Flower:

Flowers are usually single in the leaf axils all along the stem and branches, with separate male and female flowers on the same plant (monoecious); sometimes there are 2 or 3 flowers in an axil. Male flowers are on the upper part of stems and branches, oval to egg-shaped, 1.5 to 3 mm (to 1/8 inch) long, have a single stamen with a 4-chambered anther. Female flowers are all along the stems and branches, elliptic with a short beak, 1.5 to 4 mm long with a 4-parted style.

Leaves and stems:

Leaves are all submersed, opposite or whorled (usually in 3s), flexible, straight to somewhat arching, 1 to 2.8 cm (to ~1 inch) long, .2 to 2 mm wide. Prickles along the midrib are absent.

Around the edges are minute spine-like teeth, 20 to 100 per side. Leaves have a short taper to a blunt or pointed tip with 1 to 3 spine-like teeth at the apex. The leaf base has a thin, rounded to sloping sheath wrapping the stem with a few minute, spine-like teeth along the edge. Stems are round, very slender to somewhat stout, usually much branched, green to yellowish or reddish, and lack prickles.

Fruit:

Fruit is an elliptic to spindle-shaped seed (achene), 1.2 to 2.5 mm long, widest near the middle, purple-tinged, dull and pitted, the pits regularly arranged in 20 to 40 vertical rows.

Notes:

Southern Waternymph has a scattered distribution in Minnesota with the highest concentration in the north-central lakes region, found in soft or hard water lakes and ponds, in sand or muck, shallow or deep. It is distinguished from other Najas by leaves usually opposite or whorled is 3s, .2 to 2 mm wide with a short taper to a blunt or pointed tip, 20 to 100 spine-like teeth per side and 1 to 3 teeth at the tip; sheaths rounded to sloping with a few spine-like teeth along the edge; 1 to 3 flowers in leaf axils; dull fruits covered in pits arranged in 20 to 60 vertical rows. Magnification is required to see some (or most) of these traits.

There are 4 recognized subspecies, 2 of which have been recorded in Minnesota: subsp. guadalupensis has leaves with 50 to 100 teeth per side and stems not more than .8 mm diameter; subsp. olivacea leaves have 20 to 40 teeth per side, stems at least 1 mm diameter and is considered rare in Minnesota. According to the DNR, subsp. olivacea is endemic to the Great Lakes region, has a preference for harder water with sandy bottoms, and is at risk primarily from shoreline development (what isn't??); it was listed as a Special Concern species in 2013. Interestingly, Michigan Flora also has records of both these subspecies, but note few of their specimens identified as each match these descriptions. It looks like more study is needed to sort this out. The other 2 subspecies have limited ranges: subsp. muenscheri in eastern New York, has fruits 3+ mm long with pits in 50+ rows, and subsp. floridana in Florida and adjacent states, has leaves with teeth more visible to the naked eye and male flowers have 1-chambered anthers.

In any case, most similar are Nodding Waternymph (Najas flexilis) and Slender Waternymph (Najas gracillima). N. flexilis leaves have a long, gradual taper to the leaf tip, sheaths are typically rounded, and fruit is shiny and smooth or obscurely pitted at best. N. gracillima leaves are more thread-like (rarely more than .3 mm wide) and sheaths are typically straight across the top with a few, small, spine-tipped lobes along the top edge.