1. Podostemum Michaux, Fl. Bor.-Amer. 2: 164, plate 44 [as Podostemon]. 1803.
Threadfood, riverweed, orchid-of-the-waterfall [Greek podos, foot, and stemon, stamen, alluding to stamens elevated on foot-stalk (andropodium)] Threadfood, riverweed, orchid-of-the-waterfall [Greek podos, foot, and stemon, stamen, alluding to stamens elevated on foot-stalk (andropodium)]
Roots green, photosynthetic; haptera (fleshy disclike or fingerlike outgrowths) arising along flanks of roots serving as anchorage. Leaves with 1–2 basal sheaths; stipules extensions of boat-shaped leaf base; petiole round to elliptic; blade symmetric, ultimate divisions linear, awl-shaped, or spatulate, apex rounded, blunt, acute, or apiculate. Inflorescences apical or lateral, 1–12-flowered, pedicellate; spathella clavate, smooth or papillate, apex rounded or with nipple, rupturing apically into 1–6 irregular segments. Pedicels expanded at apex, elongating during anthesis. Flowers zygomorphic; tepals scalelike, 2 at base of ovary on either side of andropodium (lateral tepals), shorter than ovary, 1 (andropodial tepal) arising at top of andropodium from fork between 2 stamen filaments; anthers basifixed, quadrangular; thecae equal or inner longer than outer, apices rounded, distinct, dehiscence introrse; ovary oriented obliquely on pedicel, 2-locular, ellipsoidal; ovules 25–200; placentation axile; stigmas distinct, simple, bent toward anthers prior to spathella rupture, divergent and elongating during anthesis. Capsules 2-locular, persistent valve oriented obliquely on pedicel, each valve 3-ribbed, suture margins thickened, riblike. Seeds 0–42[–203].
Species 11 (1 in the flora): c, e North America, Mexico, West Indies (Dominican Republic), Central America, South America.
SELECTED REFERENCES Philbrick, C. T. 1984. Aspects of floral biology, breeding system, and seed and seedling biology in Podostemum ceratophyllum Michx. (Podostemaceae). Syst. Bot. 9: 166–174. Philbrick, C. T. and G. E. Crow. 1983. Distribution of Podostemum ceratophyllum Michx. (Podostemaceae). Rhodora 85: 325–341. Philbrick, C. T. and G. E. Crow. 1992. Isozyme variation and population structure in Podostemum ceratophyllum Michx. (Podostemaceae): Implications for colonization of glaciated North America. Aquatic Bot. 43: 311–325. Philbrick, C. T. and A. Novelo R. 2004. Monograph of Podostemum. Syst. Bot. Monogr. 70: 1–106. Rutishauser, R. et al. 2003. Developmental morphology of roots and shoots of Podostemum ceratophyllum (Podostemaceae–Podostemoideae). Rhodora 105: 337–353.
