This floating aquatic plant consists of a single thallus (a body
that combines the functions of leaf and stem). This thallus is 3-9 mm.
long and 2.5-7 mm. across; it is oval, broadly obovate, or orbicular in
shape and its outer margin is smooth. The texture of the thallus is
slightly succulent and it is filled with minute pockets of air,
enabling it to float. The upper thallus surface is light to medium
green, while the lower thallus surface is usually purplish red
(rarely light green); both surfaces are glabrous and nearly flat.
Toward one side of the upper surface of each thallus, there is a single
node that is often red. About 5-12 veins originate from this node,
curving inward (may require at least 10x magnification to see).
occasions, tiny unisexual flowers are produced from 2 lateral pouches
on the thallus. One pouch produces a single female (pistillate) flower,
while the other pouch produces two male (staminate) flowers. A female
flower consists of a single pistil, while a male flower consists of a
single stamen; they have neither petals nor sepals. Both types of
flowers are surrounded by tiny membranous spathes within their
respective pouches. The blooming period can occur from early summer to
early fall. The female flower is replaced by a tiny fruit (utricle)
that is 1-1.5 mm. in length. This fruit contains 1-2 ribbed seeds.
Instead of flowers, the lateral pouches more often produce vegetative
buds that are connected to the mother plant by short slender stipes
that soon wither away. A mother plant and its vegetative offspring may
form temporary clusters of 2-5 thalli, but they eventually separate
into individual plants. The root system of each plant consists of
3-15 rootlets that originate from a node on the underside of its
slender rootlets are up to 15 mm. long and their tips
pointed. When the weather becomes cooler during the autumn, a
winter turion (a starchy bud-like offset) is produced on the underside
of the thallus near the node of the rootlets. This turion is smaller in
size than the thallus and either olive green or brown. The turion
breaks free from the mother plant and sinks to the bottom of the water
body, where it remains throughout the winter. During the warmer weather
of spring, it rises to the surface of the water to begin the growth
The preference is full or partial
sun on fresh water that is mildly acidic to alkaline. The water should
contain some nitrogen and other nutrients. Great Duckweed thrives in
locations with stagnant or slow-moving water that receive some
protection from wind and waves. Sometimes it is cultivated indoors for
The native Great
Duckweed is occasional to fairly common throughout Illinois. It has a
world-wide distribution, occurring in North
America, South America, Eurasia, and other parts of the world. Habitats
include quiet inlets of lakes, ponds, backwaters of rivers, creeks with
slow-moving currents, and marshes. This aquatic plant is found in both
sandy and non-sandy wetlands.
Many of the
insects that feed on Lemna
probably also feed on Great
Duckweed. Some turtles have been observed to feed on this plant,
(Painted Turtle), Emys blandingii
(Blanding's Turtle), Sternotherus
(Musk Turtle), and Trachemys
(Slider). Similarly, the American Coot, Sora Rail,
species of ducks also feed on this species, as do carp and probably
other omnivorous fishes. Dense mats of Great Duckweed and their
abundant rootlets provide habitat for tiny aquatic organisms of various
kinds. Because the rootlets are somewhat sticky when they are wet, such
animals as muskrats, beavers, and probably some wetland birds transport
this plant on their fur or feathers from one wetland to another.
The plants were observed at a sandy marsh of the
Heron Boardwalk in
Vermilion County, Illinois. The photographs were taken indoors.
Among plants in the Lemnaceae (Duckweed family), Great Duckweed is one
of the easiest to identify because of the large size of its thallus and
its abundant rootlets. In contrast, Lemna spp.
(Duckweed) have only one
rootlet per thallus, while Wolffia
(Watermeal) have no rootlets.
Other distinctive characteristics of Great Duckweed include its red
upper node, abundant veins of its thallus (5 or more), and the purplish
red underside of its thallus. An introduced species in Illinois,
(Asiatic Duckweed), can be distinguished by the
smaller size and more narrow shape of its thalli; it also has fewer
rootlets per thallus (typically 2-5). On the basis of genetic and other
evidence, some authors have assigned species in the Lemnaceae to
another family of plants (Araceae). Other common names of Spirodela polyrhiza
are Giant Duckweed and Big Duckweed.