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Duckweeds, or water lentils, are aquatic plants which float on or just beneath the surface of still or slow-moving fresh water bodies. They arose from within the arum or aroid family, (Araceae), and therefore, often are classified as the subfamily Lemnoideae within the Araceae. Classifications created prior to the approximate end of the twentieth century tend to classify them as a separate family, Lemnaceae.
These plants are very simple, lacking an obvious stem or leaves. They consist of a small ‘thalloid’ or plate-like structure that floats on or just under the water surface, with or without simple rootlets. The plants are highly reduced from their earlier relatives in Araceae.
Reproduction is mostly by asexual budding, but occasionally three tiny ‘flowers’ consisting of two stamens and a pistil are produced and 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 Araceae. Anatomical research regarding the mechanics of this process has not been completed or remains ambiguous due to considerable evolutionary reduction of these plants from their earlier relatives. The flower of Wolffia is the smallest known flower in the world, measuring merely 0.3 mm long.
The fruit produced through this occasional sexual reproduction is a utricle, and a seed is produced in a sac containing air that facilitates flotation.
Duckweed in various environments
Duckweed is an important high-protein food source for waterfowl and also is eaten by humans in some parts of Southeast Asia (as khai-nam). Sometimes it is cited as an overlooked source for application as a food for a hungry world that produces more protein than soybeans.
Some duckweeds are introduced into freshwater aquariums and ponds where they may spread rapidly. This introduction may be deliberate or unintended and once established in a large pond, may be difficult to eradicate. Occurring naturally by being carried on the feathers, shells, and coats of native species, the plant is introduced readily by birds, turtles, reptiles, and aquatic mammals visiting multiple ponds, rivers, and lakes. 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, yet are carried away as rainy periods ensue.
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 bluegills. They also provide shade and, although frequently confused with them, can reduce certain light-generated growths of photoautotrophic algae.
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, SANDEC, 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 waste water 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 similar size water body with a clear surface.
The duckweeds long have been a taxonomic mystery, and usually have been considered to be their own family, 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 phylogenyit was difficult to test this hypothesis. 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 twenty-first 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.
The genera of duckweeds are: Spirodela, Landoltia, Lemna, Wolffiella, and Wolffia.
In July 2008 the U.S. Department of Energy (DOE) Joint Genome Institute announced that the Community Sequencing Program would fund the sequencing the genome of the giant duckweed, Spirodela polyrhiza. This was a priority project for DOE in 2009. The research is intended to facilitate new biomass and bio-energy programs.
Duckweed is being studied by researchers around the world as a possible source of clean energy. In the United States, in addition to being the subject of study by the DOE, bothRutgers University and North Carolina State University have ongoing projects to determine if duckweed might be a source of cost-effective, clean, renewable energy. Duckweed is a good candidate as a biofuel because as a biomass it grows rapidly, has 5 to 6 times as much starch as corn, and does not contribute to global warming. Duckweed is considered a carbon neutral energy source, because unlike most fuels, it actually removes carbon dioxide from the atmosphere.
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 waste water. One study in Australia surrounding aquaculture suggests that although duckweed is initially effective as a nutrient filter, over time some nutrient build-up returns.
Duckweed is the world’s smallest flowering plant. Roughly 40 species of duckweed exist and grow worldwide; these very small aquatic plants grow so rapidly that a single floating colony can double in size in less than 48 hours. Duckweed grows best in temperate and tropical climates, and prefers areas with little wave or wake, where it is sheltered from the wind; though it has also been found in areas with extreme temperatures and growing conditions. Duckweed plants remove a very high percentage of nutrients from the water during their growth cycle, and so are potentially valuable both as a food source and as a water purification system.
Fresh duckweed is between 90 and 95 percent water; this is not surprising since it is an aquatic mass with a low enough density to float. To measure its other nutritional values, scientists look at its dry mass.
Duckweed has gained the attention of agricultural specialists interested in its potential as a feed supplement for livestock because of its high protein value. Duckweed grown in cultivated conditions can contain up to 45 percent crude protein in its dry mass. The chemical makeup of these proteins, rich in the essential amino acids lysine and methionine, make it compositionally more like an animal protein than a vegetable protein.
The plant as it naturally occurs typically has a fiber content of between 15 and 30 percent. In ideal water conditions however, duckweed can be cultivated with as little as 5 percent fiber in its nutritional composition. As growth conditions improve and the fiber mass is minimized, the total amount of protein in the plant is maximized.
Other Nutritional Elements
The dry mass of duckweed, when tested, contained between 1.8 and 9.2 percent lipid tissue, and between 14.1 and 43.6 percent carbohydrates. Cultured duckweed, specifically, has demonstrated larger concentrations of certain trace minerals and pigments, like beta carotene and xanthophyll, nitrogen and phosphorus.
Because of its rapid growth and its nutritional composition, duckweed is being studied as a potential food source for poultry, hog, cattle, and human consumption. The fiber content of popular feed grains like soy and milo can be as high as 50 percent, which is not readably digestible. Duckweed, as a feed source, could be broken down and more completely consumed by the animal, increasing feed conversion rates. Also, the whole duckweed plant can be used as feed, saving the processing expenses and plant waste associated with feeding grain. In terms of dry mass grown per acre, duckweed could be grown on 10 percent of the space required to produce a similar amount of soybeans, and would only require 20 percent of the space required to grow the equivalent amount of corn.
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