• WordNet 3.6
    • n pistia pantropical floating plant forming a rosette of wedge-shaped leaves; a widespread weed in rivers and lakes
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Century Dictionary and Cyclopedia
    • n pistia A genus of monocotyledonous floating water-plants of the order Araceæ, constituting the tribe Pistioideæ. It is characterized by the absence of perianth, and the position of the solitary or few staminate flowers on the short free apex of a spadix which is adnate below to the small white spathe, and bears at the base a single obliquely globose one-celled ovary with thick style and cup-like stigma. The only species, P. Stratiotes, is common throughout the tropics excepting Australia and the Pacific islands. It consists of a rosette of pale pea-green rounded and downy leaves. It floats unattached, its tufts of long feathery roots often not reaching the bottom, and increases by runners, often soon covering ponds and tanks. keeping the water fresh and cool. It bears the name of tropical duckweed, and in the West Indies of water-lettuce.
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In literature:

It is probably the 'Pistia stratiotes', a gigantic "duck-weed".
"Missionary Travels and Researches in South Africa" by David Livingstone
Pistia, Salvinia and Azolla are common.
"Journals of Travels in Assam, Burma, Bhootan, Afghanistan and TheNeighbouring Countries" by William Griffith