Weird Duckweeds From India,
Australia & South America

Since that fateful day nearly two decades ago when a student brought a sample of duckweeds into the General Biology laboratory at Palomar College, Professor Armstrong has received numerous samples of duckweeds from colleagues throughout the United States and abroad. But of all the different species sent to the Life Sciences Department, five of the most fascinating came from the Geobotanical Institute in Zurich, Switzerland and the University of Delhi, India. Four of the unusual samples belong to the genus Wolffia, minute, rootless, leafless flowering plants that float at the surface of quiet streams and ponds. They include W. angusta, W. australiana, W. elongata, and W. microscopica. A fifth rootless, leafless species (Wolffiella caudata) belongs to a different genus (Wolffiella), a closely related group that typically float at or slightly below the water surface. What makes these five species so noteworthy is that they are seldom-seen by mortal humans and they all grow wild in far away tropical waters of India, Malaysia, Australia and South America. In fact, several of them are relatively new to science, and have only recently been described in the taxonomic literature.

The Duckweed Family (Lemnaceae) contains five genera and 38 species. The five genera can be separated by the following indented dichotomous key:

Note:  D.H. Les and D.J. Crawford (1999) proposed the new genus Landoltia containing one species L. punctata, formerly Spirodela punctata. This species is morphologically intermediate between Lemna and Spirodela. According to Les & Crawford, it represents an isolated clade distinct from both Lemna and Spirodela. [Les, D.H. and D.J. Crawford. 1999. "Landoltia (Lemnaceae), A New Genus of Duckweeds." Novon 9: 530-533.].

The duckweed family (Lemnaceae) contains 38 species of minute flowering plants, floating at the surface of ponds, swamps and quiet streams. They are distributed throughout the world, particularly in warm temperate and tropical regions. They are greatly reduced flowering plants, without leaves or stems, and with only the remnants of vascular tissue in some species. The family contains four genera, based upon the presence or absence of roots and the shape of their plant body. Some botanists refer to the plant body as a "frond" or "thallus," but these terms are not appropriate because the plant body is not homologous to a leaf or to the bodies of fungi and algae. Most botanist consider the Lemnaceae to be closely related to the Arum Family (Araceae), and comparative chloroplast DNA studies have confirmed this taxonomic affinity. Because of their degree of reduction Professor Dr. Elias Landolt of the Swiss Geobotanical Institute, Zurich considers Wolffiella and Wolffia to be the most recently evolved offshoots in the phylogeny of this family (E. Landolt, Veroff. Geobot. Inst. ETH, Stiftung Rubel 71, 1986). Wolffia has the fewest shared characters with the presumed ancestral Spirodela and is placed farthest away in an evolutionary tree (cladogram).

Members of the genus Wolffia are the ultimate in reduction of a flowering plant consisting of tiny, rootless spheres only 1 mm long (or less). One plant will easily slip through the eye of a sewing needle and a bouquet of dozen plants in full bloom will fit on the head of a pin. Under a microscope each plant is shaped like a tiny green football with a flat or rounded top, depending on the species. [The Indian species W. microscopica actually looks more like a golf tee than a football.] Smaller, daughter plants are produced in a funnel-shaped budding pouch at the basal end. They reproduce exponentially by budding, and during the peak growing season (the warm summer months in North America) they literally cover the water surface like masses of granular, green Malto-Meal. In fact, the common name "watermeal" is often used for wolffia species because they look and feel like small, mealy particles in the water. Wolffia plants also have a sexual phase and produce flowers, although they are rarely observed in bloom. Each plant produces a microscopic flower inside a small floral cavity that develops on the upper surface of the plant body. The minute flower has no petals or sepals and consists of a single pistil and stamen. Following pollination and fertilization, a single, one-seeded fruit develops inside the floral cavity. Five species of Wolffia are now known to occur in the western United States, with at least 11 species worldwide. Exactly how all 5 species arrived at their present distribution in the Pacific northwest remains an enigma. Being carried from pond to pond on the feet of water fowl (tucked neatly under the ducks' bodies during flight), probably explains the distribution of some wolffia species. In the southeastern United States there are records of wolffia plant bodies being carried by a tornado, and they have even been reported in the water of melted hailstones.

Species of Wolffiella also have a tiny, rootless, leafless plant body, but it is clearly flattened, unlike the spherical body of Wolffia. The plant body is very thin and transparent-green, generally 2-8 mm long (larger than wolffia plants). They are often known by the rather uncomplimentary common name of "mudmidget." Like Wolffia, a reduced flower, consisting of one stamen and one pistil, is produced inside a floral cavity on the upper (dorsal) side of the plant body. After pollination and fertilization the flower develops into a minute one-seeded fruit (called a utricle). They also produce smaller daughter plants in a triangular budding pouch at the basal end. In some species the daughter plants adhere to their parent in strange little clusters. Three species of Wolffiella are known from the Pacific United States, with a total of ten species worldwide. They often grow submersed under other aquatic vegetation and require close and careful examination to find them.

1. Wolffia angusta Landolt

This very minute flowering plant is native to Australia and Malaysia. Populations of a very similar plant in Pakistan and India have been named W. neglecta (E. Landolt, Ber. Geobot. Inst. ETH, Stiftung Rubel 60, 1994). Wolffia angusta has a dark green dorsal margin (when viewed from above) which is lacking in W. neglecta. The plant body is 0.6-0.9 mm long and only 0.2-0.5 mm wide. It is one of the narrowest of all species of Wolffia, hence, the specific epithet of "angusta." It is rivaled in minuteness only by the Asian species W. globosa. Not only is it one of the smallest flowering plants on earth, but it also produces one of the smallest fruits. The fruit is shaped like an edible fig syconium except it is only about 0.3 to 0.5 mm long, slightly larger than an average grain of table salt (NaCl). Following pollination and fertilization, a tiny fruit begins to develop inside the floral cavity. At maturity, the one-seeded fruit occupies the floral cavity and takes up a large portion of the entire body of its "pregnant" mother plant (see following image).

Wolffia plants also produce the world's smallest flower, a bouquet of one dozen plants will easily fit on the head of a pin and two Wolffia angusta plants in full bloom will fit inside a small printed letter "o" on this page.

Several individuals of Wolffia angusta placed lengthwise inside the "eye" of an ordinary sewing needle. The distinctive dorsal margin is clearly visible on one of the plants. The width of one plant body is the same as a single strand of sewing thread, making this one of the smallest species of wolffia. It is rivaled in size only by the Asian species W. globosa.

Wolffia angusta from a tropical stream near Townsville, North Queensland, Australia. The ruler (right) shows the minute size of this diminutive flowering plant. Note the dark green dorsal margin (red arrow). Photo courtesy of Andi Cairns, School of Tropical Biology, James Cook University, Townsville, Australia.

A tropical stream near Townsville, North Queensland, Australia. The dense duckweed bloom contains the minute Wolffia angusta, one of the world's smallest flowering plants. Photo courtesy of Andi Cairns, School of Tropical Biology, James Cook University and David Reid, Burdekin Dry Tropics Waterwatch Coordinator.

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2. Wolffia australiana (Bentham) Hartog & Plas

This very peculiar Australian species has a swollen (gibbous) underside resembling the hull of a boat. In fact, when viewed from the side (laterally) it is unmistakable. Perhaps the pronounced underside keeps it more buoyant and stable as it floats on the water surface. These plants can actually be propelled across the water by strong winds and literally pile up along the shoreline. A European species (W. arrhiza) that is occasionally found in California has a similar shape, but not quite as swollen on the lower (ventral) side.

A side (lateral) view of the remarkable Australian species (Wolffia australiana). Note the conspicuously swollen lower (ventral) side and the smaller daughter plant developing at the basal (right) end.

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3. Wolffia elongata Landolt

This is a very unusual South American species from northern Colombia and Curacao. Like W. angusta it was introduced to the scientific community in 1980 when it was officially described by the world authority on the Lemnaceae, Professor Dr. Elias Landolt of the Geobotanical Institute in Zurich, Switzerland (E. Landolt, Veroff. Geobot. Inst. ETH, Stiftung Rubel 70, 1980). As its specific epithet implies, the plant body may be conspicuously cylindrical and elongate--up to 2.5 mm long; however, specimens growing in culture dishes may appear more globose-ovoid and resemble W. columbiana. Unlike all other Wolffia species, the daughter plant emerges from the parent plant at an acute angle pointing downwardly under the water. This is yet another example of a new species in this strange and seldom-seen group of minute flowering plants that was relatively recently reported in the taxonomic literature.

A population of Wolffia elongata from Colombia. Some individuals are reproducing asexually by budding--i.e. producing daughter plants at their basal end. The elongate, cylindrical body form is not evident in this sample.

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4. Wolffia microscopica (Griffith) Kurz

This is perhaps one of the most bizarre of all Wolffia species, and certainly one of the strangest flowering plants on earth. Its native range appears to be subtropical and tropical India. The plant body is orbicular (rounded) to polygonal, with a flat upper surface and a lower side that tapers into a conical appendage that points down into the water. This appendage resembles a short root, but it apparently does not serve the function of a true root. The unusual plant superficially resembles a minute, green, golf tee floating upright on the water surface. No other wolffia species has this unusual shape. In addition to its unique shape, W. microscopica also has one of the most rapid rates of vegetative reproduction. It can produce a smaller daughter plant in its basal reproductive pouch by budding every 30-36 hours. One plant could theoretically give rise to about one nonillion plants (1 followed by 30 zeros) in four months. This represents a spherical volume of wolffia plants roughly equal to the size of the earth.

A flowering (Wolffia microscopica) next to the tip of a sewing needle. The unusual "golf tee" shape is unique among all wolffia species. A small male organ (stamen) can be seen protruding from the upper (expanded) side of the plant body.

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5. Wolffiella caudata Landolt

The genus Wolffiella (often called mudmidgets) certainly includes some of the strangest flowering plants on earth, and W. caudata is one of the most unusual of all the 10 known species of mudmidgets. It is only known from the Amazon Basin of Bolivia, where it was discovered in a mixture of duckweeds sent to Professor Dr. Elias Landolt of the Geobotanical Institute in Zurich, Switzerland. Dr. Landolt recently introduced this fascinating new species to science by officially naming and describing it in a taxonomic journal (E. Landolt, Ber. Geobot. Inst.ETH, Stiftung Rubel 58, 1992). The specific epithet "caudata" refers to a tapering, slender, tail-like appendage at the distal (apical) end of the plant body. [Daughter plants are produced in a triangular budding pouch at the opposite (basal) end.] In fact, it is the tail-like appendage that readily distinguishes this species from all other mudmidgets on earth. When viewed from above, it superficially resembles a microscopic, green stingray. Like other species of Wolffiella the plant body is very flat and transparent-green, floating at or just below the water surface. Although the suffix "iella" often denotes the diminutive of Wolffia, most mudmidgets are actually larger than Wolffia, with a plant body 2-5 mm long. What is truly remarkable about this plant is that a new species can still be discovered in poorly explored regions of the world.

Close-up view of several (Wolffiella caudata) plants from the Amazon Basin of Bolivia. Note the slender, tail-like appendage on the two plants in center. A daughter plant is produced in a triangular budding pouch at the opposite (basal) end of the plant body.

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So the next time you are exploring your favorite pond or marsh, look carefully for minute, green flecks floating on the water surface--who knows, you might even discover a new species of duckweed. And as you explore the shoreline for duckweeds, be sure to keep your eyes peeled for a hungry crocodile or a mother hippo protecting her young.