Last weekend we visited Bathgate, a town in the Central Belt of Scotland, famous for being the place where oil was first extracted from shale by James ‘Paraffin’ Young in the 1850s. Bricks were made there from the 1880s to 1920; later came the manufacture of motor vehicles on a huge scale (1961-1986). Now it has all gone, razed to the ground and replaced by neat and tidy rows of houses, with streets bearing the name of the industrial pioneers. There are some visible remains on low lying areas of water-logged and derelict ground that developers cannot use. We’ll be plant-hunting there in the summer.
Whitlowgrass at the Tesco Supermarket, Bathgate, 4th February 2023. Photo: John Grace
I strolled around, looking for anything that might be flowering at this time of year. I visited Bathgate’s huge Tesco Supermarket and spotted these tiny plants at the edge of the car-park. The flowers were ‘whiter than white’ and the plants themselves formed a population of green rosettes, each no bigger than a 50 pence piece. I thought “it’s Erophila, Whitlowgrass – it will make a good Plant of the Week”.
Whitlowgrass is not a grass; it belongs to the Brassicaceae, the Family of mustards and cabbages. Like all members of the Family, the flowers have four petals, but in Erophila each petal is deeply bifid, so anyone may be forgiven for thinking there are eight petals. The flowers are white and quite often partially closed.
Initially I believed ‘my’ plants to be Erophila verna. It is a native plant, first described and illustrated in Gerard’s Herbal of 1597. He says “it hath been taken to heal the disease of the nails called a whitlow, whereof it took his name” It is sometimes called ‘Nailwort’.
I was surprised to find this species flowering so early in the year. It is one of the first plants to flower in the spring but most books say that Erophila flowers in March (this was the first week of February). However, the books may be out-of-date: Büntgen et al. (2022) recently compiled a large data set that shows how spring in Britain occurs about a month earlier than hitherto.
Two images of Whitlowgrass. Left: from Bathgate, West Lohian, Erophila glabrescens, the ‘Glabrous Whitlowgrass’ ; Right: from Inveresk, East Lothian, Erophila verna, the Common Whitlowgrass. Differences in the petals and the degree of hairiness can be seen. Images from John Grace (left) and Chris Jeffree (right).
Erophila is is very variable and seems to have caused much trouble for taxonomists who have sometimes given names to versions that seem to be local microspecies (see Rich and Jermy 1998). Stace (2019) says that Erophila is ‘a much-misunderstood genus’. I’ve tried my best to understand it !
At the root of the ‘trouble’ is the chromosome count. For E. verna, instead of having a constant number of chromosomes as we expect from a true species, populations have been found with 14, 28, 30, 32, 36, 38, 39, 52, 58 and 64 pairs of chromosomes. As with many plant species, it does not behave as a ‘normal’ species. It is not unique in this respect; there are many plant species which show variations in their chromosome count, the most obvious of which are polyploids where the counts are related by being multiples of a base number, eg 7, 14, 28, 56 (see Elkington 1984). But Whitlowgrasses are more complicated than this! Nearly a hundred years ago, it was found that local populations differed in their chromosome numbers, and crossing a version with 15 chromosomes with one with 32 chromosomes did produce some fertile seeds which germinated to give plants with 47 chromosomes (Winge 1933). Stace (2019) and other ‘modern’ authors recognised just three species of Erophila: one of them, Erophila majuscula ‘Hairy Whitlowgrass’ has 14 pairs of chromosomes and it is tempting to assume that this is the ‘base chromosome number’ of a hypothetical common ancestor. Erophila glabrescens ‘Glabrous Whitlowgrass’ has 48-56 pairs. Dear reader, if you like mathematics you can suggest how multiplications of 14 with several ‘accidental’ subtractions and additions can produce the range of variation that has been observed.
These three Whitlow grasses are very similar to each other. Some authors simply prefer to lump them together as Erophila verna agg. But we can do a bit better than that. I looked at several images from Chris Jeffree and I studied the key and descriptions in Rich (1991) and Rich & Jermy (1998). I concluded that ‘my’ plants were not E. verna but the less common E. glabrescens. If you compare the pair of images above, you can see that ‘my’ plants have almost no hairs on the leaf surface and flower stalk, and their petals are bifid to less than halfway down the petal, whereas Chris’s are properly hairy and the petals are deeply bifid, divided almost to their base.
The leaf-hairs of E. verna are star-like and arise from pimples. Images: Chris Jeffree.
The life cycle of Whitlowgrass is short. After flowering in very early spring, seed production occurs just a few weeks later. For most of us, we don’t notice these plants until we go plant-hunting in the early summer; then we see the characteristic seed pods (the leaves have shrivelled or vanished completely, the stalks remain). There is no special mechanism for seed dispersal, although the flower stalks elongate somewhat during fruiting, giving more chance for seeds to be dispersed at least short distances by the wind.
The seeds are dormant at first; they need about three months to ‘ripen’ on the soil surface before they can germinate in September or October. The young plants can tolerate the hard winter conditions and are ready to flower when the temperatures increase a little in the spring.
Whitlowgrass is found growing on bare ground, often on gravel or silty soil. The habitats include lime-rich rocks, walls, and dunes. The strip of soil around car parks fulfils the requirement – it remains fairly ‘open’ because it is treated with herbicide in the early summer, and these plants have completed their life cycle by then.
Erophila verna seed production. The seed pod in the Brassicaceae is termed a siliqua. The pod is in two halves and when the seeds have been shed the transparent membrane between the halves remains. Images: Chris Jeffree.
There is also a question mark over whether Whitlowgrass belongs to the genus Erophila or to Draba. Both Rich (1991) and Stace (2019) opt for Erophila, saying the evidence for removing it to Draba, as some authorities have done, is ‘equivocal’.
Distribution maps of the three Erophila species. Left: E. verna, Centre: E. glabrescens, Right: majuscula. From the BSBI.
The distribution maps (above) show that E. verna is by far the most common of the three species, and close inspection of the data (not clear on these maps) suggests E. verna is becoming more frequent. It has spread beyond Europe to distant parts of the world, most notably Australia and North America, probably taken there accidentally via mud on the boots and chattels of the early European colonists.
References
Büntgen U et al. (2022) Plants in the UK flower a month earlier under recent warming. Proceedings of the Royal Society B, v. 289, doi:10.1098/rspb.2021.2456.
Elkington TT (1989) Cytogenetic variation in the British flora: origins and significance. New Phytologist 98, 101-118. https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1984.tb06100.x
Rich TCG and Jermy AC (1998) Plant Crib. BSBI, London
Stace CA (2019) New Flora of the British Isles. C&A Floristics.
Winge Ö (1933) A case of amphidiploidy within the collective species Erophila verna Hereditas 18, 181–191.
©John Grace
Botany in Scotland 