Lemna, commonly known as duckweed, is a genus of roughly 14 species of free-floating aquatic flowering plants in the family Araceae (order Alismatales). These morphologically reduced plants are among the smallest flowering plants on Earth, with individual thalli (frond-like bodies) typically measuring no more than 5 mm across. Each thallus bears a single root, a trait that distinguishes Lemna from the rootless genus Wolffia and from Spirodela and Landoltia, which produce multiple roots.
Plants grow on or just beneath the water surface and reproduce primarily by vegetative budding: two daughter plants bud from the parent, enabling extremely rapid colonisation of still or slow-moving freshwater. Lemna species also flower and set seed under appropriate conditions; certain species (for example L. gibba) are long-day plants, while others (such as L. minor) respond to short days. Formerly placed in their own family Lemnaceae, duckweeds are now recognised as highly derived members of Araceae.
The genus has a near-cosmopolitan distribution. Lemna minor (common duckweed) and L. trisulca (ivy duckweed) are found worldwide, while other species are more regional — L. perpusilla in eastern North America, L. japonica in East Asia, and L. tenera in Indochina and northern Australia, among others.
Lemna species are significant in applied science: they are used as model organisms in community ecology, ecotoxicology, and basic plant biology, and as efficient expression systems for biopharmaceutical production. Their rapid biomass accumulation makes them valuable in bioremediation of polluted waterways and municipal wastewater treatment. Dried duckweed meal contains 25–45% protein by dry weight, making it a nutritious supplement in animal feed for livestock and aquaculture.
Distribution
Lemna has a near-cosmopolitan distribution across freshwater habitats on every continent except Antarctica. Lemna minor and L. trisulca are among the most widespread aquatic plants in the world. Other species have more restricted ranges: L. perpusilla in eastern North America and Quebec, L. japonica in Japan, China, Korea, and the Russian Far East, L. tenera in Indochina, Sumatra, and northern Australia, and L. aequinoctialis throughout tropical and subtropical regions.
Ecology
Lemna species colonise still or slow-moving freshwater — ponds, ditches, slow rivers, and wetlands. Growth is predominantly vegetative, allowing dense mats to form rapidly and cover water surfaces, which can reduce light penetration and oxygen levels below. Duckweeds are consumed by waterfowl (the common name reflects this), fish, and various invertebrates, and they are widely used as test organisms in standardised aquatic ecotoxicology assays. Their tolerance of nutrient-rich water and capacity to absorb excess nitrogen and phosphorus has led to application in the bioremediation of polluted waterways and municipal wastewater treatment.
Cultural Uses
Dried duckweed meal is a protein-rich feed ingredient, containing 25–45% protein by dry weight along with 4.4% fat and 8–10% fibre. It is used as a cattle and aquaculture feed supplement. Lemna species are also employed as efficient platforms for the production of complex biopharmaceuticals, owing to their rapid growth, simple culture requirements, and ability to perform eukaryotic protein modifications.
Taxonomy Notes
Lemna was long placed in the family Lemnaceae alongside other duckweeds and related genera such as Wolffia, Spirodela, and Landoltia. Molecular phylogenetic studies later demonstrated that these plants are highly reduced members of Araceae; they are now formally treated within that family. Linnaeus first described the genus in Species Plantarum (1753).
