H. palustris: aquatic; branches whorled, leafy, entirely submerged; leaves alternate deeply pinnatifid, with narrow-linear lobes; flower-stem erect, leafless from the centre of the whorl, rising out of the water, bearing whorls of 3-6 handsome, pale purplish-lilac flowers. - Featherfoil. - Pools and ditches. Fl. May to June.