Umbilicus rupestris, labelled with its previous name Cotyledon major. Coloured etching by the Italian artist and engraver Magdalena Bouchard 1771. Credit: Wellcome Collection. Public Domain Mark.
This is a native species with a remarkably western distribution. It was first recorded in Somerset by the Reverend W Turner in the year 1562. It has the unusual feature of peltate leaves (the leaf stalk attaches to the middle of the leaf rather than the edge). Rather few plants are like this. Of the common garden plants I can think of only nasturtium (Tropaeolum majus). There is a peltate-leaved houseplant we like, called Pilea peperomioides, first collected many years ago from China by George Forrest for the Royal Botanic Garden, Edinburgh. The only other British plant I can think of with peltate leaves is Hydrocotyle vulgaris, the marsh pennywort.
Population of Umbilicus rupestris among rocks by the roadside (the A77, 1 km north of Ledalfoot, Ayrshire, SW Scotland). Late May 2022. Photo: John Grace
The reason why natural selection has rarely favoured peltate leaves remains a mystery which several early writers remarked upon. In recent times, a group in Dresden have collected much more information, and considered the mechanics of the peltate configuration, but seem no closer to answering the fundamental question of ‘why peltate?” (Wunnenberg et al 2021).
Also from the A77 roadside site. Photo: John Grace
Umbilicus rupestris is a succulent plant, belonging to the Crassulaceae, the family of stonecrops and many of the succulent plants people have in their homes. The family gives its name to a special style of photosynthesis, called Crassulacean Acid Metabolism (abbreviated CAM). These so-called CAM plants open their stomata and take up their CO2 at night, fixing it into malic acid which is stored away in the fleshy compartments of the leaves, which become quite acid. In the daytime, when most other plants open their stomata and take up CO2, CAM plants are busy turning malic acid into sugars and then they carry on using sugars just as any ordinary plant does.
Detail of flowers and leaves. Photo: Richard Milne
But Umbilicus is somewhat different. It only shows classical CAM behaviour when it is droughted. Well-watered plants behave more or less the same as all other plants, opening their stomata by day and closing them at night (Daniel et al 1985). Its habitats include crevices on rocky surfaces and man-made building materials, with very little soil. During heat waves we may expect it to run out of water, and that is when the CAM mode is vital for survival.
U. rupestris, its British distribution, from BSBI
My main interest in this species is however its western distribution, shown above. Michael Proctor and others suggested that such species are limited in their range by January temperatures (Proctor 2013). The East of Britain tends to be colder in January than the West. Below, I’ve plotted the temperatures of the East and West of Scotland, as 10-year running means from the 1880s. The generalisation of milder winters in the west seems to hold true but it is a small effect, slightly more than 1 degree Celsius. Note too, there is a tendency for warming from the 1940s and so perhaps this will nudge the distribution eastwards. This may already be happening. Jackie Muscott discovered the species in 2009 at an eastern site, on Blackford Hill, Edinburgh, and by the end of 2019 five other sites were found in Midlothian.
Average January temperatures in Scotland plotted from data freely-available from the UK Meteorological Office. Blue is West Scotland, Red is East Scotland. The data are plotted as 10-year running means.
I explored the distribution further using the European data from GBIF (Global Biodiversity Information Facility). The 2 degree isobars creep down the western flank of Italy, and so does the species. Of course, the variability between years is huge, and perhaps several successive years with mild winters are needed for the species to extend its range.
European distribution of Umbilicus rupestris and the (approx) 2 degree Celsius January isotherm (estimated from the map found at https://imgur.com/a/wbsFqUG). Land to the West of the white line is on average warmer than 2 degrees Celsius in January.
Historical details are hard to verify. The herbalist John Gerard shows the plant with a significant bulb, just as we see in Bouchard’s picture at the top of this article. I did not want to dig one up, so I looked at around 30 specimens using an on-line herbarium. Only two have something of a bulb. Moreover, I cannot believe Gerard’s assertion that “it grows plentifully in Northampton upon every walls about the town…it grows upon Westminster Abbey over the door that leads to Chaucer’s tomb to the old palace”. The explanation could however be that it was cultivated for medicine outside its natural range. In Gerard’s words “it is good for kibed heels, being bathed therewith, and one or more leaves laid upon the heel. The leaves and roots eaten do break the stone, provoke urine, and prevail much against dropsy”. Tully (undated blog) gives additional details of its uses in traditional medicine.
What about the names, Navelwort and Umbilicus? Its leaves do indeed look like the navel, and Navelwort was Gerard’s preferred name for the plant. He gives alternative English names: Ladies Navell, Hipwort, Kidney-woort. Another popular name is Wall Pennywort. The latin name Umbilicus rupestris was given by the controversial botanist Richard Anthony Salisbury (1761-1829): so we write Umbilicus rupestris Salisb. The specific name, rupestris is new Latin for ‘lives on cliffs or rocks’.
Postscript Yesterday I checked the site at Blackford Hill where the plant was recorded in 2009. I found it easily, alongside a main path used by dog-walkers and joggers. However the plants were mostly small and the site was more exposed than I remembered it. Possibly the population has been affected by severe winters and tidy-ups by Council workers..
One of about 40 plants at the eastern site, Blackford Hill, Edinburgh, June 1st 2022. Photo: John Grace
References
Daniel PP et al. (1985) Nocturnal Accumulation of Acid in Leaves of Wall Pennywort (Umbilicus rupestris) following exposure to Water Stress. Annals of Botany 55, 217-223.
Proctor M (2013) Vegetation of Britain and Ireland. New Naturalist Series, Collins, London.
Tully L (undated) https://www.freedom-outdoors.co.uk/navelwort-wall-pennywort/
Wunnenberg J et al. (2021) Strengthening Structures in the Petiole–Lamina Junction of Peltate Leaves. Biomimetics 6, 25.
©John Grace
Botany in Scotland 