David told me: "This rare pondweed, otherwise known from the River Loddon,
the Bristol Avon and the Dorset Stour, was thought to
have gone extinct in the Thames in the 1950s and there have been no Oxfordshire
records for over 75 years. It was found by Frank Hunt back in August, near
Marsh Lock, just south of Henley (SU774815), growing in a part of the river
sheltered from boat traffic by a footbridge. Marsh Lock is less than 1km
downstream from where it was seen by J.E. Lousley in 1941 and about 5km below
where the Loddon joins the Thames. Frank also tells me that there is a patch in
the the Berkshire part of the river too.
This is such a distinctive species that the BSBI referee for pondweeds, Dr ChrisPreston, was quite happy to verify the record from the photograph shown on the right.