Wolffia arrhiza

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.[1][2][3] It is native to Europe, Africa, and parts of Asia, and it is present in other parts of the world as a naturalized species.[4][5] It 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.[2] 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.[2][6] In cooler conditions the plant becomes dormant and sinks to the bed of the water body to overwinter as a turion.[7] 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.[3]

Wolffia arrhiza
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.

Detail on the water surface

This tiny plant is a nutritious food. Its green part is about 40% protein by dry weight and its turion is about 40% starch.[8][9] 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.[9] It has long been used as a cheap food source in Burma, Laos, and Thailand, where it is known as khai-nam ("eggs of the water").[10] 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.[11]

It is also useful as a form of agricultural and municipal water treatment.[12] It is placed in effluent from black tiger shrimp farms to absorb and metabolize pollutants.[13] The plants grow quickly and take up large amounts of nitrogen and phosphorus from the water.[8] The plants that grow in the wastewater can then be used as feed for animals, such as carp,[14] Nile tilapia,[15] and chickens.[7]