IUCN threat status:

Least Concern (LC)

Comprehensive Description

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Description

This is a submerged aquatic plant about ¾-3' long that branches occasionally; it is a perennial. The weak stems are light green, glabrous, and filiform (about 0.5 mm. across). The leaves are arranged oppositely or in pseudo-whorls at intervals along these stems; they are 1-3" long and 0.5-1.0 mm. across. The leaves are light to medium green, linear in shape and flattened, smooth along their margins, and glabrous, tapering to acute tips. The stems and leaves often darken with age or become brown from a coating of algae. Membranous sheaths surround the stems at the bases of the leaves; these delicate sheaths disintegrate with age. Horned Pondweed is monoecious, producing separate male (staminate) and female (pistillate) flowers on the same plant. These inconspicuous flowers are located at the bases of the leaves, where male and female flowers often occur together. Male flowers occur individually; each male flower consists of a single stamen. Female flowers occur in groups of 1-5; each female flower consists of an ovary with a style and a peltate stigma. Neither male nor female flowers have petals and sepals. The blooming period can occur from late spring into the summer. Cross-pollination of the flowers occurs underwater. Fertile female flowers are replaced by beaked fruits about 3.0-3.5 mm. in length. Each slightly flattened fruit has a curved-ovoid shape that is convex and occasionally crested on one side and either straight or slightly concave on the other side. The beak of each fruit is prominent. The fruits usually have short pedicels (0.5-1.0 mm. in length) or they share a short peduncle (0.5-1.0 mm. in length). Each fruit contains a single achene that is distributed by water. The root system is fibrous and rhizomatous. Clonal colonies of plants often develop from the rhizomes.

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© John Hilty

Source: Illinois Wildflowers

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