Wolffia arrhiza is a species of flowering plant known by the common names spotless watermeal and rootless duckweed, belonging to the Araceae, a family rich in water-loving species, such as Arum and Pistia. It is the smallest vascular plant on Earth.^[2][3][4] It is native to Europe, Africa, and parts of Asia, and it is present in other parts of the world as a naturalized species.^[5][6]

Quick Facts Wolffia arrhiza, Conservation status ...

Wolffia arrhiza
Conservation status
Least Concern  (IUCN 3.1)[1]
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Angiosperms
Clade:	Monocots
Order:	Alismatales
Family:	Araceae
Genus:	Wolffia
Species:	W. arrhiza
Binomial name
Wolffia arrhiza (L.) Horkel ex Wimm.

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Wolffia arrhiza is an aquatic plant which grows in quiet water bodies such as ponds. The green part of the plant, the frond, is a sphere measuring about 1 mm wide, but with a flat top that floats at the water's surface. It has a few parallel rows of stomata.^[3] There is no root. The plant produces a minute flower fully equipped with one stamen and one pistil. It often multiplies by vegetative reproduction, however, with the rounded part budding off into a new individual.^[3][7] In cooler conditions the plant becomes dormant and sinks to the bed of the water body to overwinter as a turion.^[8] The plant is a mixotroph which can produce its own energy by photosynthesis or absorb it from the environment in the form of dissolved carbon.^[4]

This tiny plant is a nutritious food. Its green part is about 40% protein by dry weight and its turion is about 40% starch.^[9][10] It contains many amino acids important to the human diet, relatively large amounts of dietary minerals and trace elements such as calcium, magnesium, and zinc, and vitamin B₁₂.^[10] It has long been used as a cheap food source in Myanmar, Laos, and Thailand, where it is known as khai-nam ("eggs of the water").^[11] The plant is prolific in its reproduction, growing in floating mats that can be harvested every 3 to 4 days; it has been shown to double its population in less than four days in vitro.^[12]

It is also useful as a form of agricultural and municipal water treatment.^[13] It is placed in effluent from black tiger shrimp farms to absorb and metabolize pollutants.^[14] The plants grow quickly and take up large amounts of nitrogen and phosphorus from the water.^[9] The plants that grow in the wastewater can then be used as feed for animals, such as carp,^[15] Nile tilapia,^[16] and chickens.^[8]