Podostemaceae

Abstract

Annual or perennial, aquatic herbs, often bizarre in form, sometimes resembling lichens, bryophytes, seaweeds, or unlike any other plants; haptophytes, attached by adhesive hairs to rock or other hard objects in flowing freshwater, mostly in rapids and waterfalls; roots usually photosynthetic, creeping or partly floating, thread-like, ribbon-shaped, crustose (foliose), sometimes short-lived or absent. Shoots nearly always arising as endogenous buds from roots; stems reduced or elongate, simple or branched, sometimes dimorphic, occasionally only present when flowering. Photosynthesis takes place under water, flowers or even separate floral shoots develop as the water level drops, the vegetative shoots or leaves often shed as plants become exposed. Phyllotaxis variable, in Podostemoideae usually distichous. Leaves borne on elongate stems or arising fromprostrate, often disk-like stems, extremely variable in size and shape, from scale-like to well developed and compound; sheaths single or, in many Podostemoideae, double; sheath lobes sometimes elongated into stipule-like appendages; leaf blades stalked or sessile, entire, lobed or dissected; blade lobes or segments often bearing photosynthetic filaments and/or additional hairs; ultimate leaf segments filiform, linear or spathulate. Flowers bisexual, actinomorphic or zygomorphic, solitary, in clusters or in racemeor cyme-like in florescences; flower buds naked in Weddellinoideae and some Tristichoideae, surrounded by a cupula (a collar-like vascularised cup) in some Tristichoideae, or completely enclosed in a spathella (a tubular or sack-like cover) in Podostemoideae; spathellas mostly enclosing a single sessile or pedicellate flower; pedicels often elongating in fruit. Anthesis takes place in air or flowers cleistogamous under water. Perianth of 1 complete or incomplete whorl of tepals, often confined to one side of the flower; tepals in Tristichoideae and Weddellinoideae large, 5 or rarely 4 or 6, imbricate and sepal-like; tepals in Podostemoideae small, 2–20, linear or subulate, usually alternating with stamens, in flowers with only 2 basally fused stamens occasionally an additional tepal borne at top of andropodium (common stalk); stamens 1–40, in 1 or 2 complete whorls, or in 1 incomplete whorl, or confined to one side of flower and consisting of 1–3 free stamens or a Y-shaped structure consisting of an andropodium carrying 2 stamens; filaments, when in whorls, mostly free or, in Tulasneantha, their bases united to form an androecial tube; anthers dehiscing longitudinally by slits, introrsely to latrorsely or rarely extrorsely; pollen shed in monads, dyads or (rarely) tetrads, tricolporate in Weddellinoideae, tricolpate to pentacolpate in Podostemoideae, pantoporate with up to 16 pores in Tristichoideae; ovary superior, 2-or 3-locular or 1-locular in some Podostemoideae; ovules axile, anatropous, bitegmic, tenuinucellate. Fruit a capsule, smooth or ribbed, with 2 or 3, equal or unequal valves, sometimes one or more persisting; stigmas 1–3, variable in shape and size. Seeds 2 to very numerous (over 2,000); seed coat usually mucilaginous and sticky; endosperm 0; embryo straight, with 2 cotyledons and a suspensor.