This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
Remains of this typical aquatic and arctic type are found in Inter-glacial beds at West Wittering in Sussex. To-day it is found in Arctic Europe (except Greece), N. Asia, N. W. India in the North Temperate and Arctic Zones. In Great Britain it is absent in Cornwall and N. Devon from the Peninsula province, and in the Channel from the Isle of Wight, occurring throughout the Thames and Anglia; in the Severn, not in Monmouth; in S. Wales only in Pembroke; in N. Wales in Denbigh, Flint, Anglesea; and in the Trent province and Mersey except in Mid Lancs; in the Humber province; only in Durham in the Tyne province; not in the Isle of Man in the Lakes province, and in Renfrew and Lanark. Elsewhere it ranges from Cumberland to Kent and Devon. It is naturalized in Scotland, local in Ireland.
Arrow-head is always the associate of Water Plantain, Flowering-Rush, and pondweeds of different kinds, being aquatic, and likewise found in both still and running water, growing in ponds and lakes, and also in rivers and streams. Canals are also a favourite habitat, since they are intermediate between stagnant and flowing water. The plant grows in the reed swamp.
A floating plant with erect leaves and scapes only in later stages, the Latin names and the English at once point to its main characteristic, the arrow-shaped leaves, which are at first lance-shaped, with a long point, with long lobes, borne on long 3-angled leaf-stalks. The early submerged leaves are linear and streaming. The stems are stoloniferous, the stolons or creeping shoots forming winter buds, and also, like the main stem, tubers.
The flowers are white on simple scapes, in whorls, with purple anthers in the centre, and the petals have also a purple claw. The male flowers are larger. The carpels are flattened at the sides and winged.
The plant is 2-8 in. in height. Arrow-head flowers from July up to September, and is perennial, propagated by division.
The flowers resemble those in Alisma, but are unisexual, diclinous, racemose, and the male flowers are above the female. The anthers are purple and dehisce laterally, there being many stamens. The female flowers are also variable. They are visited by flies, and contain honey.
The carpels are numerous, the smooth seeds falling when ripe into the water, in which they do not sink but germinate in the mud at the side, the plant being dispersed by currents and by its own agency.