19. Podostemum A. Michaux
Podostemum A. Michaux, Fl. Bor.-Amer. 2: 164 (1803), Podostemon orth. mut.; van Royen, Acta Bot. Neerl. 3: 228--244 (1954); Philbrick & Novelo, Syst. Bot. Monographs 70: 1--106 (2004), rev.
Roots thread-like, usually flattened, branched, attached to rocks with finger-like holdfasts; stems distinct, arising from lateral margins of root, usually in opposite or subopposite pairs, simple or branched; flowering stems only in one species (P. comatum) distinct from vegetative ones, both borne along same root. Leaf sheaths simple or double; 1--3 (--11) stipular lobes or teeth per sheath, attached to associated sheath in leaf axils, or in Podostemum mülleri Warming stipule 1 per leaf borne on dorsal side of shoot; leaf blades simple or, more often, repeatedly forked into linear segments or, in P. distichum (Chamisso) Weddell and P. irgangii C. T. Philbrick & A. Novelo, leaf segments (rachides) covered with triangular to subulate scales in half-whorls or whorls. Spathellas club-shaped, splitting irregularly from top. Flowers 1--several per stem; pedicels 0.1--0.5 (--0.8) cm long; tepals 3, linear, 2 at each side of andropodium base, in P. muelleri Warming sometimes 0, and usually 1 on top of andropodium in fork between the two filaments (abnormal flowers with proliferation of tepals and stamens are uncommon); stamens usually 2, borne on an andropodium; anthers dehiscing introrsely and latrorsely; pollen in dyads. Capsules ovoid; valves unequal, the larger persistent; each valve with 3 ribs; style short; stigmas linear, equal or unequal. Seeds from ± 30 to ± 100, but P. rutifolium subsp. ricciiforme (Liebmann) A. Novelo & C. T. Philbrick rarely sets seed but if so then only 1 or 2 ripen. About seven spp. (17 recognised by P. Royen), America, extending from N Argentina to eastern North America. The Asian species are now placed in Zeylanidium and Polypleurum. Crenias (SE Brazil) and Devillea (Goiás, Brazil) are closely related to Podostemum. Crenias and Devillea are retained here as a distinct genera, although Philbrick and Novelo (2004) have sunk them into Podostemum.
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