Herbs, aquatic, glabrous, submersed in fresh or brackish waters. Stems slender, much branched, rooting at proximal nodes, sometimes armed with prickles on internodes. Leaves: sheaths variously shaped, margins usually toothed with 1–15 teeth per side, teeth similar in size and structure to those of blades; blade linear, 1-veined, sometimes armed with prickles on midvein abaxially, margins usually serrate to minutely serrulate with 5–100 teeth per side, apex acute to acuminate, with 1–3 teeth, teeth multicellular, formed by layers of cells decreasing in cell number distally, terminated by large, sharp-tipped cell, or teeth unicellular. Inflorescences: involucres mostly present in staminate flowers, rare in pistillate, clear, bronze, brown, light green, purple, or red-purple. Staminate flowers subtended by membranous involucre, involucre rarely absent; peduncle short, elongating at anthesis, pushing flower through involucre; anther sessile, 1- or 4-loculed. Pistillate flowers sessile; ovary 1-loculed; ovules basal, 1; style terminal (arising off-center at apex of ovary and fruit in Najas gracillima), 2–4-branched. Fruits dehiscing by decay of ovary wall; ovary wall extremely delicate, closely enveloping seed. Seeds fusiform to obovoid, apex occasionally asymmetric or recurved, aerolateareolate; raphe basal; testa 3 or 10–15 cell layers thick, hard, brittle; aeroleareoles formed by outer two cell layers of testa, irregularly arranged or in 15–60 longitudinal rows, end walls often raised, giving testa papillose appearance; endosperm absent. x = 6.
Seeds are necessary for certain determination of species of Najas.
Species 40 (8 in the flora).
|1||Staminate and pistillate flowers on different plants; testa pitted, 10–15 cell layers thick; midrib of leaf blade abaxially with prickles; internodes of stem usually with prickles; seeds 1.2 mm wide or wider||Najas marina|
|1||Staminate and pistillate flowers on same plants; testa smooth or pitted, 3 cell layers thick; midrib of leaf blade abaxially without prickles; internodes of stem without prickles; seeds 1.2 mm or less wide, if 1.2 mm wide, then with smooth testa.||> 2|
|2||Style offset to one side of center at seed apex||Najas gracillima|
|2||Style situated at center of seed apex.||> 3|
|3||Seeds recurved; leaves usually recurved with age.||> 4|
|3||Seeds not recurved; leaves spreading to ascending with age.||> 5|
|4||Areoles of testa broader than long, ladderlike; widespread in a variety of habitats||Najas minor|
|4||Areoles of testa longer than broad, never ladderlike; plants of Florida and Georgia in sandy-bottomed lakes||Najas filifolia|
|5||Teeth of leaf blade multicellular, 8–22 per side||Najas wrightiana|
|5||Teeth of leaf blade unicellular, 18–100 per side.||> 6|
|6||Testa smooth, glossy; seeds narrowly to broadly obovoid, deep brown to yellow; anthers 1-loculed||Najas flexilis|
|6||Testa pitted, dull; seeds fusiform, yellowish white to greenish or reddish brown or with purple tinge; anthers 1- or 4-loculed.||> 7|
|7||Leaf sheath deeply auriculate at apex; restricted in North America to California||Najas graminea|
|7||Leaf sheath rounded to truncate slightly auriculate at apex; widespread in the flora.||Najas guadalupensis|