Soleirolia, a genus of small but perfectly formed nettles

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Close-up of a flowering stem of Soleirolia soleirolia showing the male (left) and female (right) flowers. The leaves of this tiny creeping herb are about 3 mm across

I first came across this tiny creeping herb in my garden where it had been planted as an ornamental. The bright green leaves, mostly less than 3 mm across form an attractive carpet. Until now I had never been able to spot its flowers despite having checked several times over the last few years. My guess is that this species has a relatively narrow flowering time in spring and the flowers are so tiny that they are only visible with a hand-lens. For several years the genus has intrigued me, not so much because of its small size and creeping habit but because of its distribution and evolutionary relationships.

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Engraving  of a female Soleirolia flower produced by the Anglo-French botanist Hugh Algernon Weddell in the 1850s. Weddell must have had access to microscopes of the very highest quality to produce such a drawing as the flowers that are about 1.5 mm in length

Firstly because Soleirolia consists of a single species that in the wild is only known from the Mediterranean island of Corsica. This is the only genus of nettle I know that is restricted to a single island or to the Mediterranean and I am very keen to try and found out why this could be (the history of the Mediterranean basin is quite a turbulent one). Secondly Soleirolia has traditionally been grouped with the  widespread Parietaria and intriguingly with Gesnouinia,  which also includes a single species but is restricted to the Canary Islands in the Atlantic Ocean. Whilst they look very different as plants their female flowers share many similarities of form.  It might be, therefore, that Soleirolia and Gesnouinia should be viewed  as Parietaria species that have diverged morphologically as a consequence of being isolated on islands, a common phenomenon in evolutionary biology. I am currently testing this possibility using DNA sequence data and could have a better idea in a couple of months.

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Endemic to Cosrica but now an ornamental and escaped weed throughout much of the temperate World Soleirolia soleirolia forms bright green carpets of tiny leaves

 

 

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