Flower:

Flowers are single in the leaf axils all along the stem and branches, with separate male and female flowers on the same plant (monoecious); rarely flowers are paired in an axil. Male flowers are on the upper part of stems and branches, oval to egg-shaped, 1.1 to 2.7 mm (less than 1/8 inch) long, have a single stamen with a 1-chambered anther. Female flowers are all along the stems and branches, narrowly elliptic with a long beak, 2.5 to 4.7 mm long with a 3-parted style.

Leaves and stems:

Leaves are all submersed, opposite to whorled, flexible and may be arching, 1 to 4 cm (to 1½ inches) long, .2 to .6 mm wide, very gradually tapering to a pointed tip, with minute, spine-like teeth around the edges, 35 to 80 teeth per side and 1 or 2 teeth at the very tip. Prickles along the midrib are absent.

The leaf base has a thin, rounded sheath wrapping the stem with minute, spine-like teeth along the edge. Stems are round, slender, usually much branched, green, and lack prickles.

Fruit:

Fruit is a narrowly elliptic seed (achene), 2.5 to 3.7 mm long, widest above the middle, yellowish to brown, smooth to very shallowly pitted, shiny, and with a faint pattern of cells in 30 to 50 vertical rows.

Notes:

Nodding Waternymph is the most common Najas species in Minnesota and is native to parts of North America, Europe and Asia, found in soft or hard water lakes, ponds, rivers and streams. It is distinguished from other Najas by leaves gradually tapering to the tip, sheaths with rounded sides, numerous minute spine-like teeth around the blade and sheath edge with 1 or 2 teeth at the leaf tip, and smooth, shiny fruits with faint pattern of cells in rows on the surface. Magnification is required to see some (or most) of these traits. It can take on a short, somewhat bushy form or be taller and more sparsely branched.

Most similar are Slender Waternymph (Najas gracillima) and Southern Waternymph (Najas guadalupensis), both of which have leaves that more abruptly taper at the tip, dull fruits pitted on the surface, and may sometimes have 2 or 3 flowers per axil. N. gracillima has fewer teeth on leaves (fewer than 20 per side), leaves are typically narrower (rarely more than .3 mm wide), and the sheaths are more straight across the top often with a few tiny lobes (auricles), each having a spine-like tooth at the tip. N. guadalupensis is more difficult to distinguish but tends to be less leafy (leaves usually opposite or whorled in 3s) and stems are commonly brown or reddish; fruits are more helpful for ID purposes.