6. Podostemaceae Richard ex Kunth
Riverweed Family Riverweed Family
C. Thomas Philbrick
Garrett E. Crow
Herbs, [annual] perennial, aquatic, attached to rocks and other solid substrata in rapids and waterfalls, submersed in vegetative stage, becoming reproductive as water level drops, exposing plants to air. Roots prostrate, elongate, root cap asymmetric. Stems often trailing in connate basally; stamens [1–]2[–many], [1–2-whorled or incompletely 1-whorled] restricted to 1 side of flower; filaments arising from andropodium [individually, not from andropodium], distinct or connate basally; ovary 2[or 3]-carpellate; stigmas 2[or 3], apical. Fruits capsular, 2-valved [3-valved in Tristicha], [valves persistent] 1 valve falling away after dehiscence. Seeds 0–40[–numerous] per capsule, orange-brown, minute, ovoid, outer integument expanded and sticky when wet.
Genera ca. 50, species ca. 260 (1 in the flora): c, e North America, Mexico, West Indies, Central America, South America, Africa, se Asia, India, n Australia; pantropical.
Molecular evidence indicates that Podostemaceae is related to Clusiaceae within the Malpighiales (M. H. G. Gustafsson et al. 2002). Structural modifications of vegetative features has led to controversy regarding how best to interpret the structural categories of root, shoot, and leaf in Podostemaceae (for example, I. Jäger-Zürn 2005; R. Rutishauser 1997).
SELECTED REFERENCES Jäger-Zürn, I. 2005. Morphology and morphogenesis of ensiform leaves, syndesmy of shoots and an understanding of the thalloid plant body in species of Apinagia, Mourera and Marathrum (Podostemaceae). Bot. J. Linn. Soc. 147: 47–71. Les, D. H. et al. 1997c. The phylogenetic placement of river-weeds (Podostemaceae): Insights from rbcL sequence data. Aquatic Bot. 57: 5–27. Philbrick, C. T. and A. Novelo R. 1995. New World Podostemaceae: Ecological and evolutionary enigmas. Brittonia 47: 210–222. Rutishauser, R. 1997. Structural and developmental diversity in Podostemaceae (river-weeds). Aquatic Bot. 57: 29–70. van Royen, P. 1951. The Podostemaceae of the New World. Part I. Meded. Bot. Mus. Herb. Rijks Univ. Utrecht, 107: 1–151. van Royen, P. 1953. The Podostemaceae of the New World. Part II. Acta Bot. Neerl. 2: 1–20. van Royen, P. 1954. The Podostemaceae of the New World. Part III. Acta Bot. Neerl. 3: 215–263.
