- Written also: Arrow-head, Arrow Head, Hawaii Arrowhead (USA)
- Family: Water-plantain Family – Alismataceae
- Growing form: Monoecious, aquatic perennial, with wintering turions (tubers).
- Height: 20–80 cm (8–30 in.). Stem’s upper part angular.
- Flower: Regular (actinomorphic). Petals 3, separate, white with a purple blotch at the base. Sepals 3, separate, shorter than petals. In lower part of inflorescence there are female flowers with several pear-shaped carpels. Upper parts bear male flowers with numerous purple stamens. Anthers dark purple. Central flowers usu. have many stamens and some carpels. Flowers borne in whorls of three in the inflorescence.
- Leaves: Most leaves aerial, some floating, some submerged. Long-stalked. Blade arrowhead-like, basal lobes almost as large as apical part, floating leaves with more rounded blade, even oval with cordate base. Blade-margins entire. Submerged leaves strap-like.
- Fruit: Achene.
- Habitat: On muddy bottom in shallow water of lakes, rivers, and brackish water bays. Occasionally as a submerged form in flowing water.
- Flowering time: July–August.
Arrowhead usually has both submerged, floating, and aerial leaves. Their appearance is quite different. When growing in deep water it produces only submerged leaves, and cannot flower. The flowering is most successful when the plant is anchored at a depth of less than 50 cm (20 in.). The flowers are white with a purple centre. The fruit float well. Hence, they can travel long distances with flowing water. However, fruit aren’t always produced.
Arrowhead can be separated from its close relative dwarf sagittaria by the leaf-blades. Those of the latter have only small basal lobes or lack them entirely. Furthermore, it mostly has only floating leaves and lacks aerial ones. The flowers of these two species are otherwise exactly alike, but the anthers of arrowhead are purple, those of dwarf sagittaria are yellow. The hybrid between these two species (S. natans x sagittifolia) is relatively common. Its anthera are light violet.