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One spring afternoon, a botany student brought a sample of duckweeds from nearby Lake Hodges to Professor Armstrong's biology laboratory. The two proceeded to examine the intriguing sample through dissecting microscopes, when suddenly Professor Armstrong noticed a tiny green speck among the larger duckweeds. Could this be the elusive "wolffia" that he had searched for during the past two decades? Later that afternoon the two researchers found several more of the strange green specks. After several additional hours of research using every available reference in the library on aquatic plants, the two biologists were fairly certain that they had indeed discovered "wolffia," the world's smallest flowering plant. But it wasn't until another student brought a sample of pure wolffia (containing literally thousands of individual plants) from the San Dieguito River (below the Lake Hodges Dam) that Professor Armstrong realized the significance of this exciting botanical discovery. That fall, Professor Armstrong discovered four different species of wolffia in the San Dieguito River, some of them in full bloom. The species identifications were all confirmed by the world authority on the duckweed family, Professor Dr. Elias Landolt of the prestigious Geobotanical Institute in Zurich, Switzerland. Since the initial discovery of wolffia in Lake Hodges, many new duckweed discoveries have been made by Professor Armstrong, leading to a number of publications and many botanical contacts throughout the world. That fateful spring afternoon marked the beginning of Professor Armstrong's obsession with this little-known and fascinating family of flowering plants--the Lemnaceae.
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 five 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. 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). The common name "watermeal" is often used for Wolffia species because they look and feel like small, mealy particles in the water. Five species of Wolffia are now known to occur in the western United States, with 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!
| A Key To The Genera Of Lemnaceae
1a. Plant body with 1 - several roots.
2a. Root one................................................................Lemna
2b. Roots 2 - 12.
3a. Roots 7 - 12 (or more); plant 10 mm long........Spirodela
3b. Roots 2 - 3 (up to 5); plant 3 - 6 mm long........Landoltia
1b. Plant body without roots.
4a. Plant body flattened; 3 - 10 mm long.....................Wolffiella
4b. Plant body globose-ovoid; 0.6 - 1.2 mm long.........Wolffia
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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.].
Wolffia species in California and Pacific Northwest.
There are approximately 250,000 species of described flowering plants in the world, and they range in size from diminutive alpine daisies only a few inches tall to massive eucalyptus trees in Australia over 300 feet tall. But the undisputed world's smallest flowering plants belong to the genus Wolffia, minute rootless plants of the duckweed family (Lemnaceae) that float at the surface of quiet streams and ponds. Each plant is shaped like a microscopic green football with a flat or rounded top depending on the species. They reproduce exponentially by budding, and during warm summer months they literally cover the water surface like masses of granular, green Malto-Meal. Two of the smallest species are W. angusta, an Australian species recently described in 1980 by Dr. Landolt, and the worldwide tropical species W. globosa. The entire plant body of these two species is less than one mm long (less than 1/25th of an inch) and it is difficult to say which is the smaller of the two, but perhaps W. globosa may be slightly smaller. An average individual plant is 0.6 mm long (1/42 of an inch) and 0.3 mm wide (1/85th of an inch). It weighs about 150 micrograms (1/190,000 of an ounce), or the approximate weight of two ordinary grains of table salt. One plant is 165,000 times shorter than the tallest Australian eucalyptus tree and 7 trillion times lighter than the most massive giant sequoia tree. It is small enough to slip through the eye of an ordinary sewing needle, and at least 5,000 plants could be packed into a thimble. Each plant produces a microscopic flower inside a small cavity that develops on the upper side of the plant body. The minute flower consists of a single pistil and stamen. Since the stigma is generally receptive before the anther is mature, a condition known as protogyny, the flower typically requires cross pollination from a different wolffia plant with a mature anther that is ready to shed its pollen. A bouquet of one dozen plants in full bloom will easily fit on the head of a pin. After pollination the ovary develops into a tiny one-seeded fruit called a utricle, which also holds the record for the world's smallest fruit.
How Small Are Wolffia Plants?
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The average diameter of a globose wolffia plant body is about one 1/25th of an inch (1 mm), although there are species with smaller plants. To appreciate their minute size, wolffia plants are comparable in size to the multicolored candy sprinkles used for decorating cakes and cookies.
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Wolffia The Size Of A Bacterium
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A recent article by J. Travis in Science News Vol. 155 (April 17, 1999) discusses a remarkable new species of sulfur bacteria from the greenish ooze of ocean sediment off the coast of Namibia in southwestern Africa. Sulfur bacteria oxidize sulfur compounds to produce their energy-rich ATP molecules. The spherical bacteria have diameters ranging from 0.1 to 0.75 mm, definitely within the size range of some species of Wolffia. In fact, some multicellular orchid seeds are less than 0.2 mm in diameter, smaller than this bacterium. Considering that a unaided human eye with 20-20 vision has resolving power of 0.1 mm, this bacterium is visible to the naked eye. Because of their size and light-reflecting properties, the bacteria appear to be roughly the size of a printed period or the size of an average grain of table salt (NaCl). The bacteria were discovered in sediment samples by Heide Schulz of the Max Planck Institute for Marine Microbiology in Bremen, Germany (see the April 16, 1999 issue of Science). The new species is named Thiomargarita namibiensis, or sulfur pearl of Namibia. When light shines on the bacterial cells, they glisten white from light reflecting off sulfur inside them (see the image above). But who would ever believe that a multicellular flowering plant could be as small as one prokaryotic bacterial cell. This is certainly the case with Wolffia globosa, especially if you consider the size of a single daughter plant that has broken away from the parent plant by budding (see the images above). And although it is the ultimate in reduction of a flowering plant, it actually has minute guard cells and stomata on its upper (dorsal) surface. Any way you look at these amazing records; a giant bacterial cell or a microscopic flowering plant, they are truly wonders of the world.
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A remarkable flowering Wolffia borealis showing a circular (donut-shaped) stigma and a minute anther inside a small floral cavity on the upper surface. The flowering plant is about 0.6 mm long.
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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.
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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.
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The world's smallest flowering plant also has one of the most rapid rates of vegetative reproduction. The Indian species, Wolffia microscopica, 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. For those interested in astonishing comparisons, one wolffia plant has a volume roughly intermediate in size between a water molecule and the planet earth. Since they are quite palatable and about 40% protein (dry weight), similar to soybeans in their amino acid content, wolffia is a plausible food source for people. In Thailand, Wolffia globosa is known as "khai-nam" (meaning "water eggs") and is eaten by people. Who knows, an advertisement of the future might read: "Munch a bunch of wolffia."
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A population of Wolffia columbiana (A), W. globosa (B), and W. borealis (C) in the San Dieguito River of San Diego County, California. The darker, more rounded (spherical) plants are W. columbiana, a common South American species. The smallest plants are W. globosa, some of which are only 0.3 to 0.5 mm in diameter, roughly the size of the world's largest bacterium (Thiomargarita namibiensis) from the southwest coast of Africa.
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