of sweet potato and parsnip, painted with sage butter and roasted. See Katherine’s sweet potato post for the recipe.

The case for tree thinking

First we need to consider the taxonomy of our candidate botanical subjects. Taxonomy is the scientific practice of grouping related organisms in hierarchies of similarity. We shove the continuous variation of living things into discrete boxes labeled species, genus, family, order, and so on. Carl Linnaeus started the taxonomic naming system two hundred years before Watson and Crick identified the double helix shape of deoxyribonucleic acid (DNA), marking the beginning of the genomics era. Modern practitioners bring many types of data—geography, fossils, genetics, morphology–to bear toward the twin goals of illuminating the pattern of plant species evolution and defining groups based on common ancestry.

A phylogenetic tree of the major plant clades. Each branch point (node) represents the common ancestor of the organisms on the descendant branches. A single food plant species is shown here at the tip of each branch, a sort of mascot for its lineage.

The visual embodiment of this effort is the tree of life (cladogram) that represents the pattern of plant species evolution by common descent (phylogeny–see our primer on reading phylogenetic trees and using them to understand broad patterns in plant evolution). Each branching point on the tree is a node that represents a common ancestor of all the descendant taxa on the branches that come from it. The species are like the leaves on the tips of the branches. A schematic tree of life is the only illustration in Charles Darwin’s On the Origin of Species, the landmark book that provided the kernels of the core theories of evolutionary biology. Modern scientific convention tries to match old taxonomic names—because they are familiar and useful as a practical matter–with nodes on the tree of life. Small branches connect species to genus. Larger branches connect genera to families, families to order. The deep internal named nodes show the origin of the major clades. A clade is a group of organisms that descend from a common ancestor. A major clade is a significant branch on the plant tree of life that scientists have named for convenience of reference. You may remember some of these from biology class, like “monocot” and “dicot.” The former (monocots) has held up as a robust clade, but dicot is more complicated.

Green garlic, a monocot that stores its winter provisions as a bulb

As it happens, plant taxonomy before the advent of genetic data was reasonably accurate.  Even though our understanding of plant species evolution is far from complete, genomic analysis has provided few big surprises about common ancestry of plant species and membership of taxonomic groups. Early taxonomists had the wisdom to rely primarily on similarity of reproductive structures—seeds, fruit, flowers, spores, cones—to circumscribe named groups. Reproductive structures tend to change more slowly over evolutionary time than do vegetative structures in plants. So one may expect to find a fair amount of coincident similarity among distantly related species in roots, shoots, and leaves.

This is where our categorization of root vegetables by taxonomy collides with our categorization of them by morphology. In short order we will organize our root vegetable species according to which structures the plant has chosen to amplify as a subterranean or soil-adjacent storage organ. This is not the same pattern as taxonomic organization. Grouping our root vegetables by taxonomy first helps us understand similarity and difference within and between groups of closely related plants—families, in this case. In doing so we can develop gestalt for the culinary qualities within plant families and appreciation for the evolution of plant diversity evident on our own dinner tables.  Consider this intellectual nourishment, or perhaps the advent of a lens with which to view familiar foods anew.

Placing root vegetables on the plant tree of life

Around the globe humans utilize many dozens of plant species that bear underground (or near enough) storage structures. The most recent generations of people overwintering in the United States or Europe, however, chiefly engage with only a few. Perhaps only the most dedicated winter vegetable enthusiast will be familiar with all of the species on the following roster of root vegetables potentially available in Western grocery stores or farmer’s markets, although the list is unlikely to be exhaustive. I have organized the root vegetable species by families, and the families by major clade. Our list includes 15 of the 446 currently recognized plant families.

Whole sweet potatoes (Convolvulaceae)–NOT yams (Dioscoreaceae), NOT potatoes (Solanaceae), and NOT oca (Oxalidaceae)

Please take note of the disambiguation about the words “yam” and “potato.” The tubers marketed as “yams” in most American groceries are mostly actually sweet potatoes, which are also not potatoes. True yams are large tubers that are staples of tropical diets but relatively scarce in northern diets or groceries. In New Zealand the Andean oca is also known as “yam.” All of these are in different plant families.

Monocots:

  • Amaryllis family (Amaryllidaceae): onions and shallots (Allium cepa), garlic (Allium sativum), leeks (Allium ampeloprasum), and other alliums
  • Ginger family (Zingiberaceae): ginger (Zingiber officinale), turmeric (Cucurma longa)
  • Dioscoreaceae: true yams (several species in the genus Dioscorea), including the purple yam (ube; D. alata).
  • Sedge family (Cyperaceae): water chestnut (Eleocharis dulcis)
  • Arum family (Araceae): taro (Colocasia esculenta)

Eudicots: asterids

  • Goosefoot family (Amaranthaceae): beets (Beta vulgaris)
  • Sunflower family (Asteraceae): salsify (Tragopogon porrifolius), burdock root (Arctium lappa), sunchokes (Helianthus tuberosus)
  • Carrot family (Apiaceae): carrots (Daucus carrota), parsnips (Pastinaca sativa), parsley root (Petroselinum crispum), celery root (Apium graveolens)
  • Morning glory family (Convolvulaceae): sweet potatoes (Ipomoea batatas; often mistakenly called “yams” in the United States)
  • Nightshade family (Solanaceae): potatoes (Solanum tuberosum)

Eudicots: rosids

  • Spurge family (Euphorbiaceae): cassava (manioc, yucca; Manihot esculenta), the source of tapioca
  • Mustard family (Brassicaceae): turnips (Brassica rapa), rutabagas (Brassica napus), kohlrabi (Brassica oleracea), radishes (genus Raphanus), horseradish (Amoracia rusticana), wasabi (Eutrema japonicum), maca (Lepidium meyenii)
  • Nasturtium family (Tropaeolaceae): mashua (Tropaeolum tuberosum)
  • Legume family (Fabaceae): jicama (Pachyrhizus erosus)
  • Oxalis family (Oxalidaceae): oca (Oxalis tuberosa), an Andean vegetable that is confusingly called “yam” in New Zealand

True taproots: carrot, parsnip, parsley root, salsify, burdock root, horseradish

Carrots (taproot and leaves–which make a great pesto)

Now grab a carrot, parsnip, parsley root, salsify, or burdock root for your roast vegetable medley. These are the only true taproots on our list. The roots are much longer than they are wide and taper to a point. Thin lateral roots sprout from them in random locations or in discrete vertical lines. If you cut it open and examine it in cross section you see the tough core of xylem-rich pith (water conducting tissue) in the middle surrounded by a cortical layer that separates the pith from the sweet storage tissue (parenchyma) and sugar-moving phloem that surrounds it. You have likely removed the aboveground greenery from these plants but should be able to tell or recall that it appears as if the leaves grow directly out of the crown of the taproots. They almost do. The anatomical stem on carrots and parsnips is a highly reduced disk on top of the taproot that serves as a bud-studded vascular transfer station, shuttling water and nutrients from the taproot into the leaves and flowering shoots.

Horseradish is also a taproot. A little bit grated into a sauce would make a delicious accompaniment to your roast vegetable medley. Incidentally, horseradish powder is the main ingredient in cheaper “wasabi” products available in American grocery stores, as the horseradish taste is superficially similar to that of true wasabi, which is also in the mustard family. Wasabi is also a root vegetable, but its underground storage structure is a rhizome, an underground stem, not a taproot.

Roots fused with stems (hypocotyls): celery root, beet, rutabaga, turnip, radish

celery root hypocotyls

A hypocotyl is a swollen fusion of taproot and stem base. The taproot portion is covered in fibrous secondary roots, most spectacularly in celery root. Leaf scars will be visible about these lateral roots, either surrounding the entire upper portions of the hypocotyl, as in celery root, or just at the top, as in beets and the mustard family hypocotyl vegetables (turnip, rutabaga, radish). All hypocotyl vegetables aside from beets are structurally straightforward but different from the taproots. A single layer of vascular tissue lays below the skin surface and penetrates into the storage tissue.

A rutabaga hypocotyl in the ground

Beets, however, are built from concentric rings of vascular tissue (xylem and phloem) and storage tissue (parenchyma), which is visible when the beet is cut in cross section. This ring structure is unique to the taxonomic order Caryopyllales, of which beets are a member. And as Katherine notes in her excellent beet post, the vibrant colors and earthy smell of beets are also unique. The former is due to betalain pigments, which are also unique to the Caryophyllales and distinct from the anthocyanin pigments present in all the other vegetables in our list (see our pigments post for a quick rundown of the most common pigments). The earthy smell is from a compound called geosmin. Beet is the only plant known to make it, and nobody knows why. Geosmin us usually produced by microbes in the soil and is liberated after rain to create that marvelous fresh smell after a storm.

chiogga beets show concentric vascular rings in dramatic fashion

Indidentally, our hypocotyl root vegetables here are all varieties, or subspecies, of species that also produce familiar leafy vegetables: rutabagas and the Russian or Siberian kales; turnips and Napa cabbages and mizuna; beets and Swiss chard; celery root and celery stalks or seeds. In each of these cases the variety produced for leaves has a much less pronounced hypocotyl than the variety produced as a root vegetable. Similarly, while the leaves on our hypocotyl root vegetables are all edible, they will be smaller and tougher than those on the varieties that have been bred for leaves. 

Bulbs: onion, garlic, shallots, leek

red onion bulbs growing in a planter box

Onions, shallots, garlic and other alliums might be the most famous “root vegetables” of all, but their delicious parts are constructed entirely of swollen modified leaves. The papery tunicate covering surrounding the fleshy leaf bases are also constructed out of modified leaves, all arising from the basal plate (true compressed stem) that interfaces with the spindly roots on the bottom. The fleshy part of each garlic clove is a single fat modified leaf. Inside each garlic clove or onion bulb is an apical bud that will send up new leaves and flowering shoots. Everyone who has had onions and garlic sprout on them can observe this. You can of course plant these sprouting bulbs in the soil to make a new plant. A leek is a bit intermediate between a true bulb and a big herb. They call the lower white region of overlapping succulent leaf bases a “pseudobulb,” a nod to the messy continuous nature of biology and the difficulty with labels.

slices of leek pseudobulb, showing overlapping leaf bases

Unless you’re using a variety of “sweet” onion, which has been grown or bred to lack sulfurous aromatic compounds, you might tear up when you’re cutting onions and shallots. Cutting these bulbs volatilizes the irritating compounds that otherwise protect our favorite bulbs from pests.

Root tubers: sweet potatoes, cassava

A root tuber is an enlarged root that stores starch and other nutrients. Smaller lateral roots often branch from its surface and obtain water and soil nutrients. Raw sweet potatoes are readily available candidate root tuber ingredients for your botanical scrutiny and roast vegetable medley. Cassava is not, nor should it be, at least in root tuber form. Starch derived from cassava might be elsewhere in your pantry as tapioca.

A convenient aspect of our most commonly used root vegetables is that they require very little manipulation or preparation before they can be consumed. You don’t even have to peel your sweet potatoes before you cook them. Raw cassava tubers, however, are laced full to bursting with cyanide. They are the third most important source of calories throughout the tropics, behind corn and rice, but require extensive preparation before consumption to remove the cyanide, including grating, drying, leaching and cooking.

Cassava tubers develop underground from certain roots that become fleshy storage structures. They continue to acquire water and nutrients via smaller secondary roots that dot their surface. If the plant in question grows from a seed, then the harvestable storage root may develop from the taproot that grows from the seed. This, however, proves an inefficient way to farm these species, as many more storage roots can develop on a single plant when that plant is started from a shoot—a stem with leaves. This is where the visual heuristic of placing root vegetable species on the branches of the plant tree of life gets literal with sweet potatoes and cassava. The key factor is the presence of numerous nodes—leaves along the stem and their attendant axillary buds. Cassava and sweet potato are among the plant species that can generate roots from the buds in their leaf axils under the right conditions, namely being in contact with moist soil. Roots that develop from non-root tissue (like stems) are called adventitious roots. When several nodes of a shoot are planted in the soil, many adventitious roots will develop, of which some can become enlarged storage roots. In cassava the starting shoot is a cutting from a mature cassava plant. In sweet potatoes the starting shoot is called a slip. Slips grow from buds on the proximal (closest to the parent plant) end of sweet potato tubers. On sweet potatoes this is the end with the scar where the tuber was cut away from the parent plant.

sweet potato developing slips

Rhizomes: turmeric, ginger, galangal, lotus, arrowroot, wasabi

A rhizome is a fleshy underground stem. It grows horizontally and sprouts new plants. Stems grow upward from buds near the soil surface, and roots grow from buds on the underside of the rhizome. It is structurally similar to stem tubers, like potatoes (see below), but it only grows horizontally, not in any direction, like a tuber. Rhubarb, asparagus, and irises also spread by rhizomes. If you decide to get out ginger or turmeric to flavor your vegetable medley, you’ll notice structural similarities to stem tubers, including nodes with buds.

Stem tubers: potato, sunchoke, jicama, yam

The eyes may or may not be the window into the soul, but they are our most conspicuous clue that potatoes are subterranean stem tubers, not roots. Katherine’s superb post on potato anatomy will walk you through this (potato) eye exam. Observe both ends of a potato. One end (the proximal end) bears the stump of the stolon (horizontal stem) that connected it to its mother plant. The other is tightly packed with small eyes that spiral out and around the potato. This is the growing (distal) end of the potato. New eyes originate at this end, so each eye is progressively older as you move toward the middle of the potato. Each eye contains a cluster of buds subtended by a semicircular leaf scar. The leaf in question was vestigial, translucent, and a remnant of it may still be present on your potato. Eyes are most easily visible on the “waxy” potato varieties (like Yukon Golds), which have less starch overall and a different ratio of types of starch than the “starchy” varieties (like Russets)–see Katherine’s post on potato starchiness for details.

potato eyes in spiral arrangement

The buds in each eye are axillary buds, structurally the same as Brussels sprouts. If your potato is exposed to enough light or warmth, the axillary buds will grow into new leafy stems, each of which can create a new potato plant. In this case your potato might also start synthesizing chlorophyll, turning it green. It will make toxic compounds at the same time, though, so if your potato is green you should either liberally peel it or wait to plant it in the spring.

Brussels sprouts on the stalk with residual leaf petioles. Brussels sprouts are spectacular axillary buds.

Nodes and buds are also easily visible on sunchokes, less so on jicama. True yams are actually structurally intermediate between rhizomes and stem tubers in that they might sprout adventitious roots. If you get your hands on an actual yam, instead of a sweet potato, you might see these.

sunchokes

Corms: taro, water chestnut (with a note on kohlrabi, which is not a corm)

A corm is yet another method by which plants have modified their stems to store starches and nutrients underground. The storage tissue is a swollen area of the stem above the roots and below the apical bud, from which leaves and flowers develop. Lateral buds on the stem produce modified leaves that produce a protective tunicate sheath around the starchy corm tissue. A thickened basal plate on the bottom interfaces with the roots and may sprout new corms (cormels). If you get canned water chestnuts or taro corms for your vegetable medley, these structures should be visible to you. Structurally, a taro corm is most similar to kohlrabi, which is what happened when plant breeders long ago took a weedy ancestral cabbage plant (Brassica oleracea) and bred for fat, bulbous stems. The leaf scars out the outside of a kohlrabi, and the nubbin of a root on the bottom, reveals that it is entirely stem.

Kohlrabi

The geophyte lifestyle

Potatoes are in the same genus (Solanum) as tomatoes (S. lycopersicum) and eggplants (S. melongena). The potato is the only one of these close relatives that hails from high in the Andes, where its underground tubers store the starches it needs to survive the harsh alpine conditions. This is a common ecological theme. Plants that create underground storage organs to withstand winter or seasons of drought are called geophytes. Even our short list of root vegetable species demonstrates that the geophyte lifestyle independently pops up all over the plant evolutionary tree, presumably in times and places where it may be adaptive. Even in just the Andes alone, potatoes are not the only domesticated geophyte crop with lowland relatives in the same genus devoid of starchy storage organs. Oca, confusingly called “yam” in New Zealand, where it was introduced in the mid-19th century, is otherwise known as Oxalis tuberosa. It makes stem tubers, like a potato. The specific epithet “tuberosa” separates it from non-geophyte species of Oxalis that are probably familiar to hikers and gardeners throughout the northern hemisphere. American health food stores sell dried maca hypocotyl (Lepidium meyenii) as a health food supplement, even though it is a staple crop throughout montane South America. Other Lepidium species are weedy little mustard plants. In the summer your garden may be teeming with flowering nasturtiums (Tropaeolum majus). You’ll notice a distinct lack of a fat, starchy stem tuber. Not so with mashua (Tropaeolum tuberosum).

branched taproot on a carrot

You should be well on your way at this point to getting your root vegetable medley into the oven. Finish peeling your vegetables, if you must, and dice them into approximately equally sized chunks. Toss them with a small amount of oil and salt. Add herbs if you would like. Spread them in a single layer on a baking tray or roasting pan and roast in the oven at 375 degrees Fahrenheit until they are tender, about 40 minutes. It is helpful to turn the pieces and move them around with a metal spatula halfway through the cooking time.

I like to serve these roast vegetables with some kind of sauce, often a strained yogurt mixed with salt and herbs. This is a dish filled by design with concentrated energy to maintain life through harsh seasons. The geophyte lifestyle is periodically useful for us all.

Sage, rosemary, and chia: three gifts from the wisest genus (Salvia)

This essay is our annual contribution to the Advent Botany essay collection curated by Alastair Cullham at the University of Reading. We highlight three charismatic species in the large genus Salvia (in the mint family, Lamiaceae): rosemary, sage, and chia.

Two Christmases ago we pointed out the current fad in decorating pineapples for Christmas. This year, some of our gentle readers may come across potted rosemary bushes that are trimmed into a cone to resemble a conifer. These are pleasant and ostensibly can be kept alive after the holiday season.

A rosemary shrub trimmed into a conifer shape. Photo from Pottery Barn.

A perhaps less pleasant holiday botanical encounter may include a Christmas tree-shaped Chia Pet.

Christmas tree Chia Pet. Photo from Amazon.

As far as Chia Pets go, this one is fairly innocuous. In my view, however, its only saving grace is that the chia plant itself is a fabulous taxon (Salvia hispanica), as is the rest of its large genus, Salvia, which also happens to include rosemary (Salvia rosmarinus). Rosemary of course is much more likely to make a holiday appearance as a culinary ingredient than a decoration, lovely as it is. In the kitchen it is frequently joined with its congener Salvia officinalis, usually just called garden sage. That the genus Salvia is responsible for half the taxa in the title of a Simon & Garfunkel album (Parsley, Sage, Rosemary and Thyme), notwithstanding that Art Garfunkel looks like a Chia Pet on the cover, could provide enough taxonomic joy to justify leaving this examination of these plants here. The name “sage”, however, implies wisdom, and so like the wise men of old, I shall persevere.

Parsley, Sage, Rosemary and Thyme album cover by Simon & Garfunkel (1966)

We’ll start by addressing the taxonomic elephant in the room that might otherwise distract learned readers: rosemary was only brought into the Salvia fold in 2017. Before then it was in its own small genus: Rosmarinus. The reason Rosmarinus is now Salvia is that the speciose Salvia was found to be paraphyletic: the pre-2017 conscription of the nearly 1000 species in the genus did not include all of the descendants of their most recent common ancestor. When the relationships between all the Salvia species and their closest relatives were plotted on a single phylogenetic tree, it was obvious that Rosmarinus and a few other genera should more naturally be considered Salvia, and Salvia was revised accordingly.

Rosemary (Salvia rosmarinus)

Another taxonomic bookkeeping item is to clarify that the sages in Salvia are only distant relatives of the sagebrushes and sageworts in the genus Artemisia, which is in the sunflower family Asteraceae (please see our Artemisia essay for more information about that genus, which includes the herb tarragon). The phylogenetic relationships of the major groups in Salvia from the most recent revision (Drew et al., 2017) is shown below.

Figure 2 from Drew et al. (2017): “(A) Composite chronogram of subtribe Salviinae (which contains Salvia and related taxa) based on chloroplast DNA sequences from previous molecular phylogenetic analyses. Asterisks denote nodes with low support and/or conflicting resolution among previous analyses. Salvia nomenclature follows subgeneric clades described here, including three tentatively named clades that await proper circumscription. Calibrations based on Drew & Sytsma (2012; See supplementary figure S4) (B) Circle cladogram framed on larger chronogram with weakly supported nodes collapsed, depicting species diversity and generalized staminal types within each clade of Salvia; modified after Walker & Sytsma (2007) and Walker et al. (2015).” S. elegans (pineapple sage), S. sclarea (clary sage), and S. hispanica (chia) are in the American subgenus Calosphace. Rosemary is in its own subgenus, Rosmarinus.

The phylogenetic diagram above (from Drew et al., 2017) shows locations where the flower anther structure evolved into a lever-like mechanism that aids in bee pollination by physically moving the two stamens into contact with the bee’s back when a bee enters the flower (see illustration below from Walker, Sytsma, Treutlein, & Wink, 2004).

Figure 2 from Walker et al 2004: “Flower and pollination of Salvia pratensis (Salvia clade I). A flower without the lever mechanism activated (A). As the pollinator enters the flower (B), the pollen is deposited on the back of the pollinator. As the pollinator enters an older flower (stamens removed from sketch, but remain present in flower) pollen is transferred (C). The posterior anther thecae forming the lever can be fused or free and in the subg. Leonia, produce fertile pollen”

The lever mechanism independently evolved three times within Salvia. Each of these evolutionary events was followed by rapid and prolific speciation driven by this innovation in pollination biology (Drew et al., 2017): the advent of the lever mechanism led to the radiation of around 500 species in the subgenus Calosphace in Central and South America; around 250 species evolved soon after the advent of the lever mechanism in the Salvia officinalis clade in the Mediterranean and Western Asia; and around 100 species radiated following the lever in Far East Asia in the Salvia glutinosa clade.

Sage (Salvia officinalis) flowering on my deck this summer

The bee-pollinated Salvia flowers are distinct from those pollinated by hummingbirds, which are more elongate and often red, like the flowers of pineapple sage (S. elegans), and have either evolutionarily lost the staminal lever mechanism or never had it in the first place.

Pineapple sage (Salvia elegans)

The parsley, sage, rosemary, and thyme made famous by Simon & Garfunkel started their culinary careers in Europe. All but parsley are in the mint family (Lamiaceae; see our carrot top essay for a discussion of fun chemical relationships between the flavor compounds in the mint family and the parsley family, Apiaceae). This points to the profusion of aromatic mint family species common to the rocky shrublands covering much of Europe and western Asia (Rundel et al., 2016; Vargas, Fernández-Mazuecos, & Heleno, 2018).

Called “tomillar” in spanish, literally a field of wild thyme (Thymus vulgaris) and associated species growing in the Orusco de Tajuña hills (near Madrid. Spain). Other edible Lamiaceae can be found in this plant community, including Salvia rosmarinus, and Lavandula latifolia (a lavendar). Photo by Julia Chacón-Labella.

That broad area is one of the centers of Salvia species diversity, but the genus is globally widespread. The genus probably originated and dispersed first from African and then the Mediterranean (see the figure of Salvia distribution and putative dispersal history below from Will & Claßen-Bockhoff, 2017), but the full story of dispersal and species radiation within the genus requires more elucidation.  Numerous species of Salvia are utilized as culinary or medicinal herbs or garden ornamentals throughout its range.

Fig. 8 from Will et al. 2017: “Salvia s.l. in time and space. A: Distribution of Salvia s.l., putative migration routes and fossil sites; BLB = Bering Land Bridge; D = Dorystaechas; M= Meriandra; NALB = North Atlantic Land Bridge; P = Perovskia; R = Rosmarinus; Z = Zhumeria; white arrows indicate repeated colonization of S Africa and dispersal from the Eastern Cape to Madagascar; hatched arrows (dark grey) indicate the repeated colonization of the Canary Islands from two different mainland sources; red arrow illustrate the dispersal from East Asia to Eurasia reflected by S. glutinosa; black arrows correspond to dispersal events from the OW to America reflected by two distinct lineages; ? = route uncertain; template of the map provided by the German earth science portal (www.mygeo.info). B: Simplified phylogenetic tree; nodes discussed in the text are indicated by capital letters; colors reflect distribution areas. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)”

The phylogeny above shows the large number of American taxa in subgenera Calosphace and Audibertia. While many of these species have also been used as aromatic herbs and traditional medicines, the most famous of the American Salvias, chia, is known for its nutritious seeds (Jenks & Kim, 2013a). Chia is a name given to two species of Salvia: S. columbariae and S. hispanica. S. columbariae ranges from southern California to central Mexico, at which point the range of S. hispanica begins and extends to Guatemala. Indigenous groups throughout that range historically used both species of chia as a pre-Columbian staple food source. The Aztecs cultivated it, and 16th century Spanish codices indicate it may have been as widely utilized as maize (Cahill, 2003).

Chia nutlets (S. hispanica) and a dried sage (S. officinalis) leaf for scale

Technically, the chia “seeds” you can buy in the store (or harvest yourself) are fruits. The Salvia fruit, like those of all mint family species, is called a schizocarp. The ovary inside the flower has four chambers, called locules. Each locule matures into an independent, indehiscent nutlet. The shell (pericarp) of the nutlet is stratified into the same categories of outer fruit layers as are more familiar fleshy fruits (cuticle, epicarp, mesocarp, endocarp; see our pomegranate or apple essay for more details about fruit structure), but in the Salvia nutlet the outer fruit layers are dry and compressed and inseparable from the single seed inside the fruit (Capitani, Ixtaina, Nolasco, & Tomás, 2013). Salvia nutlets mature inside of papery fused calyces (see the photo below of sage nutlets and their cup-like persistent calyces).

Sage (Salvia officinalis) leaves and nutlets inside of papery, fused persistent calyces.

The word “chia” is derived from the Aztec language Nahuatl word for “oily,” a name bestowed because chia seeds do have a high oil content (Cahill, 2003). Chia oil is rich in the omega-3 fatty acid alpha-linolenic acid, which has contributed to its recent fame as a modern health food. High alpha-linolenic acid content may be a general feature of the genus: other Salvia species, including S. officinalis, garden sage, have been shown to have high alpha-linolenic acid content in their seeds (Ben Farhat, Chaouch -Hamada, & Landoulsi, 2015).

Chia nutlets are also known for the gooey mucilage they exude when wet. This polysaccharide matrix is used as a food binder and thickener (Google “vegan egg replacement”). The production of mucilaginous diaspores (the dispersing agent, a fruit or a seed) is called myxocarpy. As Katherine discusses in her essay on okra, the flagship mucilaginous food plant, the purpose of the mucilage is likely water retention in the arid regions where these plants tend to come from. The mucilage might also act as a glue to bind the nutlet to the soil or to a dispersing animal’s fur—or to the terracotta substrate of a Chia Pet. Myxocarpy is most common in plants with small seeds growing in dry, arid areas, like those where Salvia species have radiated (Ryding, 1992).

Sage growing in coastal California, a Mediterranean-type ecosystem

Within the mint family, myxocarpy only occurs in the subfamily Nepetoideae. The subfamily, incidentally, gets its name from the catnip genus, Nepeta. Most if not all of the familiar edible herbs from the mint family are in this subfamily. Katherine has taken advantage of myxocarpy in this clade by serving soaked black basil (Ocimum basilicum) nutlets as a basil-scented vegan “caviar.”

Cat in the catnip (Nepeta cataria)

Salvia aroma and flavor–and I think the psychoactive properties of catnip for cats and known hallucinogen Salvia divinorum–comes from the terpenoids and phenolics that comprise their essential oils. The terpenoids are synthesized and stored in special glandular trichomes on the leaf surface (Schuurink & Tissier, 2019). Trichomes are hair-like extensions of the epidermis, although the glandular trichomes full of essential oil look more like water balloons than hair. Salvia species have other types of trichomes in addition to the glandular trichomes that are indeed much more hair-like and give the leaves of some Salvia a downy or prickly appearance (Kamatou et al., 2006).

Scanning electron micrograph (SEM) of a rosemary leaf. Spherical oil-filled glandular trichomes are found amongst the branched hair-like trichomes covering the lower surface of the leaf, which has a greater profusion of hairs and glands than the upper surface. When the glands are damaged or broken the aromatic essential oil is released. Magnification: x1550 (x381 at 10cm wide). Photo from https://psmicrographs.com/sems/flowers-plants/

We discussed trichome function extensively in one of our kiwi essays. The hair-like trichomes may serve the leaf by protecting it from excess solar radiation and wind and otherwise creating a more mild microclimate at the leaf surface to help it retain water.

Rosemary

Terpenoid biosynthesis requires numerous steps in which intermediate chemical products are modified by a series of specific enzymes and other proteins. Small changes in the genes responsible for those proteins can lead to big qualitative changes in the final terpenoid mix in the essential oil of a given taxon. We mammals are adept at discerning aroma differences between chemically similar terpenoids. For example, in on our carrot top essay we discussed the case of spearmint and caraway. The respective versions of the terpenoid carvone that characterize the essential oils of those plants differ only in the physical configuration of the same chemical elements, but they smell radically differently to us.

Clary sage (S. sclarea)

The function of the essential oil in the glandular trichomes, however, is not to improve human well being. Plants synthesize those lovely terpenoids as chemical defense against insect herbivores and microbial pathogens.  When the hair-like trichomes fail to stop the intruders, the glandular trichomes will explode on contact, drenching the would-be attackers in a caustic-but-fragrant deluge.

rosemary

The pharmacopeia of terpenoid aromas present in the mint family—bring to mind the scents of sages, rosemary, lavender, peppermint, spearmint, savory, thyme, oregano, marjoram, shiso, basil—owes its evolutionary origins certainly in part at least to the various selection pressures imposed on those herbal taxa by their pests. Within even commonly grown domesticated Salvia species, essential oil constituent variation leads to dramatic differences in aroma. For example, consider the differences among rosemary, garden sage, clary sage (S. sclarea), and pineapple sage (Salvia elegans), which has a notably fruity smell. The fruitiness is due in part to the presence of the terpenoids charcteristic of citrus, which are widespread across plants.

Garden sage (Salvia officinalis)

The Roman historian and natural scientist Pliny the Elder coined the name Salvia, which is derived from the Latin salvare, meaning to heal and save, and salvus, meaning uninjured or whole. The common English name “sage” of these plants ultimately comes from this same Latin root. In Pliny the Elder’s time, the Mediterranean Salvia species were considered healing herbs, good for treating colds and a variety of ailments. Salvia feature prominently in the ethnomedicine of every region in which it is found (South Africa: (Kamatou et al., 2006); Central and South America: (Jenks & Kim, 2013b)). There is a Chinese proverb that asks “How can a man grow old who has sage in his garden?” I do not know which Salvia species would have been responsible for this proverb. There are over a hundred species of Salvia species native to China, and the Mediterranean import Salvia officinalis is grown throughout the country.

Bundle of dried sage, recently, recently, in Alaska

The health and wellness meaning of “sage” is etymologically independent from its other definition as a wise thing or wise person. This second meaning ultimately comes from the Latin sapere, to know or taste. I personally enjoy conflating these meanings, tying wisdom and well-being to the plant. I like that the Salvia officinalis that grew on a pot on my deck this summer and that will season comfort food this winter is a descendent from the plants that healer contemporaries of Pliny the Elder would have searched for amidst sun-drenched rocks in the Mediterranean hills.

Salvia in macarons at my local bakery (Fire Island) this week: blackberry-sage and rosemary-merlot.

Simon & Garfunkel close the Parsley, Sage, Rosemary and Thyme album with the song “7 O’Clock News/Silent Night,” in which they juxtapose jarring newscasts from the Nixon and Johnson era with the Christmas carol. This holiday season has felt a bit like that song to me, like concerted effort is required to prevent awful, omnipresent news from drowning out the joy and solemnity of marking the darkest time of the year. But perhaps honoring traditions always involves this element of deliberately carving out the space in which to do so. Perhaps sprinkling rosemary and sage into a holiday stew or stuffing can be a radical act, a defiant embrace of old wisdom to fortify ourselves to stand with each other and create something beautiful in the cold. Regardless, insane amounts of butter will be involved, at least at my house. And when the January 2nd resolutions to “eat better” come around, chia will be there.

References

Ben Farhat, M., Chaouch -Hamada, R., & Landoulsi, A. (2015). Oil yield and fatty acid profile of seeds of three Salvia species. A comparative study. Herba Polonica, 61(2), 14–29. doi:10.1515/hepo-2015-0012

Cahill, J. P. (2003). Ethnobotany of Chia, Salvia hispanica L. (Lamiaceae). Economic Botany, 57(4), 604–618. doi:10.1663/0013-0001(2003)057[0604:EOCSHL]2.0.CO;2

Capitani, M. I., Ixtaina, V. Y., Nolasco, S. M., & Tomás, M. C. (2013). Microstructure, chemical composition and mucilage exudation of chia ( Salvia hispanica L.) nutlets from Argentina. Journal of the Science of Food and Agriculture, 93(15), 3856–3862. doi:10.1002/jsfa.6327

Drew, B. T., González-Gallegos, J. G., Xiang, C. L., Kriebel, R., Drummond, C. P., Walker, J. B., & Sytsma, K. J. (2017). Salvia united: The greatest good for the greatest number. Taxon, 66(1), 133–145. doi:10.12705/661.7

Jenks, A. A., & Kim, S. C. (2013a). Medicinal plant complexes of Salvia subgenus Calosphace: An ethnobotanical study of new world sages. Journal of Ethnopharmacology, 146(1), 214–224. doi:10.1016/j.jep.2012.12.035

Jenks, A. A., & Kim, S. C. (2013b). Medicinal plant complexes of Salvia subgenus Calosphace: An ethnobotanical study of new world sages. Journal of Ethnopharmacology, 146(1), 214–224. doi:10.1016/j.jep.2012.12.035

Kamatou, G. P., van Zyl, R. L., van Vuuren, S. F., Viljoen, A., Figueiredo, A. C., Barroso, J. G., … Tilney, P. M. (2006). Chemical composition, leaf trichome types and biological activities of the essential oils of four related Salvia Species indigenous to Southern Africa Analysis of plant volatile using 2D gas chromatography View project Chemometrics View project. Journal of Essential Oil Research. Retrieved from https://www.researchgate.net/publication/236850867

Rundel, P. W., Arroyo, M. T. K., Cowling, R. M., Keeley, J. E., Lamont, B. B., & Vargas, P. (2016). Mediterranean Biomes: Evolution of Their Vegetation, Floras, and Climate. Annual Review of Ecology, Evolution, and Systematics, 47, 383–407. doi:10.1146/annurev-ecolsys-121415-032330

Ryding, O. (1992). Pericarp structure and phylogeny within Lamiaceae subfamily Nepetoideae tribe Ocimeae. Nordic Journal of Botany, 12(3), 273–298. doi:10.1111/j.1756-1051.1992.tb01304.x

Schuurink, R., & Tissier, A. (2019). Glandular trichomes: micro-organs with model status? The New Phytologist, nph.16283. doi:10.1111/nph.16283

Vargas, P., Fernández-Mazuecos, M., & Heleno, R. (2018). Phylogenetic evidence for a Miocene origin of Mediterranean lineages: species diversity, reproductive traits and geographical isolation. Plant Biology, 20, 157–165. doi:10.1111/plb.12626

Walker, J. B., Sytsma, K. J., Treutlein, J., & Wink, M. (2004). Salvia (Lamiaceae) is not monophyletic: implications for the systematics, radiation, and ecological specializations of Salvia and tribe Mentheae. American Journal of Botany, 91(7), 1115–1125. doi:10.3732/ajb.91.7.1115

Will, M., & Claßen-Bockhoff, R. (2017). Time to split Salvia s.l. (Lamiaceae) – New insights from Old World Salvia phylogeny. Molecular Phylogenetics and Evolution, 109, 33–58. doi:10.1016/j.ympev.2016.12.041

 

Botany lab of the month: Contrasting brassica plants in the garden

This is just a quick post about some instructive cruciferous vegetable (family Brassicaceae) anatomy and within-species diversity apparent in my garden at the moment.

Red Russian kale, rutabagas, and canola oil are all different varieties of Brassica napus. Red Russian kale and rutabagas are in my garden now, and the amplification of leaves and roots, respectively, through domestication is evident.

Red Russian kale (Brassica napus)

The rutabaga leaves are large, lobed, and somewhat grayish, like the Russian kale, but they are tougher and not as numerous as on the kale.

rutabaga plant (Brassica napus; Brassicaceae)

You’ll just have to take my word for it that there is no giant rutabaga-like root (technically a swollen hypocotyl, the fused lower stem and taproot, like a turnip, radish, or maca) straining the soil surface on the kale plant.

rutabaga

Anatomical differences amplified through domestication on otherwise vaguely similar-looking cruciferous vegetable plants is also visible on Brussels sprouts and collard greens, two different varieties of Brassica oleracea. A farmer or gardener familiar with the gestalt of the plants will easily identify a Brussels sprouts plant from afar as distinct from a collard greens plant, although the large plant and leaf size are similar.

Brussels sprouts plant (Brassica oleracea)

collard greens plant (Brassica oleracea)

Up close, though, you’ll see that the larger, more tender collard greens leaves have only a very tiny bud in their leaf axils (where the leaf joins with the stem).

Giant collard green leaves subtend very tiny axillary buds.

The developing Brussels sprouts, though, are not nearly done growing and are already much larger than the axillary buds  in any other variety of B. oleracea.

Young Brussels sprouts are really just giant axillary buds developing on the stalk.

While red Russian kale is Brassica napus, most of all the other kales are leafy varieties of Brassica oleracea, along with collard greens, Brussels sprouts, cabbage (which is the enlarged terminal bud, similar to the axillary bud), kohlrabi, broccoli, and cauliflower (read about this diversity and more about the anatomy involved in our essay The extraordinary diversity of Brassica oleracea). Last year we let one of the B. oleracea kales, a curly green winterbor variety, overwinter. Many of these brassicas retain the biennial life cylce of their weedy Mediterranean ancestor (read about it in our essay Caterpillars on my crucifers: friends or foes?), so overwintering is something for which a kale plant can prepare itself. The term biennial means that the plant’s life cycle requires two years to complete. In the first year the plant produces a profusion of leaves (the “rosette”). In the second year the plant flowers, sets seed, and dies. The leaves from the first year die over the winter. It is the job of those axillary buds to survive the winter as tightly wrapped bundles of overlapping leaves that will be familiar to Brussels sprouts fans. In the spring those leaves in the axillary buds unfurl and grow as the tiny stem that supports them elongates. This unfurling of leaves from otherwise small axillary buds was apparent this spring in our overwintering kale.

This winterbor kale stem overwintered. Above each leaf scar (from last year) new leaves are expanding on a new lateral stem from the axillary buds.

If you’d like to read even more about cruciferous vegetables in the mustard family (Brassicaceae), we have a few other longer essays that fill in some of this back story:

Thanksgiving turnips and the diversity of the genus Brassica

The most political vegetables: a whirlwind tour of the edible crucifersGreens: why we eat the leaves we do

Maca: A Valentine’s Day call for comparative biology

Angelica: Holiday fruitcake from a sometimes toxic family

Angelica archangelica may be the most festive species in a crowded field of charismatic relatives. Just watch out for the toxic branches of the Apiaceae family tree. This essay is one of our two contributions to this year’s Advent Botany holiday essay collection.

For scientific names of plants associated with the winter holidays, I think it would be hard to beat Angelica archangelica. Commonly known just as angelica or garden angelica, A. archangelica is one of the few cultivated members among the 60-ish species of large biennial herbs in the genus. They are distributed primarily across the northern reaches of Europe, Asia, and western North America.

Angelica stalks candied and photographed by hunter-harvester-gardener Hank Shaw. Recipe on his blog

Candied angelica stalks (young stems and petioles from first-year plants) have long been prized in Western Europe as a unique confection or addition to baked goods. If your path this holiday crosses with a fruitcake studded with bright green chunks, those are unfortunately dyed pieces of candied angelica stalk. Or you may have a qualitatively different experience with angelica in the form of delicious liqueurs that include the root or fruit of the plant as an ingredient. The floral, spicy, and fresh flavor of angelica graces gins, vermouths, absinthes, aquavits, bitters, and Chartreuse, among others (Amy Stewart’s website accompaniment to her book The Drunken Botanist has some tips for growing and using angelica for the DIY mixologist). Continue reading

Kiwifruit 2: Why are they green?

Why are some kiwifruits green when they are ripe? Or avocados or honeydew melons? The answer involves genetic accidents, photosynthesis, hidden pigments, and the words “monkey peach.”

In our kiwifruit fuzziness essay we described how the type and density of trichomes—the hairlike projections from the fruit’s skin that create the fuzziness—in the Actinidia chinensis species complex is correlated with the habitat in China to which a particular population is adapted and the ploidy level of its genome. Only polyploid (having multiple genome copies) Actinidia chinensis occupy the harshest environments—the high, arid reaches of western China—and have the highest trichome density and the longest trichomes. And those fuzzy, resilient, polyploid kiwifruits are all green on the inside (1). They are the plant kingdom’s version of an unshaved vegan after backcountry skiing for a week. The hardy plant had no trouble growing outside its plateau of origin and became the most common commercial kiwifruit in the world (A. chinensis var. deliciosa), followed closely by yellow-fleshed (“golden”), less fuzzy variants of the same species (A. chinensis var. chinensis).

An expanded view of the dozens of Actinidia species reveals orange, red, and purplish pigments that color fruits in the genus. While beautiful, this warm palette strikes me as noteworthy only in contrast to the bright green displayed by the fuzzy A. chinensis var. deliciosa that initially grabbed my attention, and, later, in green kiwiberries (A. arguta). A non-green (for lack of better terminology, “colorful”) ripe fruit, after all, is a common end point for species with fleshy fruit.

Fig. 1 from Crowhurst et al. (2008) of some fruit diversity in the kiwifruit genus Actinidia. We describe the botany and anatomy of kiwifruits in our kiwifruit fuzziness essay.

It is not difficult, however, to bring to mind other examples of species with green-ripe fruit: avocado, green grapes, some citrus, honeydew melon (I’m specifically thinking here of the pericarp or mesocarp tissue under the skin and exclude from this discussion immature fruits that lose their greenness when fully ripe, like green beans and olives). Green ripe fruit, then, in Actinidia and other taxa, seems to me to be something to explain. What, if any, function might it serve, and what are the mechanisms responsible?

While the literature on the subject is far from exhaustive, there is a fairly pedestrian explanation at least for the mechanism, if not any adaptive function, of unusually green fruit flesh outside of Actinidia: fruits start green, and straightforward mutations in a few key genes cause them to remain so. Like that intrepid, hirsute montane vegan, though, Actinidia performs the task a little differently, and it is a bit of a mystery. To understand why that is, we need some backstory on pigments in fruit and how and why they change as fruit ripens, with a focus on Actinidia. Continue reading

Kiwifruit 1: Why are they so fuzzy?

Kiwifruit is not covered in hairs. It’s covered in trichomes. And only if you’re talking about green Actinidia chinensis var. deliciosa. But, why? One answer is: pretty much to keep it from drying out. Another is: because it’s a polyploid from western China and was kind of chosen at random to be the most commonly grown kiwifruit, and they’re not all fuzzy. Those aren’t mutually exclusive answers. Put on your ecophysiology hats and grab a paring knife.

Think of fruit growth as a balancing act between ingoing and outgoing fluxes. When the balance is positive, fruits grow. When it is negative, they shrink—or shrivel. The main fluxes in question are carbon and water, which enter the fruit from the xylem and phloem of the plant vascular system. Water is lost mainly to the atmosphere via transpiration (evaporative water lost through stomata and other pores and from the skin surface). Keeping the ledger positive isn’t an easy job for a fruit. Hot, dry, and windy weather encourages transpiration and thereby increases the odds that a fruit will experience water stress. Excessive sunlight may cause sunburn. Fruits also need to avoid attack from pathogens and herbivores before the seeds within mature. A fruit’s skin—its cuticle and epidermis—is its first line of defense against abiotic and biotic threats. Some fruits resort to creative coverings to get the job done.

Here I’ll take a close look at the skin of kiwifruits. Why, exactly, are they so fuzzy?

A heart-shaped green kiwifruit (Actinidia chinensis var. deliciosa), covered in fuzzy trichomes

Continue reading

A holiday pineapple for the table

This deep dive into pineapple anatomy is our contribution this year to the very fun Advent Botany essay collection, a celebration of plants that are at least somewhat tangentially connected to the winter holidays. In previous years we’ve contributed essays on figs, peppermint, and sugar.

December is the time to bring out the fancy Christmas china, polish the silver pitchers, and . . . bedeck your best bromeliads. In 2017, as in 1700, no proper hostess can be without a pineapple for her centerpiece. Here we unpack the botany of pineapple, which is as complicated and fabulous as its cultural history. A proper hostess, after all, should also be able to dazzle her guests with tales of tropical fruit morphology. Continue reading

Botany Lab of the Month: Jack-O-Lantern

Happy National Pumpkin Day! Turn carving your Halloween Jack-O-Lantern into a plant dissection exercise.

IMG_7963

The first Jack-O-Lanterns were carved out of turnips in 17th-century Ireland. While the large, starchy hypocotyls (fused stem and taproot) of cruciferous vegetables are anatomically fascinating, this post will be about the stuff you are more likely cutting through to make a modern Jack-O-Lantern out of squash. Continue reading

Carrot top pesto through the looking glass

Isomers are molecules that have the same chemical constituents in different physical arrangements. Some terpenoid isomers have very different aromas and are important food seasonings. A batch of carrot top pesto led to an exploration of intriguing terpenoid isomers in the mint, carrot, and lemon families.

“Oh, c’mon. Try it,” my husband admonished me with a smile. “If anyone would be excited about doing something with them, I should think it would be you.”

The “them” in question were carrot tops, the prolific pile of lacy greens still attached to the carrots we bought at the farmer’s market. I have known for years that carrot tops are edible and have occasionally investigated recipes for them, but that was the extent of my efforts to turn them into food. My excuse is that I harbored niggling doubts that carrot tops would taste good. Edible does not, after all, imply delicious. My husband had thrown down the gauntlet, though, by challenging my integrity as a vegetable enthusiast. I took a long look at the beautiful foliage on the counter.

“Fine,” I responded, sounding, I am sure, resigned. “I’ll make a pesto with them.”

Carrot tops, it turns out, make a superb pesto. I have the passion of a convert about it, and not just because my carrot tops will forevermore meet a fate suitable to their bountiful vitality. The pesto I made combined botanical ingredients from two plant families whose flavors highlight the fascinating chemistry of structural and stereo isomers. Continue reading