Plant of the week – 31st August – Toothwort (Lathraea squamaria)

I think I first became aware of the toothwort through the I-Spy book of British Wild Flowers.  Most plants depicted in the book were relatively common, but the toothwort was one of just two whose account ended with the sombre statement “it is not common.”  The other was the bee orchid, and I became somewhat obsessed with seeing both, and have been ever since.  I was about nine when I found my first bee orchid, but I had to wait till the middle of my A-levels to finally track down a toothwort, thanks to a chemistry field trip to the Peak District, which brought me my first encounters with several exciting plants, including also Chrysosplenium alternifolium, Viola lutea and female Petasites hybridus.  As for what one actually learns, academically, on a chemistry field trip, I really can’t recall a single thing, other than empirical confirmation that limestone rocks are good for plants.

Toothworts belong to the genus Lathraea, one of only three genera of full parasites that are native to Britain, the others being Orobanche and Cuscuta (in fact Orobanche has been broken into two pieces by recent classification changes, but we won’t talk about that).  The toothwort itself is Lathraea squamaria, the only native British species in the genus.  It gets its common name from the calyces, which resemble molar teeth, especially when the flowers have gone over: they are sort of like eggs that have been cracked open, forming four large, neat triangular teeth.  The whole plant is a milky pink colour.  It grows as a parasite on the roots of hazel, elm and sometimes other woody plants, scattered across much of Britain.  The leaves are reduced to scales so all we see is the little flowering stem, usually near the base of the host plant.

Since my first encounter with it, I’ve seen the toothwort in Bristol (right under the suspension bridge), Italy, Estonia (a rubbish specimen, mind) and Humbie Woods south of Edinburgh, where I spotted the plants during an epic walk along the river from Pencaitland.  A few years later I led a group of half a dozen Botsoc council members back to see the plant, but completely failed to refind it, which was a little embarrassing.  I now know why: I’d found the plant in Humbie at the start of May 2012 (the timing is crystal clear because it was just before my son was born), but the season that year was very late, and generally May is too late to see it.  Hence when I returned a few years later during the easter break, I finally relocated it, and last year took some students along to see it.  One of them, Jude Stone, responded to the visit with the rather lovely painting seen here, which is of plants from this very site.  I had planned to take an Estonian nature documentary maker to the site this year, but Covid 19 put the kybosh on that.  Still the 2019 excursion also produced Adoxa moschatellina, and the only other Lathraea that occurs in Britain – L. clandestina, the purple toothwort.

The purple toothwort is not native, but occurs occasionally as a naturalised alien in various parts of Britain.  Midlothian is actually one of the best places to see it, as it occurs sporadically near rivers in the area, including by Pencaitland and along the cycle path that runs along the river Esk as it approaches Musselburgh.  There are also masses of it in Edinburgh Botanic Gardens on the east side of the largest pond, and like its relative it flowers around Easter.  It is native to western Europe and I have seen beautiful patches of it in the limestone hills of northern Spain.

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As its English name suggests, L. clandestina is a vivid purple – not just the corolla but the calyx too, which are pretty much the only parts to appear above ground.  While L. squamaria has a normal looking stem, the stem of L. clandestina is shortened to almost nothing, with the flowers appearing as a cluster at ground level, having seemingly appeared out of nowhere, which I think explains the “clandestine” of the Latin name.

A curious feature of this species is the thick white rims to the four corolla teeth.  When closed, these form a cross-like structure on top of a little purple egg.  If you’ve seen the movie Alien, it does look rather like a miniature version of the sinister alien egg that gave John Hurt such a bad case of indigestion.  But what bursts from this egg is of course the corolla, though it is several times larger than the egg, so it’s a botanical wonder that it fits inside at all.  The corollas of the native toothwort, by contrast, protrude only slightly from the calyx. 

Taxonomically, the toothworts have been somewhat faithless, jumping ship from one family to another in a manner reminiscent of the current UK prime minister.  Originally, they were classified in the family Orobanchaceae, along with the broomrapes and other genera of full parasites not found in Britain.  Then, as systematists began to test plant relationships using DNA evidence, it was discovered that Lathraea was not especially closely related to Orobanche; in fact its closest relative within the British flora appears to be the yellow rattle, Rhinanthus.  Rhinanthus is one of seven British semi-parasitic genera that were classified in Scrophulariaceae at the time, and the inflating calyces that give it its common name are a bit like those of the toothwort.  Hence Lathraea was moved to Scrophulariaceae for a little while. 

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Further molecular work, however, showed the scrophs themselves to be an unnatural group.  The characters that link them together are ancestral among a larger grouping of plants including several other British families, notably the mint family, Lamiaceae (see diagram).  The Scrophulariaceae as was comprise four distinct evolutionary lines, and so broke into four pieces.  Monkey flowers moved to Phrymaceae, all the semi-parasites went to Orobanchaceae, and most other species deserted to the unglamorous destination of Plantaginaceae.  So jarring is this strange combination of foxgloves, speedwells and toadflaxes with the humble plantains that certain authors refuse to accept it, erecting the family Veronicaceae to contain the refugees from Scrophulariaceae.  Meanwhile, Scrophulariaceae is reduced to a rump including just three native British genera: Verbascum, Scrophularia and Limosella, though it did at least acquire Buddleia as a booby-prize.

Amidst this blizzard of family hopping, Lathraea crawled back with its tail between its legs into the newly mighty Orobanchaceae.  The family now contains both full parasites and semi-parasites including eyebrights, bartsias, cow-wheats and louseworts as well as yellow rattles.  We can be confident that this time, the family will stay together.  In an evolutionary sense, it is much more difficult to acquire a new ability than to lose one.  To become a semi-parasite, a plant must evolve a complete system of detecting a host, attaching to it and consistently drawing nutrients from it without triggering the host’s defences.  This has been achieved ten times across the flowering plants, if you exclude parasites on fungi which are a whole other story.  One such event occurred in the common ancestor of the expanded Orobanchaceae.  These semi-parasites hedge their bets, getting food from both chlorophyll and the host.

To evolve a full parasite from a semi-parasite is much, much easier.  All you need to do is lose the ability to make chlorophyll, and not die.  The former can occur through a simple genetic mutation, the latter will follow if enough food is being drawn from the host.  Hence multiple transitions from semi to full parasites occurred in Orobanchaceae.  One gave us the broomrapes, another the toothwort. 

The native toothwort remains a rare treasure, and seldom grows in the most accessible places, but when life here gets back to normal I’ll be happy to take people to see it in Humbie.  Otherwise, you can get your fix of purple toothwort with a trip to Musselburgh – look for it between the cycle path and the river, often right by the water, just downstream from the weir.

Richard Milne