Water-starworts
What are they?
The water-starworts are a small group of rather underwhelming waterplants which, for a long time were placed in their own family, the Callitrichaceae, but which have more recently been merged into a new and enlarged plantain family (Plantaginaceae).
Where are they found?
Most species are typically aquatic in nature and may be found growing more or less submerged in water, but with the leafy branches often reaching up to the surface and spreading out to look rather like duckweed. Some species will grow quite happily as prostrate mats on damp mud.
Identification
This is a very difficult group to identify to the species level for two main reasons; firstly, the leaves and growth form can be very variable and there is much overlap in the species. Secondly, ripe fruits are the best means of certain identification, but these are often not produced or can be very hard to find much of the time. As a group, water-starworts have tiny flowers that have no sepals or petals. The flowers sit solitarily in the leaf axils and may have a pair of bracts at their base, or may be without bracts. Each flower will be either male only or female only, with one flower of each sex often situated in opposite pairs of leaf axils. Only the female flowers will, of course, produce fruits. When examining the tiny, four-parted fruits, it is necessary to note the colour of ripe fruits and the presence or absence of wings along the margins - and the location and relative size of these wings.
Terrestrial plants are worth comparing with Blinks and Common Water-purslane, which grow in similar, damp habitats and can look superficially similar.
Common Water-starwort Callitriche stagnalis
Native. Widespread throughout the region in watery places, both in permanent water bodies such as lakes and rivers (where not too deep) and in wet, muddy spots in shady places. Flowers May to September. Leaves typically around 2.5-4.5mm long, larger in the terminal rosettes of aquatic plants, almost always rounded or spoon-shaped and not linear. A highly variable plant and the one most frequently found growing as a terrestrial form on wet mud. Ripe fruits greyish and with broad wings.
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Various-leaved Water-starwort Callitriche platycarpa
Native. Thinly scattered across the region and most frequently recorded around Cambridge and in the Norfolk Broads, where it prefers alkaline waters. Flowers May to September. Leaves most often of two types, being linear with a notched tip in lower leaves and becoming more broadly rounded towards the top of the stems. Ripe fruits pale brown and with narrow wings.
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Blunt-fruited Water-starwort Callitriche obtusangula
Native. Thinly scattered across the region but mostly scarce and only becoming frequent in the Fens, which is probably its stronghold in the UK. Flowers May to September. Leaves typically rather broad and frequently forming extensive, floating mats at the water surface. Ripe fruits pale brown and without wings.
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