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I came across Menyanthes trifoliata on the BSS trip to Aberlady Bay in East Lothian, led by Richard Milne. We encountered the Bogbean via a left turn from the main path through Aberlady Bay Local Nature Reserve. The beauty of this plant blew me away! The petals had a pinkish tint and were covered in swirling strands of hair.
Menyanthes trifoliata L. Photo: Chris Jeffree
There weren’t many in full flower when we were there, but there were many individuals showing their characteristic trifoliate leaves. The plant was almost the sole occupier of a large body of water, as well as growing among the surrounding grass. Some of the plants had pink buds and others had partially open pinkish-white flowers.
Dense population of M. trifoliata L. Photo: Richard Milne
Upon returning home I was curious to learn more about this plant, especially the function of the hairs on the petals. I found a study which provided evidence that the hairs might deter nectar thieving ants (Tagawa, 2018). The study showed that when the hairs were absent from the petals the number of ants that successfully acquired nectar increased. Ants which take nectar from the flower do so at no benefit to the plant, as they do not pollinate it and, moreover, that nectar might have lured pollinators. So having a mechanism to deter nectar-thieving ants would be beneficial. However the study does acknowledge that the hairs could rather be a side effect of evolution, as the habit of Menyanthes trifoliata individuals to live in water should already act as a deterrence to nectar-thieving ants. Furthermore, the hairs increased the time that ants were present on the flower, which could be a disadvantage by deterring pollinators, outweighing the comparative advantage of preventing the thieving of nectar.
The lack of flowers when we were there may have been due to the plant opening its flowers sequentially. This characteristic is seen in many species. A study by Firmage and Cole (1988) provided evidence that sequential flowering is an evolutionary advantage; it increases the blooming period and consequentially the probability that a pollinator will visit the plant and that the plant will produce seed.
Menyanthes trifoliata has pin and thrum flowers, like Primula vulgaris. Pin and thrum flowers have different forms, with pin flowers having a longer stigma and shorter filaments, and the thrum flowers having longer filaments and a shorter stigma. This increases cross pollination, as a pollinator visiting a ‘pin’ flower is more able to successfully fertilise a ‘thrum’ flower; this is an evolutionary advantage as it increases variation in the population.
Menyanthes trifoliata with fruit, growing in Silverflowe bog, Dumfries and Galloway. Photo: Chris Jeffree
Menyanthes trifoliata is found throughout the northern hemisphere; across the UK, Europe and Japan, with a different closely related variety growing in North America (Menyanthes trifoliata minor- characterised by fewer hairs on the leaves). It is a perennial plant and grows in pools, bogs and marshes and is a diagnostic species for these habitats, being used to infer the health and presence of wetlands. The dense mat of trifoliate leaves I saw at Aberlady is the result of a highly efficient mode of vegetative reproduction via rhizomes. The length and number of solon branches in the rhizome is a further diagnostic characteristic which can be used to infer the quality of the wetland habitat (Haraguchi 1996). The plant also spreads via seeds which form in 6-9 mm fruits and are dispersed through the water and by birds (Olesen, 1987). It is not considered invasive however it can spread quickly via rhizomes so care must be taken when introducing this species. It is one of only two plants in the family Menyanthaceae native the UK, the other being Nymphoides peltata (Fringed Water-lily), which has a fringe of hairs around the border of the petals.
The other British member of the Menyanthaceae, Nymphoides peltata Photo: Richard Milne
Historically, Menyanthaceae has been grouped with Gentianeaceae due to shared characteristics (dimorphic flowers, absence of mycorrhiza, sympetalous corolla and syncarpous gynoecium) and the similarity in pollen morphology between Manyanthes trifoliata L. and the Gentianaceae (Wood, 1983, Blackmore et al., 1984). However DNA sequencing has provided evidence to the contrary, with Menyanthaceae belonging to the order Asterales, not the Gentianales as once thought and it is the closest relative to the Asteraceae within the British Flora (APG II 2003).
Whilst Menyanthes trifoliata is of ‘least concern’ on the IUCN list in the UK, the species is endangered in Korea, Bulgaria and North Macedonia (Bancheva et al., 2019 and National Red List, 2022). The declining population in these areas is attributed to habitat disturbance and climate change (Lee et al., 2011). It appears similar concerns were shared in the UK during the 1960s, as mentioned in the BES Journal of Ecology as drainage of wetlands decreased the extent of its preferred habitat (Hewett 1964). The map below shows the distribution of this species in the UK; it is widespread across the UK however at a low density due to the sparsity of wetland habitats. Menyanthes trifoliata can be present in both nutritionally rich and deficient sites, and it can coexist with multiple species too so phenotypic differences are common (Haraguchi, 1993). The same BES Journal of Ecology entry mentioned above describes the plant individuals having up to 29 flowers on the scape- I did not see anywhere near this many on the plants on the individuals at Aberlady bay, but what a sight this would be!
The distribution of Menyanthes trifoliata in the British Isles. From BSBI.
References
APG II 2003. An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG II. Botanical Journal of the Linnean Society, 141(4), pp.399-436.
Bancheva, S., Natcheva, R., Vladimirov, V., Tanev, A. and Gospodinov, G., 2019. September. New data on the distribution of Menyanthes trifoliata, Carex limosa and Comarum palustre in” Torfeno Branishte” Reserve,” Vitosha” Natural Park, Bulgaria. In ARPHA Conference Abstracts (Vol. 2, p. e46606). Pensoft Publishers.
Blackmore, S. and Heath, G.L.A., 1984. Menyanthaceae. Review of palaeobotany and palynology, 42(1-4), pp.121-132.
Firmage, D.H. and Cole, F.R., 1988. Reproductive success and inflorescence size of Calopogon tuberosus (Orchidaceae). American Journal of Botany, 75(9), pp.1371-1377.
Haraguchi, A., 1993. Phenotypic and phenological plasticity of an aquatic macrophyte Menyanthes trifoliata L.. J. Plant Res. 106, 31–35 https://doi.org/10.1007/BF02344370
Haraguchi, A., 1996. Rhizome growth of Menyanthes trifoliata L. in a population on a floating peat mat in Mizorogaike Pond, central Japan. Aquatic botany, 53(3-4), pp.163-173.
Hewett, D.G., 1964. Biological flora of the British Isles. Menyanthes trifoliata, pp.723-735.
Lee, G.M. and Kim, J.G., 2011. Effects of habitat substrates and companion plants on the growth of Menyanthes trifoliata. Journal of Wetlands Research, 13(3), pp.613-621.
National Red List. 2022. Buckbean. [online] Available at:<http://redlist.moepp.gov.mk/buckbean/#:~:text=Menyanthes%20trifoliata%20has%20a%20very,management%20regimes%2C%20and%20tourism%20development.> [Accessed 18 June 2022].
Olesen, J.M., 1987. Heterostyly, homostyly, and long-distance dispersal of Menyanthes trifoliata to Greenland. Canadian journal of botany, 65(7), pp.1509-1513.
Tagawa, K., 2018. Repellence of nectar-thieving ants by a physical barrier: Adaptive role of petal hairs on Menyanthes trifoliata (Menyanthaceae). Journal of Asia-Pacific Entomology, 21(4), pp.1211-1214.
Wood, C.E., 1983. The genera of Menyanthaceae in the southeastern United States. Journal of the Arnold Arboretum, 64(3), pp.431-445.
©Hannah Udall
Botany in Scotland 