Callitriche

Lansdown, R.V. 2008: Water-starworts Callitriche of Europehandbook no. 11
1Leaf-bases not joined; fruit 1.4-1.8 mm x 1.6-2.2 mm, wider than long
1'Leaf-bases joined by a ridge of tissue extending across the node; fruit (0.7-)1.0-1.5(-1.8) x (0.7-)1.0-1.6, more or less as wide as long