Podostemaceae, moss-like, alga-like, or lichen-like river-weeds, are a family of ecologically and morphologically unusual aquatic angiosperms. They grow on rocks in rapids and waterfalls in the tropics and subtropics of the world.
They are distributed in east, southeast and south Asia, Papua New Guinea, north Australia, Africa, Madagascar and adjacent islands, and Central and South Americas. Central and South Americas harbor about 18 genera and 155 species, while 17 genera and 75 species, and 13 genera and 40 species are distributed in Africa and Asia, respectively (Cook, 1996).
Vegetative plants grow submerged in swift-running water during the rainy season and subsequently emerge and eventually dry during the dry season when the water level drops. Generally flowers open and seeds are set shortly after emergence.
Podostemaceae are attached to submerged rock-surfaces exposed to fast-flowing water. The plants comprise horizontal leading roots or shoots and adventitious lateral shoots or clusters of leaves, whose morphological identities differ with taxa. The horizontal roots or shoots are creeping on rock surfaces (in some species floating and attached at the base), adhering with rhizoids or holdfasts, and are variously flattened. Such a body plan is a remarkable adaptation to the unique environment, i.e. the border between hard rocks and fast-flowing water.
Roots are more or less flattened and dorsiventral in most of Podostemaceae, but absent in Dalzellia and Indotristicha tirunelveliana (Tristichoideae). Primary roots are reduced in Indotristicha and Terniopsis (Tristichoideae) or absent in all Podostemoideae. Instead, adventitious roots develop from the hypocotyls and give rise to adventitious shoots on the upper surface or marginally.
Primary shoots are reduced or do not develop. Adventitious shoots arise endogenously from the roots, while in rootless Dalzellia the adventitious shoot develops near the axil of the cotyledon. In Dalzellia, Indotrischa ramosissima and Terniopsis (Tristichoideae) and Weddellina (Weddellinoideae), like in most plants, the shoot has an apical meristem. The shoot of subfamily Podostemoideae lacks an apical meristem. The leaves arise without the shoot apical meristem.
Flowering shoots are borne on the root or shoot. The flowers are much reduced (at most 5 mm long) with one or a few tepals, stamens and pistil. Some American species have showy flowers with many tepals and stamens.