Pistia L.
SUMMARY
Laticifers absent. HABIT : small, free-floating evergreen herbs with pendent feathery roots, stem very short, acaulescent, stoloniferous. LEAVES : several in a rosette, densely pubescent. PETIOLE : very short, almost absent, sheath very short, ligulate, very thin, scarious at base. BLADE : somewhat spongy, obovate-cuneate to obovate-oblong, apically rounded, truncate or retuse; midrib absent, primary veins subparallel, all arising from base, diverging towards apex, running into margin near apex, strongly prominent on lower surface, higher order venation reticulate. INFLORESCENCE : solitary, very small, much shorter than leaves. PEDUNCLE : very short, pubescent. SPATHE : pubescent without, glabrous within, somewhat constricted centrally, lower margins connate with each other and with ovary wall forming tube, free margins between tube and blade folded between stigma and male organs forming a partition between an upper male chamber and a lower female one, blade erect, ovate, expanded, acute-acuminate. SPADIX : shorter than spathe, mostly adnate to spathe, only the apical male zone free, female zone bearing single gynoecium at base and a thin, green, pouch-shaped flap just below spathe partition, male zone subtended by a thin, marginally lobed, green, annular flap, composed of a basally naked spadix axis supporting a single whorl of 2-8 flowers, axis sometimes extending a little beyond. FLOWERS : unisexual, perigone absent. MALE FLOWER : synandrium composed of 2 connate stamens, thecae dehiscing by single apical slit. POLLEN : inaperturate, ellipsoid-elongate to -oblong, medium-sized (mean 27 µm.), exine plicate with strongly undulate frills running the length of the grain. FEMALE FLOWER : gynoecium obliquely adnate to spadix axis, ovary ovoid, 1-locular, ovules numerous, orthotropous, placenta broad, apparently parietal, probably morphologically basal, style attenuate, bending inwards towards male flowers, stigma small, discoid-subcapitate. BERRY : thin-walled, utricular, several-seeded, ellipsoid, irregularly breaking up and decaying to release seeds. SEED : barrel-shaped, ± subtruncate and excavated at apex and base, testa thick, reticulate-alveolate, thicker and with operculum at micropylar end, embryo obovoid to conoid, endosperm copious.