Pistia stratiotes
Family
Araceae
Synonyms
None
Vernacular Names
| Malaysia | Kiambang. |
| English | Water lettuce, tropical duckweed, Nile cabbage. |
| Indonesia | Ki apu (Sundanese), kayu apu (Javanese), kiambang (West-Kalimantan). |
| Philippines | Kiapo, apon (Tagalog), loloan (Iloko). |
| Thailand | Chok (Central), kaa kok, phak kok (Northern). |
| Vietnam | B[ef]o c[as]i, b[ef]o tai t[uw][owj]ng, d[aj]i ph[uf] b[if]nh. |
| French | Lettue, d’eau. |
Geographical Distributions
P. stratiotes has a pantropical distribution, and occurs throughout South-East Asia.
Description
It is a small perennial, fast-growing, free-floating aquatic herb with unisexual flowers. Its stem is almost absent, present as trailing stem to produce roots and shoots. The roots are adventitious, 20-80 cm long, form a dense tuft and root hairs spread plumosely.
The leaves are clustered in a rosette, nearly overlapping, erect when crowded. It varies in shape and size; either spoon-shaped or tongue-shaped, 2-10 cm x 2-9 cm. The leaf is pale to yellowish-green in colour with whitish velvety hair. The leaf blade is succulent and the veins are flabellate, Leaf stalk is absent.
The inflorescence has a flower spike with fleshy axis surrounded by 2 or more bracts, several with 12-15 mm long peduncle in the upper part of the rosette. The male and female flower is small and located separately on the flower spike with bracts that are narrowly ovate, 7-9(-20) mm x 3 mm that acuminate at the apex. Its outside is covered with short soft hairs, white colour while the inside is smooth. The lower margins connate with each other and with the ovary wall, free margins folded between stigma and stamens to form a constriction. Just below them are bracts partitioning a thin, green pouch-shaped flap. A flower spike is mostly united to bracts, but free at the apex, with 1 naked male flower. 4-6(-8) stamens united in a cohesion of the anthers of each male flower, subtended at the base by a thin, cup-shaped ring. There are 4 pores in the anthers, in 2 superposed pairs. The base of its bracts holds 1 naked female flower with a 1-celled ovary while the ovules are many and located in 4-6 rows on a convex parietal placenta. The style is free from flower spike, thick with bearded subglobose stigma.
The fruit is a dry, ellipsoid berry and irregularly rupturing. It contains few to numerous seeds. The brown seed is heart-shaped, 2 mm long, wrinkled, tapering towards the base while its apex is truncated and depressed in the centre.
The seedling is germinating under water, emerging with a 2-lobed, cotyledon-like structure, filled with aerenchyma that allows for floating. No primary root is developed, but 2-3 adventitious roots emerge from the base of the cotyledon. It is broadly egg-shaped and densely hairy. The first leaf appears between the 2 lobes.
Ecology / Cultivation
P. stratiotes floats in stagnant or slowly flowing fresh water, ponds and tidal areas, from sea-level up to 1200 m altitude. It is sensitive to cold weather and thus cannot exist far beyond the tropics of Cancer and Capricorn. It can become a part of dense aggregations of free-floating vegetation, called sudds, which are common in wide, slow-flowing rivers and in extensive swampy areas. Sudds are formed by thick, floating mats of P. stratiotes, and/or Eichhornia crassipes (Mart.) Solms, which are then colonized by hydrophytes, mainly grasses and sedges, which often invade from the shore. When the raft is large enough, it may be torn away from the shore by wind or flood, to become a floating island. It performs best at pH 4, and cannot grow at pH 3; its range of pH tolerance is much narrower than that of E. crassipes, which has its optimum growth at pH 7. P. stratiotes is a noxious weed like E. crassipes, obstructing navigation in rivers and grills of hydroelectric plants. It interferes with fisheries by hindering the nets and lowering the oxygen content of the water and the pH.
Line Drawing / Photograph
References
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Plant Resources of South-East Asia No 12(2): Medicinal and poisonous plants 2.
