Plant of the Week – June 14th 2021- Sea Campion, Silene uniflora

This is yet another plant of shingle beaches. We’ve already had Oyster Plant and Sea Sandwort, and this week I’ve chosen a companion of theirs, Sea Campion (Silene uniflora, which used to be called Silene maritima). We see all these plants together on the foreshore of the Ayrshire coast and they never cease to amaze me. The Sea Campion has just gone past its very best; two weeks ago it was flowering so profusely that it looked like snow from afar and shimmering silk closer up. It grows in shingle alongside the other two species, and also on rocks – and sometimes even on mountains and man-made habitats such as mine spoil, ballast and disused railway lines.

Growing on a shingle beach, Ballantrae, South Ayrshire. Photo: John Grace

It’s a native, a perennial, and it forms cushions on shingle beaches all around the coast of Northern Europe.  The leaves are somewhat succulent and glaucous. It belongs to the Pink family, the Caryophyllaceae, and it has the usual character of opposite sessile leaves and five petals (but beware, in S. uniflora each is deeply divided so you might count ten). It’s much like Bladder Campion, Silene vulgaris, with its bladder-like calyx but on uniflora there are fewer flowers per shoot (1-4 rather than ‘numerous’) and the calyx is much less narrowed at its mouth. The most obvious difference is that the shoots of vulgaris are all erect whilst uniflora has trailing non-flowering shoots. Earlier authors sometimes considered uniflora to be a mere subspecies of vulgaris. Indeed they have the same chromosome number (2n=24) and sometimes they hybridise but these hybrids are extremely rare.

The genus Silene is named after Seilenos, one of the Greek gods. Those early botanists who named plants were much better at Greek mythology that we are today. I learned from Wikipedia that the etymology of the God Seilenos is “from σείω – shake, move to and fro and ληνός – wine, and meaning shattering, drunken god of wine. Seilenos presides over other demons and is related to musical creativity, prophetic ecstasy, drunken joy, drunken dances and gestures”. Well, campions do move to and fro rather like dancers and uniflora in particular seems to shimmer in the breeze. I discovered a scholarly article in which the authors tested the hypothesis that campion plants wave about to attract pollinators (Warren and James 2008). The authors attached flowers of uniflora to artificial stalks of varying length. The more mobile (long-stalked) flowers were visited by insect pollinators more frequently and by more species than were the more stable flowers. It was also shown that stalk-length is an inherited character. I had always supposed that flower stalks were there to assist seed dispersal or possibly to disperse their scent, but evidently there is much more to stalk-length. The compound eye of insects is presumably an excellent detector of motion.  

Showing five very deeply-cut petals and up to 10 stamens. Photo: John Grace

Many Silene species are dioecious (‘two houses’ i.e. separate male and female plants) and some have sex chromosomes XX/XY just like mammals (Bačovský et al. 2020).  The species most often reported to have sex chromosomes are S. latifolia and S. dioica, but S. uniflora is not one of those. Instead, it exhibits the strange intermediate sexual world of gynodioecy, where there are female plants and hermaphrodite plants. It seems an impossible system, as the hermaphrodites will be able to freely produce seed whilst the females will receive much less pollen. Not surprisingly, it is rare: only about 1 % of angiosperms have this system which, in evolutionary terms, may be ‘on the way’ to being fully dioecious.

Predominantly coastal distribution of Silene uniflora (with permission from the BSBI)

Campions are susceptible to attack by several microfungal pathogens that cause anther smut. The fungus lives on the anthers and its spores are carried from plant to plant by pollinating insects. No pollen is produced by the infected plant and so these organisms are referred to as plant-castrating fungi (Abbate et al. 2018).  There are several species of them, at least three in the genus Microbotryum (Chung et al. 2012), and also more recently Thecaphora melandrii (Smith et al. 2020). The smuts are evident from the dark spores that can smother the flower, but so far I haven’t seen this on my local population of S. uniflora. I shall keep looking and may report any positive sightings.  

References

Abbate JL et al. 2018 Co-occurence among three divergent plant-castrating fungi in the same Silene host species. Molecular Ecology 27, 3357-3370.

Bačovský B et al. (2020) The formation of sex chromosomes in Silene latifolia and Silene dioica was accompanied by multiple chromosome re-arrangements. Front. Plant Sci., 28 February 2020 | https://doi.org/10.3389/fpls.2020.00205

Chung E et al. (2012) Variation in resistance to multiple pathogen species anther smuts of Silene uniflora.  Ecology and Evolution doi: 10.1002/ece3.346

Smith P (2020) The distribution and host range of Thecaphora melandrii, with first records for Britain. Kew Bulletin 75, article 39.

Warren J & James P (2008) Do flowers wave to attract pollinators. A case study with Silene. Journal of Evolutionary Biology OI:10.1111/j.1420-9101.2008.01543.x

John Grace

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