Duckweeds, or water lenses, are flowering aquatic plants which float on or just beneath the surface of still or slow-moving bodies of fresh water and wetlands. Also known as "bayroot", they arose from within the arum or aroid family (Araceae), so often are classified as the subfamily Lemnoideae within the Araceae. Other classifications, particularly those created prior to the end of the 20th century, place them as a separate family, Lemnaceae.
These plants have a simple structure, lacking an obvious stem or leaves. The greater part of each plant is a small organized "thallus" or "frond" structure only a few cells thick, often with air pockets (aerenchyma) that allow it to float on or just under the water surface. Depending on the species, each plant may have no root or may have one or more simple rootlets.
Reproduction is mostly by asexual budding (vegetative reproduction), which occurs from a meristem enclosed at the base of the frond. Occasionally, three tiny "flowers" consisting of two stamens and a pistil are produced, by which sexual reproduction occurs. Some view this "flower" as a pseudanthium, or reduced inflorescence, with three flowers that are distinctly either female or male and which are derived from the spadix in the Araceae. Evolution of the duckweed inflorescence remains ambiguous due to the considerable evolutionary reduction of these plants from their earlier relatives.
The flower of the duckweed genus Wolffia is the smallest known, measuring merely 0.3 mm long. The fruit produced through this occasional reproduction is a utricle, and a seed is produced in a bag containing air that facilitates flotation.
|Close-up of two different duckweed types: Spirodela polyrrhiza and Wolffia globosa: The latter are less than 2 mm long.|
One of the more important factors influencing the distribution of wetland plants, and aquatic plants in particular, is nutrient availability. Duckweeds tend to be associated with fertile, even eutrophic conditions. They can be spread by waterfowl and small mammals, transported inadvertently on their feet and bodies, as well as by moving water. In water bodies with constant currents or overflow, the plants are carried down the water channels and do not proliferate greatly. In some locations, a cyclical pattern driven by weather patterns exists in which the plants proliferate greatly during low water-flow periods, then are carried away as rainy periods ensue.
Duckweed is an important high-protein food source for waterfowl. The tiny plants provide cover for fry of many aquatic species. The plants are used as shelter by pond-water species such as bullfrogs and fish such as bluegills. They also provide shade and, although frequently confused with them, can reduce certain light-generated growths of photoautotrophic algae. Duckweed is also eaten by humans in some parts of Southeast Asia. As it contains more protein than soybeans, it is sometimes cited as a significant potential food source. Some initial investigations to what extent duckweed could be introduced in European markets show little consumer objection to the idea.
The plants can provide nitrate removal, if cropped, and the duckweeds are important in the process of bioremediation because they grow rapidly, absorbing excess mineral nutrients, particularly nitrogen and phosphates. For these reasons, they are touted as water purifiers of untapped value.
The Swiss Department of Water and Sanitation in Developing Countries, associated with the Swiss Federal Institute for Environmental Science and Technology, asserts that as well as the food and agricultural values, duckweed also may be used for wastewater treatment to capture toxins and for odor control, and that if a mat of duckweed is maintained during harvesting for removal of the toxins captured thereby, it prevents the development of algae and controls the breeding of mosquitoes. The same publication provides an extensive list of references for many duckweed-related topics.
These plants also may play a role in conservation of water because a cover of duckweed will reduce evaporation of water when compared to the rate of a similarly sized water body with a clear surface.
Despite some of these benefits, because duckweed thrives in high-nutrient wetland environments, they can be seen as a nuisance species when conditions allow them to excessively proliferate in environments that are traditionally low in nutrients. This is the case within the Everglades, where surface runoff and agricultural pollution have introduced increased levels of nutrients into an otherwise low-nutrient wetland system, which allows fast growing species such as duckweed to establish themselves, spread, and displace other native species such as sawgrass.
The duckweeds have long been a taxonomic mystery, and usually have been considered to be their own family, the Lemnaceae. They primarily reproduce asexually. Flowers, if present at all, are small. Roots are either very much reduced, or absent entirely. They were suspected of being related to the Araceae as long ago as 1876, but until the advent of molecular phylogeny, testing this hypothesis was difficult.
Starting in 1995, studies began to confirm their placement in the Araceae and since then, most systematists consider them to be part of that family.
Their position within their family has been slightly less clear, but several 21st-century studies place them in the position shown below. They are not closely related to Pistia, however, which also is an aquatic plant in the family Araceae.
Duckweed genome sizes have a 10-fold range (150 to 1,500 MB), potentially representing diploids to octaploids. The ancestral genus of Spirodela has the smallest genome size (150 MB, similar to Arabidopsis thaliana), while the most derived genus, Wolffia, contains plants with the largest genome size (1,500 MB). DNA sequencing has shown that Wolffiella and Wolffia are more closely related than the others. Spirodela is at the basal position of the taxon, followed by Lemna, Wolffiella, and Wolffia, which is the most derived.
To identify different duckweed genomes, a DNA-based molecular identification system was developed based on seven plastid-markers proposed by the Consortium for the Barcode of Life. The atpF-atpH non-coding spacer was chosen as a universal DNA barcoding marker for species-level identification of duckweeds.
Research and applications of duckweeds are promoted by two international organizations, The International Lemna Association and the International Steering Committee on Duckweed Research and Applications.
In July 2008, the U.S. Department of Energy (DOE) Joint Genome Institute announced that the Community Sequencing Program would fund sequencing of the genome of the giant duckweed, Spirodela polyrhiza. This was a priority project for DOE in 2009. The research was intended to facilitate new biomass and bioenergy programs. The results were published in February 2014. They provide insight into how this plant is adapted to rapid growth and an aquatic lifestyle.
Duckweed is being studied by researchers around the world as a possible source of clean energy. In the U.S, in addition to being the subject of study by the DOE, both Rutgers University and North Carolina State University have ongoing projects to determine whether duckweed might be a source of cost-effective, clean, renewable energy. Duckweed is a good candidate as a biofuel because it grows rapidly, produces five to six times as much starch as corn per unit of area, and does not contribute to global warming. Duckweed removes carbon dioxide from the atmosphere, and it may have value for climate change mitigation.
Duckweed also functions as a bioremediator by effectively filtering contaminants such as bacteria, nitrogen, phosphates, and other nutrients from naturally occurring bodies of water, constructed wetlands, and wastewater.
Turning the canals of the Poitevin Marsh (Marais Poitevin, France) into the "Green Venice":
The Araceae are a family of monocotyledonous flowering plants in which flowers are borne on a type of inflorescence called a spadix. The spadix is usually accompanied by, and sometimes partially enclosed in, a spathe or leaf-like bract. Also known as the arum family, members are often colloquially known as aroids. This family of 114 genera and about 3750 known species is most diverse in the New World tropics, although also distributed in the Old World tropics and northern temperate regions.
The largest collection of living Araceae is maintained at the Missouri Botanical Gardens. Another large collection of living Araceae can be found at the Munich Botanical Garden, due to the efforts of researcher and aroid authority Josef Bogner.Lemna
Lemna is a genus of free-floating aquatic plants from the duckweed family. These rapidly growing plants have found uses as a model system for studies in community ecology, basic plant biology, ecotoxicology, and production of biopharmaceuticals, and as a source of animal feeds for agriculture and aquaculture. Currently, 14 species of Lemna are recognised.Lemna aequinoctialis
Lemna aequinoctialis Welw. (lesser duckweed) is a tiny, floating aquatic plant found in quiet waters in tropical and subtropical regions. Fronds are generally 3-nerved, green, up to 6 mm long. Flowers are 1-ovulate, the small utricular scale open on one side. Seeds have 8–26 ribs.Lemna gibba
Lemna gibba, the gibbous duckweed, swollen duckweed, or fat duckweed, is a species of Lemna (duckweed). It has a simple plant body, known as a thallus, which floats on the surface of the water and measures 3 – 5 mm in diameter. A single root hangs down into the water. Found in a wide range of still or slow-flowing water bodies, this common duckweed can also grow on mud or damp rocks.Lemna minuta
Lemna minuta is a species of duckweed known by the common name least duckweed. It is the smallest Lemna species. It is native to parts of the Americas, and naturalized in others; the exact native range is not known. It is found on other continents as a non-native introduction as well. The plant's distribution is ever-expanding; it has been spreading in Europe and it was described from Poland for the first time in 2007. In many areas it is a noxious weed, such as in Belgium.This tiny plant varies in shape depending on growth conditions. In the shade it is a single green translucent oval body no more than 2.5 millimeters long, and in full sunlight it generally grows in pairs. There is a central vein usually visible under magnification and microscopy. The plant produces an ephemeral membrane-bound flower.
This duckweed grows in slow-moving, calm, and stagnant freshwater habitats. It affects the ecology of its habitat by forming mats on the water surface, reducing sunlight penetration and oxygen exchange.Lemna obscura
Lemna obscura is a species of plant in the family Lemnaceae. It is endemic to Ecuador.Lemna trisulca
Lemna trisulca L. (syn. Staurogeton trisulcus (L.) Schur; star duckweed; ivy-leaved duckweed) is a species of aquatic plants in the genus Lemna (duckweed) with a subcosmopolitan distribution, occurring in quiet, freshwater habitats in cool, temperate regions. L. trisulca normally does not occur in warm, temperate regions. Unlike other duckweeds, it has submerged rather than floating fronds, except when flowering or fruiting.Spirodela
Spirodela is a genus of aquatic plants, one of several genera containing plants commonly called duckweed. Spirodela species are members of the Araceae under the APG II system. They were formerly members of the Lemnaceae.Spirodela species are free-floating thalli; two to five plants may remain connected to each other. Plants are green, but may have a red or brown underside. Multiple roots (seven to 12) emerge from each thallus. Spirodela is larger (10 mm (0.39 in)) than Lemna (2 mm (0.079 in) – 5 mm (0.20 in), one root per thallus).Certain species of Spirodela overwinter as turions, a dormant form that lacks air pockets, so sinks to the bottom of the pond. In spring, turions rise to the surface and germinate to start a new population.
Spirodela often forms floating mats with related species, e.g. Lemna and Wolffia.
The genus is virtually cosmopolitan in distribution. Spirodela punctata is sometimes treated as Landoltia punctata.Spirodela polyrhiza
Spirodela polyrhiza (orth. var. S. polyrrhiza) is a species of duckweed known by the common names common duckmeat, greater duckweed, great duckmeat, common duckweed, and duckmeat. It can be found nearly worldwide in many types of freshwater habitat. It is a perennial aquatic plant usually growing in dense colonies, forming a mat on the water surface. Each plant is a smooth, round, flat disc 0.5 to 1.0 cm wide. It produces several minute roots. It also produces a pouch containing male and female flowers. The top part dies in the fall and the plant often overwinters as a turion.
S.polyrhiza is an ideal system for biofuels, bioremediation, and carbon cycling due to its aspects of fast-growing, direct contact with media, and smallest genome size (~150 Mb). A comprehensive genomic study of S. polyrhiza was published in February 2014. The results provide insights into how this organism is adapted to rapid growth and an aquatic lifestyle.S. polyrhiza, living in pond, differs development from terrestrial plants in morphology and physiology. It undergoes mainly vegetative growth called fronds in spring and summer, while switching into a dormant phase represented by turions in autumn and winter due to nutrition starvation and freezing temperatures. Turions could also be induced by plant hormone ABA in the lab. Researchers reported that turions were rich in anthocyanin pigmentation and had a density that submerged them in liquid media. Transmission electron microscopy l of turions showed in comparison to fronds shrunken vacuoles, smaller intercellular space, and abundant starch granules surrounded by thylakoid membranes. Turions accumulated more than 60% starch in dry mass after two weeks of ABA treatment.Spirodela punctata
Spirodela punctata (or Landoltia punctata, common name dotted duckmeat) is a species of duckweed (Lemnaceae). The species is morphologically intermediate between Lemna and other species of Spirodela. In 1999 D.H. Les and D.J. Crawford proposed segregating the species to a new genus Landoltia containing just the species L. punctata, on the basis of biochemical and DNA studies.
S. punctata originally was found in Australia and South Asia, but today it can also be found in the Southern and Eastern United States.Thallus
Thallus (plural: thalli), from Latinized Greek θαλλός (thallos), meaning "a green shoot" or "twig", is the undifferentiated vegetative tissue of some organisms in diverse groups such as algae, fungi, some liverworts, lichens, and the Myxogastria. Many of these organisms were previously known as the thallophytes, a polyphyletic group of distantly related organisms. An organism or structure resembling a thallus is called thalloid, thallodal, thalliform, thalline, or thallose.
A thallus usually names the entire body of a multicellular non-moving organism in which there is no organization of the tissues into organs. Even though thalli do not have organized and distinct parts (leaves, roots, and stems) as do the vascular plants, they may have analogous structures that resemble their vascular "equivalents". The analogous structures have similar function or macroscopic structure, but different microscopic structure; for example, no thallus has vascular tissue. In exceptional cases such as the Lemnoideae, where the structure of a vascular plant is in fact thallus-like, it is referred to as having a thalloid structure, or sometimes as a thalloid.
Although a thallus is largely undifferentiated in terms of its anatomy, there can be visible differences and functional differences. A kelp, for example, may have its thallus divided into three regions. The parts of a kelp thallus include the holdfast (anchor), stipe (supports the blades) and the blades (for photosynthesis).
The thallus of a fungus is usually called a mycelium. The term thallus is also commonly used to refer to the vegetative body of a lichen. In seaweed, thallus is sometimes also called 'frond'.
The gametophyte of some non-thallophyte plants – clubmosses, horsetails, and ferns is termed "prothallus".Wayne P. Armstrong
Wayne P. Armstrong (aka "Mr. Wolffia") is a natural historian, author, photographer, and creator of the extensive online natural history textbook, Wayne's Word: An Online Textbook Of Natural History.
He was a professor of botany at Palomar College, now adjunct professor. He is an expert on the flora of North San Diego County. He wrote the section on duckweeds, subfamily Lemnoideae, in the revised Jepson Manual. He specializes in macrophotography of unusual and little-known plants and animals.Wolffia
Wolffia is a genus of nine to 11 species which include the smallest flowering plants on Earth. Commonly called watermeal or duckweed, these aquatic plants resemble specks of cornmeal floating on the water. Wolffia species are free-floating thalli, green or yellow-green, and without roots. The flower is produced in a depression on the top surface of the plant body. It has one stamen and one pistil. Individuals often float together in pairs or form floating mats with related plants, such as Lemna and Spirodela species. Most species have a very wide distribution across several continents. Wolffia species are composed of about 40% protein on a dry-matter basis, about the same as the soybean, making them a potential high-protein human food source. They have historically been collected from the water and eaten as a vegetable in much of Asia. The genus was first recorded in New Zealand by Ruth Mason.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. It is native to Europe, Africa, and parts of Asia, and it is present in other parts of the world as a naturalized species. 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. 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. In cooler conditions the plant becomes dormant and sinks to the bed of the water body to overwinter as a turion. 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.This tiny plant is a nutritious food. Its green part is about 40% protein by dry weight and its turion is about 40% starch. 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. 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"). 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.It is also useful as a form of agricultural and municipal water treatment. It is placed in effluent from black tiger shrimp farms to absorb and metabolize pollutants. The plants grow quickly and take up large amounts of nitrogen and phosphorus from the water. The plants that grow in the wastewater can then be used as feed for animals, such as carp, Nile tilapia, and chickens.Wolffia borealis
Wolffia borealis is a species of flowering plant known by the common name northern watermeal. It is native to North America including sections of Canada and the United States. It grows in mats on the surface of calm water bodies, such as ponds. It is a very tiny plant with no leaves, stems, or roots. The green part is up to 1.2 millimeters long with one rounded end and one pointed end. On the flattened top of the plant is a single stamen and pistil. Like other Wolffia, it is edible and makes a nutritious food.Wolffia brasiliensis
Wolffia brasiliensis is a species of flowering plant known by the common name Brazilian watermeal. It is native to North and South America, where it grows in mats on the surface of calm water bodies, such as ponds. It is a very tiny plant with no leaves, stems, or roots. The green part is up to 1.2 millimeters long with a flat surface with a bump in the center.Wolffia columbiana
Wolffia columbiana (Columbian watermeal) is a perennial aquatic plant in the Duckweed family (Lemnaceae). This plant is distributed widely throughout North, Central, and South America, and also occurs in Curaçao.Wolffia globosa
Wolffia globosa is a species of flowering plant known by the common name Asian watermeal. It is native to Asia and is found in parts of the Americas, where it may be native or naturalized. It grows in mats on the surface of calm, freshwater bodies, such as ponds, lakes, and marshes. It is a very tiny, oval-shaped plant with no leaves, stems, or roots. The body of the plant, a transparent green frond, is less than a millimeter wide. Like other Wolffia, the plant is edible and makes a nutritious food.
Wolffia globosa has been described as the world's smallest flowering plant, at 0.1–0.2 mm (0.004–0.008 in) in diameter.Known in Thai as Pham (ผํา), it is a popular item in Thai cuisine, especially in Isan.Wolffiella
Wolffiella is a genus of aquatic plants commonly called duckweeds in the Araceae family. Common names for plants in this genus include bogmat and mud-midget. They are rootless and have a keel that allows them to maintain their orientation in the water. They are small, measuring 2 mm (0.079 in) to 10 mm (0.39 in) in width.