Parentucellia viscosa – March 3rd 2025 – Yellow Bartsia

This plant is rare in Scotland. It belongs to a Family of parasitic plants, the Orobanchaceae, which includes the broomrapes and toothworts. Members of this family make underground connections with a host plant, drawing sugars and mineral nutrients from the host. We were excited in 2022 when we found a rare broomrape on the west coast of Scotland, at Ardrossan, in plain sight at a derelict military installation (see the report here). Broomrapes lack chlorophyll altogether and are totally reliant on the host plant, but Yellow Bartsia is merely a hemiparasite; it has its own green leaves and is capable of fairly normal rates of photosynthesis (Press et al 1993).

Parentucellia viscosa, Yellow Bartsia. Note the glandular hairs that make the plant sticky, and see the seeds of other species that are sticking to it. Image: Chris Jeffree.

Worldwide, parasitism by means of an underground connection (a ‘haustorium’) has evolved 12 times independently, resulting in at least 292 genera and about 4750 species (Nickrent, 2020). In the British Isles the most common hemiparasites are the eyebrights (Euphrasia) the Yellow rattle (Rhinanthus minor) and Red Bartsia Odontites vernus. Chris Jeffree found the Yellow and Red Bartsias growing together  (see below) – surely an extremely rare sight, leading us to wonder about possible subterranean competition. Perhaps they can parasitise each other.

Parentucellia viscosa, Yellow Bartsia, growing with Odontites vernus, the Red Bartisia (pink flowers). Other species in the image include clover, Trifolium repens, which may be the host plant for both species. Image: Chris Jeffree.

Parentucellia viscosa is considered to be a native plant; Pearman’s account of when it was first observed in Britain is more detailed than usual, as there some uncertainty (Pearman 2017). Evidently it was previously considered to be a large Euphrasia. Most likely, John Ray the English parson-naturalist saw it first, in Cornwall, more or less at its present headquarters.

It grows up to 50 cm tall, and is covered with sticky (viscid) glandular hairs, hence its specific name viscosa and the alternative English name Glandweed. The stickiness is presumably to deter herbivores although recently a Japanese researcher has suggested it traps seeds that have been caried in the wind, and these seeds will germinate around it, thus becoming new host plants (Yamazaki 2023). (Far-fetched I think!).

Flowers of Parentucellia viscosa. Note the glandular hairs on all parts of the plant. Image: Chris Jeffree.

The plant is pollinated by bees, butterflies and flies. The best information I could find on the life history was from Humbolt Bay in California, where the plant is invasive on sand dunes (Pickart and Wear 1999). Many small seeds are produced (the authors estimated as many as 12,000 per plant) and they germinate late-March to late-May. Plants flower in June and the plants die in August. Some seeds remain in the soil, as a ‘seed bank’.

In the absence of an available host, the plant is capable of weak growth. Normally, it finds a suitable host, which it infects to its advantage. It is not especially selective in its choice of host, but seems to have a preference for grasses and members of the Fabaceae (especially Lotus and Trifolium). Perhaps someone reading this can help me to understand the evolutionary status of hemiparasites: why waste resources on making the photosynthetic machinery when you can rely on photosynthates from the host?  Were hemiparasites once completely parasitic and have recently evolved with green leaves as ‘an insurance policy’ or are they en route to being completely parasitic like their relatives the broomrapes and toothworts. Perhaps members of the Fabaceae are favoured hosts because these species are nitrogen-rich. Certainly, there are unanswered questions.

Developing seed capsules of Parentucellia viscosa. Image: Chris Jeffree.

Why is it rare in Scotland? Some authorities have suggested it is warmth-loving yet its British and World distributions do not support this view. In Scotland, it has been recorded in Orkney and Shetland; elsewhere in the world it occurs in some rather cold places including the Falkland Islands (Islas Malvinas) and Scandinavia. Moreover, the types of habitats  where it is found do not seem especially rare. Quoting from the BSBI Atlas 2020 we have:

“A hemiparasitic annual herb of damp, open grassy places on sandy soils, often by tracks. It normally occurs in drier dune-slacks and in reclaimed heath-pasture, but is also found on pathsides, rough and scrubby grassland and field-borders, and increasingly in re-seeded amenity grasslands and waste places. It thrives on disturbance”.

Distribution of Parentucellia viscosa in Britain and Ireland, from BSBI/Maps.

Perhaps we will see more of it, as it is sometimes included in commercially-available wild-flower mixtures. One can buy 100 seeds for £3.84 on ebay!

Distribution of Parentucellia viscosa in the world, from GBIF. Note a tendency for oceanic regions.

In older books and Floras we find that Parentucellia is placed in a large Family that contains many species with bilateral flowers, the Scrophulariaceae. But evidence from DNA indicated it should not be the case, as it did not reflect true relatedness. So, many genera were unceremoniously removed and placed in other Families. Cymbalaria, Euphrasia, Rhinanthus and Parentucellia were all placed in the Orobanchaceae (APG 2016).

How did Parentucellia get its name? It was to honour Pope Nicholas V, who was really Tommaso Parentucelli (1397-1455). He founded the Vatican Library and established a Botanic Garden in Rome. The English name Bartsia honours a Dutch physician-botanist, Johann Bartsch (1709-38).

References consulted

 APG, Angiosperm Phylogeny Group (2016), An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG IV, Botanical Journal of the Linnean Society, 161 (2): 105–20, doi:10.1111/boj.12385

Lacerda R and Ascensão L (2015) Glandular trichomes on the leaves and flowers of Bellardia trixago and Parentucellia viscosa (Orobanchaceae). Microscopy and Microanalysis 21.S6 58-59.

Nickrent DL (2020) Parasitic angiosperms: how often and how many?. Taxon 69.1 (2020): 5-27.

Pearman D (2017) The discovery of the native flora of Britain and Ireland. BSBI.

Pickart AJ and Wear KS (1999) The ecology of Parentucellia viscosa invasion in dune wetlands. pages 57-70 in Proceedings of the Californian Exotic Pest Control Symposium 5, edited by M Kelly et al. https://www.cal-ipc.org/wp-content/uploads/2017/12/Proceedings_1999NoNames-reduced.pdf#page=58

Press MC et al. (1993) Gas exchange characteristics and nitrogen relations of two Mediterranean root hemiparasites: Bartsia trixago and Parentucellia viscosa. Oecologia 95, 145-151.

Tank DC et al. (2006). Review of the systematics of Scrophulariaceae sl and their current disposition. Australian Systematic Botany, 19(4), 289-307.

Upson R and Lewis R (2014). “Updated Vascular Plant Checklist and Atlas for the Falkland Islands, 2014” (PDF). Falklands Conservation.

Yamazaki K (2023) The sticky hemiparasitic plant Parentucellia viscosa catches hostplant seeds that may provision its descendants. Acta Oecologia 121, 103949.

See also a good description at

https://fermanagh.bsbi.org/parentucellia-viscosa-caruel

Attribution for image at the head of the article: Patrice78500, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons

©John Grace