Najas, the water-nymphs or naiads, is a genus of aquatic plants. It is cosmopolitan in distribution, first described for modern science by Linnaeus in 1753. Until 1997, it was rarely placed in the Hydrocharitaceae, and was often taken as constituting (by itself) the family Najadaceae.

The APG II system, of 2003 (unchanged from the APG system, of 1998), places the genus in family Hydrocharitaceae, in the order Alismatales of the monocots.

An infrageneric classification of two sections is proposed: Section Americanae and sect. Caulinia.

Species

1. Najas affinis Rendle - South America, Senegal, Guinea-Bissau
2. Najas ancistrocarpa A.Braun ex Magnus - China, Japan, Taiwan
3. Najas arguta Kunth - Cuba, Costa Rica, Panama, South America
4. Najas australis Bory ex Rendle - India, Madagascar, Mauritius, KwaZulu-Natal, Seychelles
5. Najas baldwinii Horn - West Africa
6. Najas brevistyla Rendle - Assam
7. Najas browniana Rendle - southern China, India, Taiwan, Java, Cavern Island in Northern Territory of Australia
8. Najas chinensis N.Z.Wang - Primorye, China, Taiwan, Japan
9. Najas conferta (A.Braun) A.Braun - Cuba, Hispaniola, Panama, Brazil
10. Najas faveolata A. Br. ex Magnus
11. Najas filifolia R.R.Haynes - southeastern United States (Georgia, Alabama, Florida)
12. Najas flexilis (Willd.) Rostk. & W.L.E. Schmidt - temperate Northern Hemisphere
13. Najas gracillima (A.Braun ex Engelm.) Magnus - Asia, North America
14. Najas graminea Delile - Africa, Asia, New Guinea, Melanesia, northern Australia; naturalized in California and parts of Europe
15. Najas grossareolata L.Triest - Sri Lanka
16. Najas guadalupensis (Spreng.) Magnus - North and South America, Caribbean
17. Najas hagerupii Horn - Ghana, Mali
18. Najas halophila L.Triest - Java, New Guinea, Queensland
19. Najas heteromorpha Griff. ex Voigt - eastern India
20. Najas horrida A.Braun ex Magnus - Africa, Madagascar, Sinai
21. Najas indica (Willd.) Cham. - Indian Subcontinent, China, Southeast Asia, New Guinea
22. Najas kurziana Rendle - Bihar, East Timor
23. Najas madagascariensis Rendle - Madagascar; naturalized in Mauritius
24. Najas malesiana W.J.de Wilde - India, Bangladesh, Indochina, Malaysia, Indonesia, Philippines; naturalized in eastern Brazil
25. Najas marina L. - widespread and nearly cosmopolitan
26. Najas minor All. - widespread in Europe, Asia, Africa; naturalized in eastern North America
27. Najas oguraensis Miki - East Asia, Himalayas (Pakistan, Nepal, northern India)
28. Najas pectinata (Parl.) Magnus - Sahara
29. Najas pseudogracillima L.Triest - Hong Kong
30. Najas rigida Griff. - eastern India
31. Najas schweinfurthii Magnus - Senegal, Cameroon, Ethiopia, Sudan, Tanzania
32. Najas tenuicaulis Miki - Honshu Island in Japan
33. Najas tenuifolia R.Br. - Hong Kong, Southeast Asia, Australia
34. Najas tenuis Magnus – India, Sri Lanka, Myanmar
35. Najas tenuissima (A.Braun ex Magnus) Magnus - Finland, Russia, Hokkaido
36. Najas testui Rendle - western + central Africa
37. Najas welwitschii Rendle - tropical Africa, western India
38. Najas wrightiana A.Braun - Mexico, Central America, Cuba, Bahamas, Venezuela; naturalized in Florida

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