With thanks to Tiggrx, then, that's another one for the list;
Fat Duckweed (
Lemna gibba)
I've also managed to add three more over the past couple of days; this
Hjelmqvist's Cotoneaster (
Cotoneaster hjelmqvistii), growing by the side of a suburban road, I'd previously dismissed as just another
Cotoneaster horizontalis until I decided it was worth a second look
(
Hjelmqvist's Cotoneaster, Cotoneaster hjelmqvistii)
The glabrous (hairless) leaves on this one identify it as "common"
Mock-orange (
Philadelphus coronarius); every plant I've found in this area before has turned out to be Hairy Mock-orange,
P x virginialis, with densely pubescent leaves.
(
Mock-orange, Philadelphus coronarius)
And this rose, which had scrambled over a sizeable section of bramble hedge alongside an old railway line near Cheddar, north Somerset, I've identified as
Noisette Rose (
Rosa noisettiana) - I'd presume it must have been bird-sown, a very attractive plant anyway, the flowers gradually fading from pale yellow to white as they age
(
Noisette Rose, Rosa noisettiana)