1. Najas Linnaeus, Sp. Pl. 2: 1015. 1753; Gen. Pl. ed. 5; 445, 1754.
Bushy-pondweed, naid, water-nymph, naiade [Greek Nnaias, a water-nymph]
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.
Species 40 (8 in the flora): nearly worldwide.
Seeds are necessary for certain determination of species of Najas.
Chase, S. S. 1947. Preliminary Studies in the Genus Najas in the United States. Ph.D. thesis. , Cornell University. Chase, S. S. 1947b. Polyploidy in an immersed aquatic angiosperm. [Abstract.] Amer. J. Bot. 34: 581--582. Haynes, R. R. 1979. Revision of North and Central American Najas (Najadaceae). Sida 8: 34--56. Haynes, R. R. and C. B. Hellquist. 1996. New combinations in North American Alismatidae. Novon 6: 370--371. Lowden, R. M. 1986. Taxonomy of the genus Najas L. (Najadaceae) in the Neotropics. Aquatic Bot. 24: 147--184. Triest, L. 1988. A revision of the genus Najas L. (Najadaceae) in the Old World. Mém. Acad. Roy. Sci. Belgique, Cl. Sci. (8° ), n. s. 22: 1--172, 29 plates. Wentz, W. A. and R. L. Stuckey. 1971. The changing distribution of the genus Najas (Najadaceae) in Ohio. Ohio J. Sci. 71: 292--302.