Lemnoideae

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. 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.