Podostemum ceratophyllum Michaux, Fl. bor.-amer. 2: 164, t. 44. 1803. Lacis ceratophylla (Michaux) Bongard, Mem. Acad. Imp. Sci. Saint-Petersbourg, ser. 6, Sci. Math., Seconde Pt. Sci. Nat. 1: 78. 1835.-TYPE: U.S.A. Kentucky: in rupibus saltus fluvii Ohio, juxta Louisville, Michaux s.n. (holotype: P, photo: MEXU!; isotypes: B, G-DC).

Podostemum abrotanoides Nuttall ("Podostemon abrotanoides"), J. Acad. Nat. Sci. Philadelphia 7: 105. 1834. Podostemum ceratophyllum var. abrotanoides (Nuttall) Weddell in DC., Prodr. 17: 73. 1873. Podostemum ceratophyllum f. abrotanoides (Nuttall) Fassett, Rhodora 41: 526. 1939.-TYPE: U.S.A. Georgia: Chatahoochee River, Nuttall s.n. (lectotype, here designated: PH!; isolectotypes: K! P! PH!).

Podostemum ceratophyllum f. chondroides Fassett, Rhodora 41: 527. 1939.-TYPE: U.S.A. Pennsylvania: Winona Falls, near Bushkill, 6 Sep 1938, Fassett & Calvert 19488 (holotype: WIS!; isotypes: DAO! C! F! GH-2 sheets! L! MO! SMU!).

Podostemum ceratophyllum var. circumvallatum P. Royen, Acta Bot. Neerl. 3: 258. 1954.-TYPE: HONDURAS. Comayagua: vicinity of Siguatepeque, 1080-1400 m, 14-27 Feb 1928, Standley 56080 (holotype: US!; isotype: F!).

  Plant Description: Roots 0.3-1.1 (0.7) mm wide. Stems monomorphic, arising 1.8-9 (5) mm apart along root, 0.04-30 (2) cm long. Leaves petiolate, perpendicular to the stem axis or upright, arising in a ca. 130° distichous arrangement, 1-13 (5) times dichotomously divided or lobed, 1.7-142 (31.5) mm long; leaf divisions arising in a 2-dimensional manner; ultimate leaf divisions 0.2-40 (2.4) mm long, 0.05--0.8 (0.3) mm wide, spatulate, parallel-margined or awl-shaped, flattened, apices rounded, blunt, acute or apiculate, with a faint central vein or central vein absent; petioles 0.4-59 (7.5) mm long, round to elliptical in cross section; leaf bases symmetrical, attached perpendicular to the stem axis; stipules 0.5-3.6 (1.7) mm long, composed of an extension of boat-shaped leaf base, entire, caducous, absent from leaves 3-9 (5) nodes back from stem apex. Flowers 1-12 (1) per stem; spathella 1.5-6.2 (3.3) mm long, 0.8-1.8 (1.3) mm wide, smooth to minutely papillate, apex rounded or with a nipple; tepals 3, linear or awl-shaped, straight or curved, apex acute; lateral tepals 0.7-2 (1.2) mm long; andropodial tepal 0.1-1.1 (0.7) mm long; andropodium 0.1-3.3 (1) mm long prior to anthesis, during anthesis to 0.7-4.3 (2.6) mm; stamens 2, filaments 0.2-0.9 (0.4) mm long prior to anthesis, during anthesis to 0.3-2 (0.7) mm; anthers 0.5-1.5 (0.8) mm long, 0.4-0.8 (0.6) mm wide; pollen dyads 27-33 (30) µm long, 15-20 (18) µm wide; ovary 0.7-2.6 (1.6) mm long, 0.4-1.4 (0.9) mm wide; stigmas entire, 0.3-1.4 (0.6) mm long prior to anthesis, during anthesis to 0.4-1.5 (0.7) mm; pedicels 0.4-2.9 (1.1) mm long prior to anthesis, during anthesis to 0.6-10.5 (3.5) mm. Capsules 1.4-3.1 (2.3) mm long, 0.7-1.7 (1.2) mm wide; 6 ribbed (3 per valve), suture margins also rib-like; pedicels in fruit 0.5-9 (4.3) mm long; seeds 0--42 (8) per capsule, 0.4-0.8 (0.6) mm long, 0.5-0.8 (0.8) mm wide. Eastern North America, from eastern Oklahoma and Louisiana (U.S.A.) northeast to Nova Scotia and New Brunswick (Canada); disjunct in the Dominican Republic and Honduras; sea-level to ca. 800 m, the record from Honduras at ca. 1100 m. See Royen (1954) and Philbrick and Crow (1983) for additonal comments on the range of this species. Podostemum ceratophyllum is the only species of Podostemaceae in North America; it is unknown whether other members of Podostemaceae occur in the same habitat with P. ceratophyllum in Honduras and the Dominican Republic.

  Podostemum ceratophyllum can be distinguished from all other species in the genus by the combination of two characters: an entire (vs toothed) extension of the boat-shaped sheathing leaf base and ribbed capsules. The Brazilian P. flagelliforme possesses the same type of leaf base but has ribless (smooth) capsules.

Podostemum ceratophyllum has been studied more extensively than any other species in the genus. The morphology and biology P. ceratophyllum have attracted considerable attention (e.g., Graham & Wood 1975; Hammond 1936, 1937; Philbrick 1981, 1984; Philbrick & Bogle 1988; Philbrick & Crow 1992; Capers & Les 2001). Several authors re­ ported on the structure and developmental morphology of P. ceratophyllum. Warming (1881, 1883) presented the first detailed accounts of the morphology of this species, whereas Hammond (1936, 1937) considered developmental issues and the remarkable ability of plants to regenerate. Rutishauser et al. (2003) described the development of vegetative structures and used P. ceratophyllum to illustrate the unusual (non-axillary) stem branching pattern in many New World Podostemoideae. Stem branching and flower production are associated with the production of leaves having two sheaths (dithecous leaves).

Philbrick and Bogle (1988) reported on floral variation in two P. ceratophyllum populations. Although three tepals and two anthers per flower are typical, these authors documented the presence of seven tepals and seven anthers per flower. The structure and placement of the "extra" tepals and anthers varied considerably.

Philbrick (1984) reported on the pollination and seed germination biology of P. ceratophyllum based on a study of three New Hampshire populations. Although all three populations flowered extensively, seed was produced in only one. Evidence of pollen movement between flowers, either via biotic or abiotic means, was lacking. Differences in the time of anther dehiscence were proposed as a primary factor limiting seed production. Seeds were produced via self-pollination when anther dehiscence occurred prior to spathella rupture.

Philbrick (1984) also reported on seed biology of the species and indicated that seeds lacked an innate dormancy. Rhizoidal outgrowths of the radicle were associated with the initial seedling attachment to the substratum. Only the initial stages of the endogenously developed horizontal root were observed by Philbrick (1984).

Podostemum ceratophyllum has been shown to be important in the ecology of rivers where it is the dominant macrophyte (Everitt & Burkholder 1991). In a study of stream segments in North Carolina, Everitt and Burkholder (1991) found that P. ceratophyllum, in association with the macroalgae Lemanea australis Atkinson (Rhodophyta), was a primary colonizer of open, sunny habitats during cool seasons. Podostemum ceratophyllum was the only primary colonizer during warm seasons. Meijer (1976) reported that the species may be a good indicator of clean streams, although he provided no supporting em­ pirical evidence.

Some river biota are dependent on plants of P.  ceratophyllum.  The  riverweed darter, a perch-like fish (Etheostoma podostemone Jordan, Perciformes, Percidae), occurs in habitats closely tied to P. ceratophyllum (Connelly et al. 1999). The plant is part of the critical habitat for two fish species (amber darter, Percina antesella Williams & Etnier; Conasauga logperch, Percina jenkinsi Thompson,  Perciformes,  Percidae)  that are listed as federally endangered in the United States (U.S. Fish and Wildlife Service 1985).

Podostemum ceratophyllum is listed as a rare or endangered species or as a species of special concern in several states in the eastern U.S.A. Factors such as the paucity of appropriate habitat in some regions and detrimental impact of human activities on rivers (e.g., siltation, nutrient pollution, dam building; cf., Quiroz et al. 1997) con­tribute to conservation concerns.

There are two type specimens in PH of P. abrotanoides. We have selected one as the lectotype, which has been annotated accordingly. The other is an isolectotype. Isolecto­ types are also located in BM, K, and P.