Flower:

Flowers lack petals, are tiny and inconspicuous, nestled in the leaf axils along much of the stem, with separate male and female flowers on the same plant (monoecious), often one male and one female in one axil and a female in the opposite axil. Male flowers have have a single yellow-tipped stamen, females have 2 styles. At the base of the flower is a scale-like bract that may or may not persist.

Leaves and stems:

Leaves are opposite with pairs arranged at right angles to the pairs above and below (decussate), connected at the base around the stem (connate), and variable in shape. Submersed leaves are linear and ribbon-like, 1-veined, up to 1 inch (2.5 cm) long, up to 1 mm wide, notched at the tip, and stalkless. Floating leaves are more spoon or spatula-shaped, 3-veined, up to 3/8 inch (1 cm) wide, toothless, mostly rounded at the tip, tapering at the base. In between are transitional leaves that are commonly narrowly club-shaped. Leaves on plants that are land-locked may be more oblong-elliptic. Stems are green and smooth.

Fruit:

Fruit is somewhat heart-shaped, .8 to 1.4 mm long, usually longer than wide, blackish at maturity, 4-parted with a distinct groove between each of the 4 seeds. Seeds are covered in tiny pits, more or less arranged in lines, and have a narrow, slightly bumpy wing or ridge along the top edge, though this wing is often obscure and sometimes absent, and the arrangement of pits can be difficult to discern even under magnification but is probably easiest to see around the wing.

Notes:

Callitriche palustris is one of the most common aquatic species in the world and will grow almost anywhere it's seasonally or continually wet. In Minnesota it's been found in sandy, gravelly, muddy and mucky soils, in wet ditches, bogs, marshes, swamps, pools in rock outcrops, prairie and woodland swales, as well as lakes, ponds, streams and rivers, in slow or fast-moving water. It's considered amphibious, persisting in terrestrial forms as well as aquatic forms.

Of the 3 Callitriche species in Minnesota, it is most likely to be confused with C. heterophylla, a rare species of rock outcrops in our southwest counties and protected bays of lakes in the northeast. They can be distinguished most reliably by the fruit: C. heterophylla fruits usually do not exceed 1 mm in length, the groove between seeds is obscure or absent as is the wing along the top edge, and the pits are not arranged in lines. C. hermaphroditica, the third species, lacks floating leaves, leaf pairs are not connected at the base, and seeds are more broadly winged, the wing extending to the base.