Hippuris vulgaris; The Mare's-tail is rather strange plant. It grows on very wet soils, often in water. On first glance one might think this plant is a horsetail species (these are primitive ferns - check out that list). This however, is not true: it is a perfectly regularly flowering plant. The flowers are very small (in the leaf axils) and as such often go unnoticed. In wet areas they are common.
Traditionally it was a member of the Hippuridaceae family but is now regarded as to be a member of the Plantain-family (Plantaginaceae). The Icelandic name of this species is Lófótur.