The botany of henna

Photos: Henna body art NYHENNA via flickr.com CC BY-NC-SA 2.0; Henna flowers and leaves J.M. Garg via wikipedia.org CC BY-SA 4.0

Note: This is a science outreach piece belonging to a series I wrote for the newsletter of the Fredericton Botanic Garden.  I’d be happy to see it modified for use elsewhere and so am posting the text here under a CC BY-NC 4.0 license. If you use it, though, I’d appreciate hearing where and how.

 

Henna-based body art has thousands of years of history in India, Africa, and the Middle East, and it’s an increasingly common sight in the Western world too.  The intricate designs are beautiful, and many traditional designs are packed with symbolism and story.  But the henna itself has a story, too – a botanical one.  For those of us who love plants, history, and naming, there’s a lot to like about henna.

The paste henna artists use is reconstituted from dried and ground leaves of the henna plant (sometimes called Egyptian privet).  It’s a heat-loving tropical plant that’s related to loosestrife (in the family Lythraceae).  It originated in western India, where it still grows wild, but because it’s been prized and widely cultivated, it now occurs worldwide in the tropics.  Our tropical floras are thoroughly sprinkled with such human-assisted migrants – not only henna, but coconut, mango, tamarind, guava, bougainvillea, and others.

Henna’s Latin name is Lawsonia inermis.  The genus Lawsonia was named by Linnaeus – the brilliant pioneer who, in the 18^th century, regularized the scientific organization and naming of plants and animals. He named henna for his friend Isaac Lawson, a Scottish physician and botanist.  (The “physician and botanist” combination was very common at the time, partly because medicine was a profession that afforded plenty of leisure, and partly because so many remedies were plant-based*.)  Linnaeus was very much in Lawson’s debt, because Lawson was a patron as well as a friend.  In particular, Lawson financially supported publication of Linneaus’ masterwork, Systema Naturae, in 1735.  It’s hard to exaggerate the role of the Systema Naturae in kickstarting modern biology – perhaps it’s best to say that the Systema Naturae was to biology’s 18^th century what Darwin’s Origin of Species was to its 19^th and 20^th.  Every undergraduate learns about Linnaeus (and his house, in Uppsala, Sweden, is a fascinating museum).  Lawson, though, seems otherwise to have been forgotten – not an uncommon fate for the supporting characters without which science wouldn’t progress.  It’s heartening, therefore, to see him honoured in henna’s Latin name.

Henna can be used for body art because its leaves contain lawsone, a chemical that binds to proteins in skin cells and produces an orange-brown colour.  (While the binding is permanent, skin cells are naturally and gradually shed, along with their bound lawsone, which is why the henna art is temporary).  Why the lawsone? Like most of the interesting chemistry that plants produce, lawsone is almost certainly defensive. In lab experiments, it has antifungal and antibacterial properties; in nature, it may defend Lawsonia against microbial pathogens or against insect herbivores (or both). In this function it’s joined by thousands of chemicals we use in industry, medicine, cuisine, and recreation.  Think of latex from the rubber tree, aspirin from willow bark, eugenol from clove flower buds, capsaicin from hot peppers, caffeine from the coffee bean – all chemicals that plants have evolved to synthesize as weapons against their enemies, be they microbial, insect, or even mammalian.

Henna isn’t the only familiar dye to have plant origins. Perhaps the dye we now know best is indigo – the colour of blue jeans – and while most indigo is now synthetic, it was originally extracted from the indigo tree (Indigofera tinctoria) or from dyer’s knotweed (Persicaria tinctoria).  Before indigo, woad (from the European mustard Isatis tinctoria) was the common blue; the Picts of ancient Britain are reputed to have used it in much the same way that other cultures used henna**. Among familiar reds, madder is of plant origin (Rubia tinctorum***), while cochineal (the dye that put the “red” in British redcoats) is extracted from scale insects that live only on their cactus hosts.  In each case the dye molecule is likely defensive, but we’ve appropriated it to add colour to our lives.

Henna: source of colour, art, tradition, and through its naming, a memorial to an otherwise unsung figure in the history of science.  What more could you ask from a plant?

© Stephen Heard (sheard@unb.ca) April 20, 2017 but licensed CC BY-NC 4.0

Other Botanic Garden newsletter outreach pieces:

• The biology of plant galls
• Nature’s pharmacy?  Medicinal plants in the Garden
• Why goldenrods don’t make you sneeze: the biology of pollination
• How plants prepare for winter
• What’s in a (Latin) name?
• Spring, light, and strategy on the forest floor
• The garden of insects
• There’s fungus among us
• Life under the snow

---

*^They still are.  I don’t mean so-called “natural” or “herbal” remedies – which are problematic either if they don’t work (in which case they’re worthless) or if they do work (in which case inconsistent dosing and unknown drug interactions can be dangerous).  I mean, instead, the large part of our drug arsenal based on chemicals originally extracted from plants.  Think about aspirin (willow), digitalin (foxglove), quinine (fever tree), taxol (yew), codeine (opium poppy), scopalomine (jimsonweed), and many, many more.

**^I was surprised to learn that there’s actually considerable controversy about whether the Picts used woad in this way.  The every-schoolchild-knows story is based on Roman accounts, but (imagine!) the stories conquerors tell about the peoples they attempt to subjugate are not always true.

***^Note the “tinct” in all four names – it comes from the Latin for dyeing, but also as an English word means to faintly or subtly colour.

 

 

Advertisement