Duckweed family (Lemnaceae)
Description: This floating aquatic plant consists of a single oval or oval-obovate thallus (a body that combines the functions of leaf and stem); it is about 2-5 mm. long and 1.5-3.5 mm. across. The thallus has a slightly succulent texture and smooth margins; it is able to float on water because of numerous tiny air bubbles that are imbedded within its interior. The upper thallus surface is medium green and slightly convex along a faint longitudinal ridge; the lower thallus surface is light green and flat. Both surfaces are glabrous. Under magnification, 3 interior veins are usually visible (a central vein and 2 lateral veins) within the body of the thallus. A single rootlet up to 2 cm. long (rarely longer) develops near the center of the thallus' lower surface. This rootlet is slender and white with a tip that is usually obtuse. At the base of the rootlet, there is a short cylindrical sheath.
On rare occasions, a single tiny flower is produced that spans about 1 mm. across. This flower consists of a membranous cup-shaped scale, a single pistil, and 2 anthers. Flowers can be produced anytime between late spring to early fall. The flower is replaced by a single fruit (1 mm. in length or a little less) that contains a single ribbed seed. However, this plant reproduces primarily by a budding process from two lateral reproductive pouches. The offsets of this budding process are identical genetically to the mother plant. An offset is connected to the mother plant by a slender white stipe, but this soon withers away. During the summer, Common Duckweed often forms dense colonies of plants from budding. During the cooler weather of fall, small starchy buds called 'turions' are produced that sink to the bottom of the water body. This dormant state continues until the warmer weather of spring, when the turions rise to the water surface and the growth process begins again.
Cultivation: This plant adapts to fresh water that is stagnant or slow-moving and relatively high in nutrients. It has a fairly wide pH tolerance. Full or partial sun is required. This plant can spread aggressively and may cover the entire surface of a small body of water in some circumstances.
Range & Habitat: Common Duckweed occurs throughout Illinois; it is the most common Lemna sp. (Duckweed) in the state. This species is native to Illinois and it is widely distributed throughout the world. Habitats include ponds and lakes, slowing moving creeks and rivers, vernal pools, marshes, and ditches. This duckweed is common in disturbed and degraded wetlands, although it also occurs in higher quality wetlands.
Faunal Associations: Müller (1873/1883) has speculated that insects living on the surface of the water are the pollinators of the rarely produced flowers. A small number of insects feed on the thalli of Lemna spp. (Duckweeds). These species include Rhopalosiphum nymphaeae (Pond Lily Aphid), Scirtes tibialis (Duckweed Beetle), Tanysphyrus lemnae (Duckweed Weevil), the caterpillars of the Pyralid moths Monroessa icciusalis and Synclita obliteralis, and the maggots of Lemnaphila scotlandae (Lemna Fly). Two springtails that feed on the thalli are Podura aquatica and Sminthurus aquaticus. Some vertebrate animals feed on Common Duckweed and other duckweeds, especially ducks and turtles. More information about the ducks and other wetland birds that feed on these plants can be found in the Duck Table. The following turtles are known to feed on duckweed to varying degrees: Chelydra serpentina (Snapping Turtle), Chrysemys picta (Painted Turtle), Emydoidea blandingii (Blanding's Turtle), Graptemys ouachitensis (Ouachita Map Turtle), Graptemys pseudogeographica (False Map Turtle), Kinosternum flavescens (Yellow Mud Turtle), and Trachemys scripta (Slider). There are also records of carp and muskrats using the thalli as a food source. Because the rootlets of duckweed are sticky when they are wet, ducks and other wetland birds can carry this plant from one body of water to another, introducing it to new wetland areas.
Photographic Location: The photograph was taken at a pond of Crystal Lake Park in Urbana, Illinois.
Comments: To some people, duckweed may appear to be some kind of algae, but they rank among the smallest of all flowering plants. The different species of duckweed are often difficult to distinguish from each other, and an instrument with 10x magnification or higher is often necessary to make a correct identification. Two other groups of similar-looking plants are the Wolffia (Watermeal) and Spirodela (Greater Duckweed). Like duckweed, these species are floating aquatics consisting of small individual thalli. However, species of Watermeal tend to be smaller in size and lack rootlets, while species of Greater Duckweed tend to be larger in size and have 2 or more rootlets per thallus. Another common name of Lemna minor is Lesser Duckweed.