Pistia L.

Scientific Description

Family Araceae.

Habit and leaf form. Herbs; non-laticiferous and without coloured juice. Perennial; plants with a basal concentration of leaves; each plant with a very short rhizome and with several long, hanging roots suspended beneath the tuft, producing radiating stolons that terminate in new plantlets. Hydrophytic; free floating. Leaves floating. Not heterophyllous. Leaves medium-sized; alternate; congested; sessile; simple. Leaf blades obovate; 7–15 -nerved; parallel-veined; cross-venulate, or without cross-venules. Leaves ligulate, or eligulate. Axillary scales present, or absent. Leaves without a persistent basal meristem. Vegetative anatomy. Plants without silica bodies. Leaf anatomy. Guard-cells not ‘grass type’. Hairs present. Stem anatomy. Secondary thickening absent. Roots. Roots with velamen, or without velamen.

Reproductive type, pollination. Fertile flowers functionally male and functionally female. Unisexual flowers present. Plants monoecious. The unisexual flowers aggregated in different parts of the same inflorescence. Female flowers solitary (fused to the lower part of the spadix, reduced to a naked pistil); without staminodes. Male flowers solitary. Anemophilous, or entomophilous. Pollination mechanism conspicuously specialized, or unspecialized.

Inflorescence and flower features. Flowers aggregated in ‘inflorescences’; in spikes. The terminal inflorescence unit seemingly racemose. Inflorescences scapiflorous; axillary; spadix much reduced, partly fused to the spathe; the male flower separated from the female flower by a cup-shaped membrane, an appendix absent; pseudanthial; spatheate (spathe tubular in lower part, opening above). Flowers ebracteate; ebracteolate; small; (these or the spadix) fragrant, or malodorous; cyclic. Perianth absent. Fertile stamens present, or absent (when female). Androecium 4–6. Androecial members coherent (united into a synandrium). Androecium exclusively of fertile stamens. Stamens 4–6; with sessile anthers. Anthers cohering; dehiscing via pores; extrorse; four locular; bisporangiate, or tetrasporangiate. Pollen shed as single grains. Fertile gynoecium present, or absent (when male). Gynoecium 1–8 carpelled. The pistil 1 celled. Gynoecium monomerous, or syncarpous; of one carpel, or eu-syncarpous; superior. Carpel apically stigmatic. Ovary unilocular; 1 locular. Gynoecium stylate. Styles apical. Stigmas wet type, or dry type; non-papillate; Group II type. Placentation basal to parietal. Ovules in the single cavity 15(–50) (? i.e. ‘many’); pendulous, or horizontal, or ascending; orthotropous, or hemianatropous, or anatropous.

Fruit and seed features. Fruit fleshy to non-fleshy (juiceless); green; indehiscent; a drupe; few to many seeded. Seeds more or less obloid; non-endospermic. Perisperm absent. Seeds with starch. Cotyledons 1. Embryo straight (linear). Seedling. Hypocotyl internode present, or absent. Mesocotyl absent. Seedling collar not conspicuous. Cotyledon hyperphyll compact; assimilatory, or non-assimilatory. Coleoptile absent. Seedling non-macropodous. First leaf dorsiventral. Primary root ephemeral.

Physiology, biochemistry. Photosynthetic pathway: C₃.

Geography, cytology, number of species. Native of Australia. Not endemic to Australia. Australian states and territories: Western Australia, Northern Territory, Queensland, and New South Wales. South-West Botanical Province.

Additional characters Fruit rostrate (by the persistent style). Leaves not solitary.