The Lemnaceae (Duckweed Family)

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Table Of Contents
  1. Some Notes On The Identification Of Duckweeds
  2. A Brief Technical Description Of The Lemnaceae
       Aerenchyma: Tissue With Intercellular Air Spaces
       Bisexual Flowers & One-Seeded Fruits Of Duckweeds
  3. Some Generalizations About The Duckweed Family
       Stomata On The Upper Surface Of Duckweed Species
       The Leaflike Prophyllum Of Spirodela & Landoltia
       Winged Root Sheath In Two Species Of Lemna
  4. Cladograms Of The Duckweeed Family (Lemnaceae)
  5. Updated Key To The Five Genera Including Landoltia
  6. ID Of Species That Are Morphologically Very Similar
       Nerves (Veins) & Air Spaces In Duckweed Identification
  7. Importance Of Backlighting When Identifying Duckweeds
       Elongated Tract Of Cells (Costa) In Wolffiella Identification
  8. Photoperiodism (Day Length) In The Duckweed Family
  9. Aseptic (Axenic) Culture Of Duckweeds In Agar Media
  10. Control Of Duckweed Blooms In Ponds And Reservoirs
  11. Index And Keys To The Genera Of Lemnaceae
  12. List Of All Western North American Species
  13. Flow Chart Of All Western North American Species
  14. Worldwide List Of All Species Of Lemnaceae
  15. View Images Of All North American Species
  16. Identifying Dried Duckweeds On Herbarium Sheets
  17. How To Cite Wayne's Word Articles In Reports
  18. Publication Updates For Lemnaceae & Wayne's Word
  19. Wayne's Word & Lemnaceae On-Line Copyright Policy
You May Also View Additional Duckweed Articles And Photos By
Wayne Armstrong At Another Web Site Through Palomar College:

Refer to The Charms Of Duckweed by Dr. John Cross. This site has information
and illustrations about duckweed anatomy, population growth, laboratory projects,
and practical applications using duckweeds for waste water reclamation.

1. Some Notes On Duckweed Identification

Since flowering and fruiting are rarely observed in most species of Lemnaceae, the following keys and descriptions are based primarily on vegetative characteristics. Minor traits which might seem insignificant in morphologically complex plants assume greater importance in the Lemnaceae. Ideally, it is best to observe living plants under a 30X dissecting microscope, preferably with substage lighting to view veins and the shape of budding pouches (dried herbarium specimens can be hydrated in water to obtain a resemblance of their former shape). For difficult species it is often necessary to grow them in containers to observe the development of diagnostic features such as shape, size, number of plants cohering, nervation, anthocyanin pigmentation and turions. Some species may exhibit considerable morphological variation, particularly when growing under less than optimal environmental conditions, making their precise vegetative identification very difficult.

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

See Straight Pin & Sewing Needle Used In Wayne's Word Articles

2. A Brief Technical Description Of The Duckweed Family

Small aquatic herbs floating on or below the surface of quiet streams and ponds, often forming dense, homogeneous clonal populations; plant body not differentiated into a stem or leaf, reduced to a fleshy or thalluslike ovoid or flattened structure bearing one-several roots (without root hairs) on the underside or rootless; the terms dorsal and ventral are often used in the literature for the upper and lower surfaces of the plant body floating in water; [The terms adaxial and abaxial are typically used for leaves, referring to the surface adjacent to the leaf axil (adaxial) and the opposte surface away from the leaf axil (abaxial). Adaxial and abaxial also refer to the upper and lower sides of a leaf; however, the abaxial side is also the back or dorsal side. This terminology is especially appropriate for leaves arranged vertically on a stem. Since the plant body of a duckweed is not technically a leaf, the terms adaxial and abaxial are confusing for general descriptions. For duckweeds it is preferable to use upper and lower surface. Thanks to Elena George of Humboldt State University for bringing this to my attention.] plant body often with one-several layers of conspicuous air spaces (aerenchyma) and one-several veins (nerves); daughter plants produced in budding pouch at basal end or along 2 lateral margins of parent plant, often remaining attached to parent plant by a short stipe; some species producing rootless (or very short-rooted), starch-filled daughter plants, called turions, which sink to the bottom and overwinter; flowers bisexual and usually protogynous, the androecium consisting of 1 or 2 stamens and the gynoecium consisting of a single pistil; flowers are produced in a floral cavity on the dorsal surface (Wolffiella & Wolffia), or in a membranous, saclike spathe (utricular scale) within a lateral budding pouch (Spirodela, Landoltia & Lemna); Note: Some authorities consider duckweed species to be monoecious with one or two staminate flowers (each consisting of a single stamen) and a pistillate flower (consisting of a single pistil); corolla and calyx 0; ovary superior and unilocular with short style and circular concave stigma, the stigma often secreting a fluid droplet at anthesis; stamen with a short filament and unilocular or bilocular anther, transversely or apically dehiscent, bearing spinulose pollen grains; fruit an indehiscent, bladderlike utricle containing one-several seeds with prominent operculum; 5 genera, at least 38 species; worldwide distribution, especially temperate and tropical regions; the smallest and structurally simplest of all angiosperms, with greatly reduced vascular tissue (tracheids) limited to veins of plant body, filaments of stamens, and roots of some species; duckweeds and associated microfauna important food source for certain waterfowl; duckweeds are potentially valuable for waste-water reclamation and one species (Wolffia globosa (Roxb.) Hartog & Plas), known locally as "khai-nam," is eaten by people in S.E. Asia; Landolt, E. (1986) Veroff. Geobot. Inst. ETH, Stiftung Rubel 71 "The Family of Lemnaceae: A Monographic Study" Vol. 1; Landolt, E. & R. Kandeler (1987) Veroff. Geobot. Inst. ETH, Stiftung Rubel 95 "The Family of Lemnaceae: A Monographic Study" Vol. 2. Duckweed species in California are described by Landolt (1957) in "Physiologische und okologische Untersuchungen an Lemnaceen," Ber. Schweiz. Bot. Ges. 67: 271-410.

Aerenchyma tissue in the duckweed Lemna minuta (1000x). The large intercellular spaces are surrounded by layers of choroplast-bearing parenchyma cells. The air-filled spaces provide buoyancy for the duckweeds, keeping them afloat on the water surface. Although enlarged air spaces may provide a competitive advantage for increased buoyancy, some species have greatly reduced air spaces and float below the water surface.

Dorsal view of Lemna gibba in full bloom. Two stamens and a short style are projecting from a lateral budding pouch at the base of the plant. The androecium consists of two pollen-bearing stamens. The gynoecium consists of a single pistil with a concave stigma, slender style and basal ovary bearing a one or two ovules. The bisexual flower is enclosed within a membranous saclike spathe within the budding pouch. Note: Some authorities consider the duckweeds to be monoecious species with one or two staminate flowers (each consisting of one stamen) and one pistillate flower (consisting of a single pistil) on the same plant body.

Dorsal View of the Bisexual Flower of Landoltia punctata

Lateral view of flowering Wolffia borealis showing the dorsal floral cavity containing one anther-bearing stamen and one pistil (gynoecium). The pistil has a seed-bearing ovary, a slender (short) style and a circular, concave stigma. The flowers are protogynous, with the stigma becoming receptive before the anther matures and sheds pollen. A daughter plant protrudes from a funnel-like budding pouch at the basal end. The entire flowering plant is only one millimeter (1/25th of an inch) in length. It weighs approximately 200 micrograms (roughly 1/150,000 of an ounce).

Dorsal view of several budding Wolffia borealis in full bloom. The floral cavity on the dorsal side reveals a circular concave stigma (nearest the basal end) and a single, pollen-bearing anther. Unlike Lemna, Spirodela and Landoltia, the flower is not enclosed within a membranous spathe. The flowers are protogynous, with the stigma becoming receptive before the anther matures and sheds pollen. The far right plant shows only the stigma, while the far left plant shows only the anther. The top and bottom plants show both the stigma and a faint anther.

Wolffia Plants Are The Size Of Candy Sprinkles
See Candy Sprinkles Used For Decorating Cakes & Cookies

Utricles of the duckweed family (Lemnaceae). The utricle is a small, bladderlike, thin-walled fruit. It is often compared with a one-seeded achene, except the utricle has a pericarp that is loose and fragile. Because of their small size (usually only 1-2 mm or less), utricles of the duckweed family are seldom seen. In fact, the one-seeded utricles of Wolffia species are the undisputed smallest fruits on earth. The smallest are from the Australian W. angusta and the Asian/African W. globosa.

The world's smallest fruits are produced by species of Wolffia, including the Australian W. angusta. The above image shows a mature fruit within the plant body. The larger fruit of Lemna shows a thin, transparent pericarp surrounding a ribbed seed. A pericarp layer is not evident on the wolffia fruits.

Germinated seeds of Lemna perpusilla showing seedlings with attached seeds.

Seeds Of A Duckweed (Lemna perpusilla)
Seedlings Of A Duckweed (Lemna perpusilla)

Two of the Wolffia species included in Landolt's 1986 Monograph of the Lemnaceae (Vol. 1) have each been split into two species (E. Landolt, 1994, Ber. Geobot. Inst. ETH, Stiftung Rubel 60). The justification for two additional Wolffia species is based on allozyme studies by D.J. Crawford, Columbus, Ohio (Crawford, D.J. & E. Landolt, 1995, Allozyme Diversity Among Species of Wolffia (Lemnaceae), Plant Systematics & Evolution 197: 59-70). South African populations of W. globosa (Roxb.) Hartog & Plas are now recognized as W. cylindracea Hegelm., an older name used in the literature since Hegelmaier (1868). The widespread Asian W. globosa (also from California and southern Florida) has been retained as W. globosa. Populations of W. angusta Landolt in Pakistan and India have been named W. neglecta Landolt. The Malaysian and Australian populations of W. angusta have been retained as W. angusta. In addition, a new species of Wolffiella from the Amazon Basin has been named W. caudata Landolt (E. Landolt, 1992, Ber. Geobot. Inst. ETH, Stiftung Rubel 58). The specific epithet for this latter curious species refers to the tail-like, tapering distal end of the plant body (See WAYNE'S WORD: Weird Duckweeds From Far Away Lands). Another new species of Lemna (L. yungensis) was also described by Landolt from vertical wet rocks of the Andean Yugas in Bolivia (E. Landolt, 1998, Bulletin of the Geobotanical Institute ETH 64). D.H. Les and D.J. Crawford (1999) have 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.] These revisions raise the total worldwide number of taxa in the Lemnaceae to 38 species in five genera.

Mudmidgets (Wolffiella lingulata) in full bloom. This is a dorsal view showing several broad, lingulate (tongue-shaped) plants with their free ends curved downward (recurved) in the water. Each plant has an immature yellow anther protruding from a floral cavity. The lower plants show a minute circular stigma adjacent to the anther. The plants are about 7 mm in length. The genus Wolffiella includes some of the most bizarre of all flowering plants. Although the generic name for mudmigets refers to the diminutive of Wolffia, they are not as small as Wolffia species.

A juice strainer filled with Wolffiella lingulata. The thousands of recurved, lingulate plants resemble translucent green leaves or shavings.

See More Images Of Wolffiella Species
Detailed Description Of The Genus Wolffiella
See Complete List Of All Species Of Lemnaceae

3. Some Generalizations About The Duckweed Family

The duckweed family is well represented in western North America with nearly half of the world's species. The plant body of duckweeds is quite unlike other flowering plants because it does not have stems or leaves. It represents the ultimate in reduction of an entire vascular plant. The terms "frond" and "thallus" are sometimes used in the literature, but these terms are not appropriate because the plant body of duckweeds is not homologous to the fronds of ferns or the body of fungi and algae. Although the body of duckweeds does have paired guard cells and stomata on its upper surface and superficially resembles a leaf (particularly the flattened duckweeds Spirodela, Landoltia and Lemna), it is morphologically and embryonically completely different. In Spirodela, Landoltia and Lemna it is a flattened structure with slender, hairlike roots on the underside. Spirodela and Landoltia are unique among duckweeds because of a minute, membranous scalelike leaf (prophyllum) enveloping the dorsal and ventral surfaces of the basal end. In Spirodela polyrrhiza the prophyllum is visible on young plants (fugacious in older plants) and on overwintering turions. This basal portion and its connecting stalk correspond to a condensed shoot that has become greatly reduced through evolution. Landoltia has a reduced prophyllum that perishes in full grown plants. A prophyllum is lacking in Lemna, Wolffia and Wolffiella. The latter two genera have been reduced through evolution to minute, rootless spheres or flattened ribbons. Wolffia has a minute globose or ovoid body one mm long or less. In Wolffiella the thalluslike body is transparent and flattened, with the free ends often curved downward in the water.

Magnified view (1000x) of the upper surface of Lemna minuta showing a pore slit (stoma) flanked by two slender guard cells. The cells surrounding the stoma resemble the subsidiary cells of true leaves. Although the plant bodies of duckweeds have stomata and carry on gas exchange with the atmosphere, they are not homologous to leaves.

Turion of Spirodela polyrrhiza. Note the minute, transparent, bractlike leaf called a prophyllum at the basal end. The prophyllum overlaps both the dorsal and ventral sides of the turion, but is more visible on the lower (ventral) surface. The prophyllum of Landoltia punctata is much smaller. If the prophyllum is homologous to a leaf in its embryonic origin, then it is one of the world's smallest leaves.

Ventral side of a hydrated herbarium specimen of Landoltia punctata. A budding pouch in the parent plant bears a younger, daughter plant extending horizontally to the right in photo. The daughter plant shows a scalelike prophyllum that is penetrated by two roots. The ventral prophyllum is very difficult to see without careful examination under a dissecting microscope. A prophyllum is present in the genera Landoltia and Spirodela. It is a membranous, scalelike leaf that envelops the dorsal and ventral surfaces of the basal end, but usually is not evident in older plants. The prophyllum portion and its connecting stalk are homologous to a condensed shoot that has become greatly reduced through evolution. More advanced genera, such as Lemna, Wolffiella and Wolffia lack a prophyllum.

Underside of a hydrated herbarium specimen of Spirodela polyrrhiza showing a small, scalelike prophyllum at the basal end of a daughter plant. This species has 7-12 or more roots, with one or two roots passing through the ventral prophyllum. Most of the roots are outside the margin of the prophyllum. The prophyllum is more evident on young daughter plants. Spirodela and Landoltia are the only duckweed genera with a prophyllum. This scalelike, basal leaf is absent in the more advanced genera, including Lemna, Wolffiella and Wolffia.

Although all species of Lemna have a basal root sheath near the attachment node, two species in section Alatae (L. aequinoctialis and L. perpusilla) have a distinctive root sheath with 2 lateral wing-like appendages.

Underside of Lemna aequinoctialis showing winged root sheath near the basal attachment node. This species has one prominent apical papule on the upper side. The seeds have 8-26 distinct ribs and generally fall out of fruit wall at maturity. The closely-related L. perpusilla of the eastern United States also has a root sheath with 2 lateral wing-like appendages at the base. It has seeds with 35-70 indistinct ribs, remaining within fruit wall after ripening.

Elongated tracts of cells called nerves are present in Lemna, Landoltia and Spirodela. They originate at the node (point of root attachment) and extend through the plant body toward the distal (apical) region. A similar tract of elongated cells (called the costa) can be seen in the triagular budding pouch of Wolffiella. The position of the coasta in relation to the budding pouch is an important characteristic used to separate W. lingulata from W. oblonga. Tracts of elongated cells also extend through the center of the roots of Lemna, Landoltia and Spirodela. Nerves and tracts of elongated cells may serve to transport minerals and sugars, similar to the function of veins. In some species of Lemna, Landoltia and Spirodela, the elongated cells of nerves contain tracheids with ring-shaped or spiral-shaped thickenings in the walls (annular tracheids). These elongated cells are not called veins because the plant bodies of duckweeds are not homologous to leaves.

4. Cladograms Of The Duckweed Family

Different genes within the nucleus and cytoplasmic organelles (chloroplast and mitochondria) can be used to construct phylogenetic trees called cladograms. One gene in the nucleolus codes for the smaller subunit of the ribosome. The gene is called SSU rDNA or small subunit ribosomal DNA. Base sequences from this gene are sometimes used to compare taxa at the species level. Chloroplast DNA, including the protein-coding rbcL gene, is often used at the family level to show the relationships between genera and species within the family. Introns are also used to construct family trees. Introns are sections of messenger RNA that are removed prior to translation at the ribosome.

Most botanists consider the Lemnaceae to be closely related to the arum family (Araceae), and comparative chloroplast DNA studies have confirmed this taxonomic affinity (Duvall, et al. Annals of the Missouri Botanical Garden Vol. 80, 1993). In fact, several authorities have proposed some drastic and significant changes in the classification of many traditional angiosperm families, including the placement of all duckweeds in the Araceae rather than the Lemnaceae. [See: Angiosperm Phylogeny Group. 1998. "An Ordinal Classification For The Families Of Flowering Plants." Annals of the Missouri Botanical Garden 85: 531-553; Judd, W., C. Campbell, T. Kellogg and P. Stevens. 2002. Plant Systematics: A Phylogenetic Approach. Sinauer Associates, Inc., Sunderland, MA. Some of these proposed changes are summarized in an article by E. Dean in Fremontia 30 (2): 3-12, 2003. If accepted by the botanical community, the incorporation of these changes into botany textbooks, floras, checklists and herbarium collections will be a formidable task.

Computer-generated evolutionary trees or cladograms have been used to show the taxonomic relationships of duckweed species within the family. The cladograms are based on thousands of data characters, including morphology, anatomy, flavonoids, allozymes, and DNA sequences from chloroplast genes and introns. The branch (clade) length and position in the tree correspond to the number of character differences between taxa. The characters are numerically weighted according to their evolutionary importance. For example, a root would have a higher value than a papule. Cladograms are generated multiple times, and they don't always come out the same. The term "bootstrapping" refers to a cladogram or phylogenetic tree that comes out the same way out of a total number of times. For example, one thousand cladogram "trees" are generated and the same pattern comes out 900 times. This cladogram would have a bootstrap value of 90 percent. The following cladogram shows all the five genera and 38 species within the duckweed family (Lemnaceae). It was generated from DNA sequences of rbcL genes from all known members of the the family using the computer program PAUP:

A cladogram of the duckweed family based on the chloroplast gene rbcL. Five genera and 38 species are shown. According to the cladogram, the ancestral genus is Spirodela and the genusWolffia is placed farthest away because it has the fewest shared characters with Spirodela. Spirodela, Landoltia and Lemna are more closely related, while Wolffia and Wolffiella have more characters in common. With the exception of one new genus Landoltia and a few changes within sections of the family, most of the results are consistent with previous studies based solely on morphological characteristics made by meticulous botanists. Cladogram modified from Les, D.H., Crawford, D.J., Landolt, E., Gabel, J.D. and R.T. Kimball. 2002. "Phylogeny and Systematics of Lemnaceae, the Duckweed Family." Systematic Botany 27 (2): 221-240.

Because of their degree of reduction, Landolt (1986) considers the two diminutive genera Wolffia and Wolffiella to be the most recently evolved offshoots in the phylogeny of this family. Wolffia has the fewest shared characters with the presumed ancestral Spirodela and is placed farthest away in an evolutionary tree (cladogram). The new genus Landoltia is morphologically intermediate between Lemna and Spirodela. According to D.H. Les & D.J. Crawford (Novon 9: 530-533, 1999), it represents an isolated clade distinct from both Lemna and Spirodela. DNA comparisons of all members of the Lemnaceae by Les, et al. (Systematic Botany 27 (2): 221-240, 2002) indicate that all five genera represent distinct clades. With the exception of Landoltia and a few changes in sections, the 38 taxa recognized in the study by Les et al. (2002) are remarkably consistent with those recognized as morphologically distinct by Landolt.

Duckweeds Now Placed In The Arum Family (Araceae)

Phylogenetic studies by G.W. Rothwell et al. (2004) and L.I. Cabrera (2008) indicate that Pistia plus the five genera of Lemnaceae form a monophyletic group within the arum family (Araceae). In other words, they are derived from a common ancester. Maintaining Lemnaceae and Araceae as distinct families would make the arum family paraphyletic, with a common ancestor but not all of its decendants (i.e. duckweeds are excluded). Their cladograms are based on sequences of the trnL-trnF intergenic spacer region of the chloroplast genome. This spacer region is non-coding DNA between the trnL and trnF loci. Because it is non-coding, it is not under selection (not highly conserved), compared with highly conserved genes that code for structural products, regulatory proteins, or transfer RNAs. It is interesting to note that the duckweeds belong to the same plant family as the titan arum (Amorphophallus titanum). This remarkable plant has a 2.4 m erect spadix that protrudes from a vase-shaped, pleated spathe 4 m in circumference.
  • Rothwell, G.W., Van Atta, M.R., Ballard Jr., H.E. and R.A. Stockey. 2004. "Molecular Phylogenetic Relationships among Lemnaceae and Araceae Using the Chloroplast trnL-trnF Intergenic Spacer." Molecular Phylogenetics and Evolution 30: 378-385.

  • Cabrera, L.I., Salazar, G.A., Chase, M.W., Mayo, S.J., Bogner, J., and P. Dávila. 2008. "Phylogenetic Relationships of Aroids and Duckweeds (Araceae) Inferred From Coding and Noncoding Plastid DNA." American Journal of Botany 95 (9): 1153-1165.

Pistia stratiotes: An aquatic member of the arum family (Araceae) with characteristics similar to the duckweed genus Spirodela. Evidence from chloroplast DNA suggests that the Lemnaceae evolved from an ancestral member of the order Arales, similar to the present-day Pistia stratiotes. Note the small white spathe (red arrow) surrounding the anthers at the apex of a reduced spadix.

Complete List Of All Species Of Lemnaceae
PCR & Cladogram Of The Lemnaceae
Stinking Arums That Attract Flies

5. An Updated Key To The Duckweed Family

The following simplified, indented dichotomous key separates the duckweed family (Lemnaceae) into five distinct genera:

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

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.].

According to Professor Dr. Elias Landolt (personal communication, 2001), the creation of the new genus Landoltia is not necessary based on a purely morphological point of view; however, based on DNA and enzymatic studies, the change is warranted in order to form phylogenetically consistent taxa. The inclusion of a fifth genus Landoltia appears to be necessary based upon a comprehensive analysis of the Lemnaceae by D.H. Les, D.J. Crawford, E. Landolt, J.D. Gabel, and R.T. Kimball (2002). More that 4,700 characters were studied, including data from morphology and anatomy, flavonoids, allozymes, and DNA sequences from chloroplast genes (rbcL, matK) and introns (trnK, rpl16). See: Les, et al. 2002. Phylogeny and Systematics of Lemnaceae, the Duckweed Family. Systematic Botany 27 (2): 221-240.

See Images Of Landoltia punctata

Depending on the genus, daughter plants are produced vegetatively in 2 lateral, flattened, budding pouches (Spirodela, Landoltia & Lemna), a flattened, triangular budding pouch at the basal end (Wolffiella), or a funnel-shaped budding pouch at the basal end (Wolffia). Each plant produces up to a dozen daughter plants during its lifetime of 1-2 (or more) months. The daughter plants repeat the budding history of their clonal parents, resulting in exponential growth. It has been estimated that the Indian Wolffia microscopica (Griff.) Kurz may reproduce by budding every 30 hours under optimal growing conditions. At the end of 4 months this would result in about 1 nonillion plants (1 followed by 30 zeros) occupying a total volume roughly equivalent to the planet earth. This astronomical vegetative growth and the ability of some species to grow in stagnant, polluted water is why some duckweeds are well suited for water reclamation. Some species not only thrive on manure-rich water, but can be fed back to livestock, thus completing the recycling process. In addition, some species (such as Wolffia) are a potential source of food for humans because they contain about 40 percent protein (dry weight) and are equivalent to soybeans in their amino acid content (with high levels of all essential amino acids except methionine).

Although flowers are rarely observed in some species, all duckweeds bloom and reproduce sexually; however, some populations in small ponds may be clones of each other and not able to produce viable seeds. Since the flowers are typically protogynous with the stigma receptive before the anther is mature, the plants must be cross pollinated by genetically different individuals with mature pollen-bearing anthers in synchronization with the receptive stigmas. During the summer months, 2 stamens (androecium) and one pistil (gynoecium), all enclosed in a membranous saclike spathe, appear within budding pouches at the edge of the plant body in Spirodela, Landoltia and Lemna. In Wolffiella and Wolffia, a minute floral cavity develops on the upper side of the plant body containing a single stamen and pistil (not enclosed by a spathe). The tiny bisexual flowers have no sepals or petals, and are barely discernible without magnification. Because of the sweet (sugary) stigmatic secretions and spiny pollen grains (covered with minute protuberances), there is evidence that certain species may be pollinated by insects. In fact, Lemnaceae pollen has been detected on flies, aphids, mites, small spiders, and honey bees on the surface of dense duckweed layers. With floral sex organs projecting from the surface or lateral budding pouches, many duckweed species may be contact-pollinated as flowering individuals bump together or become piled up in windrows along the edges of ponds and lakes.

See contact pollination in Lemna gibba
Stigmatic Secretion Of Wolffiella lingulata

6. Identification Of Morphologically Similar Species

Since flowers and fruits are rarely observed, most taxonomic keys to the Lemnaceae are based on relatively few diagnostic vegetative characteristics that may vary under different environmental conditions. This often makes precise identification of some species difficult, or in some cases, practically impossible. All North American species have been separated by their flavonoid spot patterns using two-dimensional paper chromatography [see McClure & Alston (1966), Amer. J. Bot. 53: 849-860]. It should be noted that flavonoid chemistry is not always reliable for taxon distinction because chromatographic patterns may be influenced by environmental factors [see Ball, Beal & Flecker (1967), Brittonia 19: 273-279]. In addition, R. Scogin of RSA and J.L. Platt of OSU studied two-dimensional chromatography on clonal populations of Lemna minuta Kunth from San Diego County and came up with patterns identical with McClure & Alston's L. valdiviana Phil. According to Landolt (1987), the original clones of L. valdiviana studied by McClure & Alston may have actually been L. minuta. During the past century, the taxonomy of L. minuta Kunth has been complicated by different names used by different authors. Several of the synonyms commonly found in the literature include L. valdiviana var. minima Hegelm., L. minima Phil. ex Hegelm. and L. minuscula Herter. James L. Reveal (Taxon 19: 328-329, 1990) neotypified the oldest name L. minuta Kunth and cleared up some of the confusion and controversy about this widespread species.

The plant bodies (fronds) of Lemna valdiviana are often connected in clusters of four to seven, and the nerve ("vein") typically extends 3/4 of the distance from the node (point of root attachment) to the apex. The closely related L. minuta has one faint nerve that only extends about 1/2 the distance from the node to the apex. When growing in full sunlight, plant bodies of L. minuta are often only 2 mm long and are connected clusters of two. One of the most difficult duckweeds to identify in the field is the growth form of Lemna minuta found in shady habitats. The plant bodies are often connected in clonal clusters of four and are slightly longer than typical L. minuta growing in full sunlight. The shade form of L. minuta can be separated from L. valdiviana by the extent of the nerve. The obscure nerve of L. minuta only extends about 1/2 the distance from the node to apex.

Dorsal view of Lemna validiviana with backlighting, showing extent of nerve in relation to node (point of root attachment) and apex of plant body. The single nerve extends beyond the midpoint to about 3/4 of the distance between the node and apex. The nerve clearly extends beyond the region of air spaces (aerenchyma tissue). These characteristics rule out L. minuta, at least the typical form that grows in full sunlight. In L. minuta, the nerve rarely extends beyond the aerenchyma tissue and only extends about half the distance from the node to apex. These may seem like relatively minor morphological differences, but DNA sequencing studies clearly separate these two closely-related species.

General shape and extent of nerve in Lemna valdiviana compared with L. minuta. Plants of L. valdiviana are connected in clonal clusters of four to seven, while in L. minuta the plants are typically connected in two's. Each daughter plant is connected by a short stalk (stipe).

Note: Sometimes placing difficult species in an observation dish and examining them over several days can be helpful. Digital images can also bring out subtle differences. The following duckweeds were photographed through a dissecting microscope using a Sony digital camera with backlighting:

Three duckweeds from Pinnacles National Monument in central California. A. Lemna minor: Three veins arising from point of root attachment (n), without dorsal row of papules and reddish anthocyanin on ventral side (as in L. turionifera) and without winged root sheath (as in L. aequinoctialis). B. Lemna valdiviana: One faint vein extending more than 3/4 distance from root node (n) to apex (red arrow), plant body very thin and transparent throughout and floating on or just below water surface (slipping under plant bodies of L. minor and L. minuta in an observation dish). C. Lemna minuta: One vein extending less than 2/3 distance from root node (n) to apex, vein not extending beyond region of larger air spaces (red arrow), plant body slightly thicker in middle (not as uniformly thin and transparent as L. valdiviana), small size (only 1-2 mm long) or larger when growing in shade, floating on water surface (not submersed as in L.valdiviana). Photo taken with substage illumination.

Image Of Typical Lemna valdiviana Growing In The Shade
Image Of Typical Lemna minuta Growing In Full Sunlight

7. Importance Of Backlighting For Duckweed Identification

When identifying duckweed species (especially Lemna, Landoltia and Spirodela), it is very important to view the plant bodies with backlighting (substage illumination) in order to see the number and the extent of the nerves. Backlighting is also crucial in order to see the tract of elongated cells (costa) in the budding pouch of Wolffiella. The position of the costa within the triangular budding pouch is very important in order to distinguish between W. lingulata and W. oblonga.

Illustration of Wolffiella lingulata compared with W. oblonga. With backlighting the shape of the budding pouch and relative position of the costa can be observed. In W. lingulata the budding pouch angle is 80 to 120 degrees, with the costa situated between the middle and edge of the lower wall of the pouch. In W. oblonga the budding pouch angle is 40 to 70 degrees, with the costa situated along the edge of the lower wall of the pouch. Without backlighting under a microscope or good quality hand lens, it is virtually impossible to see these characteristics. Illustration modified from photos by W.P. Armstrong. 1993. Lemnaceae. In The Jepson Manual of Higher Plants of California, J.M. Hickman, Editor. University of California Press, Berkeley, California.

Dorsal views of Lemna turionifera. The left image has illumination from above and below. The right image has only substage illumination. To observe the number and position of nerves, it is best to use substage illumination only. The lateral dark bodies at the base of the mother plant are overwintering starch-filled bodies called turions. Because the specific gravity of starch is about 1.5, the turions sink to the bottom of quiet streams and ponds during the fall where they survive the freezing winter months. In the spring when the temperatures are once again suitable for growth, the turions produce bubbles of carbon dioxide and rise to the surface. They give rise to daughter plants by budding, and soon clonal colonies of this remarkable duckweed once again cover the water surface. Without turions, it is sometimes difficult to distinguish this species from the closey related L. minor. The dorsal surface of L. turionifera has a row of minute papules along the midline which are absent in L. minor. In addition, blotches of reddish anthocyanin sometimes develop on the ventral surface of L. turionifera which are absent from the underside of L. minor.

See Images of Lemna turionifera
See Image of Lemna minor

In these times of high technology, as botanical research moves toward a molecular emphasis, it is very important to have specimens verified by a taxonomist. It is also imperative to have carefully prepared voucher specimens on file in a nationally recognized herbarium. Modern molecular techniques, such as DNA sequencing, may lead to a better understanding of these fascinating species.

8. Photoperiodism In The Duckweed Family

Although some duckweed species superficially resemble each other, they may have significantly different biochemical patterns, such as an entirely different photoperiodism in response to day length (hours of darkness). During the hours of daylight the protein leaf pigment called phytochrome 730 (P-730) is formed. During the hours of darkness P-730 is slowly converted into phytochrome 660 (P-660). In short-day plants P-730 inhibits flowering. Short-day plants typically need about 15 hours of darkness to convert all the P-730 present at sundown into P-660. In these plants, P-660 stimulates the release of the essential flower stimulant "florigen" which induces flowering. The P-660 pigment is very sensitive to specific wavelengths of light, and a flash of light during the 15 hours of darkness can instantaneously convert all the P-660 back into P-730. Lemna aequinoctialis is clearly a short-day plant because it requires 16 hours of darkness (8 hours of light) to flower. The closely related L. perpusilla is also a short-day species that exhibits maximum flowering with 13-18 hours of darkness, and no flowering with 9 hours of darkness (15 hours of light). These species will generally not bloom during the longest days of summer or in a pond next to a bright street light.

Long-day plants require 15 hours of daylight and 9 hours of darkness in order to flower. In these plants P-730 stimulates the release of florigen and subsequent flowering. If the nights are long enough to convert all the P-730 into P-660, no florigen will be released and flowering will not occur. Lemna gibba is a long day plant that flowers with 9 hours of darkness. This species typically flowers during the longest days of summer. It will generally not flower with 12 hours of darkness, such as at the equator or during the vernal equinox, because the nights are too long. The physiology of these long-day and short-day species of duckweeds can definitely affect their range and potential for flowering and seed production.

Exactly how some duckweed species are dispersed and how they survive intermittent streams and ponds that dry up during summer is 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 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! Some species have been carried by rivers and streams, and in the shipment of fish and aquarium cultures. Professor Dr. Elias Landolt (1997) discusses some of the ways duckweeds survive dry conditions (Bulletin of the Geobotanical Institute ETH, Stiftung Rubel 63). Seeds of all Lemnaceae investigated so far tolerate drying for at least a few months to several years; however, seeds are rarely produced by clonal populations of some species. Although vegetative plant bodies are unable to withstand desiccation for more than a few hours, they may survive days (or weeks) embedded in wet mud and debris. According to Dan Richards (The Distributional Ecology Of Duckweeds (Lemnaceae) In Local Populations Of Northern California, MA Thesis, Humboldt State University, 1989), vegetative plants of two species survived up to six hours of desiccation (out of water). The two species tested by Richards (1989), Lemna minor and Landoltia punctata, had a much higher survival percentage when they were in large clumps compared to individually dried plants. Richard's experiments clearly show that these species could easily be carried short distances by migratory water fowl. Species that do not readily form seeds can also survive weeks or months of drought as turions, especially if the turions are imbedded in mud, silt and debris. This is especially true of the minute turions of Wolffia species. According to Landolt (1997), the South African Wolffia cylindracea may survive seasonally dry ponds for at least 16 months if the minute turions are firmly imbedded in clayey soil.

9. Axenic Culture Of Duckweeds In Nutrient Agar

The following methods are summarized from E. Landolt and R. Kandeler (1987): "The Family of Lemnaceae: A Monographic Study (Volume 2)." Veroff. Geobot. Inst. ETH 95: 1-638.

Species of Lemnaceae can be grown aseptically in nutrient agar similar to the methods used in plant tissue culture. The transfer techniques are similar to bacterial cultures using a flamed inoculation loop. The plants must first be cleansed (sterilized) before transfer to the sterile agar. Plants connected in clonal clusters should be separated from each other. Individual plants should be dipped in a 0.5% solution of sodium hypochlorite (10% Clorox® or Purex® solution) for at least one minute, washed in aseptic distilled water, and then transferred to an aseptic nutrient solution containing 1% sucrose (see recipe for Hutner's solution below). Contamination by fungi will show up in this dilute sugar solution within several days. If all the plants die, or if the solution becomes cloudy or covered by fungi, the treatment must be performed again. Plants that survive may be transferred to another aseptic nutrient solution containing 1% sucrose, 0.5% casein amino acids and 0.004% yeast extract. This solution will reveal contaminations at once. According to Landolt (1987), about 1-10% of the plants normally succeed in staying alive and become aseptic. Some species (such as Wolffiella) may need more attempts than others. Plants that survive this sterilization technique (and are not contaminated or infected by fungal molds) can be transferred to an aseptic nutrient agar in test tubes or Petri dishes. One of the best nutrient solutions for preparing the agar is 20% Hutner's solution (see table below). The mineral components of Hutner's solution are similar to some commercial plant tissue culture media. J.W. McClure ("Taxonomic Significance of the Flavonoid Chemistry and the Morphology of Lemnaceae in Axenic Culture," Ph.D. Dissertation, University of Texas, 1964) maintained stock cultures of Lemnaceae clones in a 33% Hutner's solution fortified with 1% sucrose and 1.25% "Bacto-Agar" (Difco Laboratories) per 100 ml of medium.

Recipe For 20% Hutner's Nutrient Medium:

Mineral Nutrient Mg per Liter

For More Information See The Charms Of Duckweed by Dr. John Cross

10. Control of Duckweed Blooms In Ponds and Reservoirs

One of the most common questions received at this site is how to control population explosions or "blooms" of duckweeds in which ponds, lakes and reservoirs become covered with a thick green layer of Lemna, Spirodela, Landoltia and Wolffia. Lemnaceae blooms typically occur in waters rich in nutrients, especially phosphorus and/or nitrogen. The nutrients originate from pollution from excessive use of fertilizers or possibly by an imbalance in the populations of fish or water fowl resulting in excessive nitrogenous waste products in the water. The recirculation of nitrogen and phosphorus from the cycle of growth and decomposition of duckweeds may also contribute to the high levels of these elements. Destroying the duckweed layer with herbicides does not solve the problem of excess nutrients in the water. In addition, the chemical herbicides may be toxic to the animal life, either directly or through biological magnification. Because of the exponential growth rate of Lemnaceae, herbicides must be used repeatedly (perhaps several times a year). Ideally, it is best to eliminate the influx of concentrated nitrates and phosphates into the water and avoid the use of concentrated fertilizers.

The manual or mechanical removal of the duckweed cover can also remove a lot of the nitrogen and phosphorus nutrients. The duckweed mats can be composted and used as "green manure." They can also be fed to livestock, rabbits, poultry and fish. It has been estimated that 10 acres of duckweeds could theoretically supply 60 percent of the nutritional needs of 100 dairy cows, the manure of which could be recycled to provide fertilizer for the thriving duckweeds. According to R.M. Harvey and J.L. Fox, 1973 ("Nutrient Removal Using Lemna minor," J. Water Poll. Control Fed. 45: 1928-1938), one hectare of water area is sufficient to raise 4000-7000 chickens and ducks during a vegetation period. And according to E. Rejmankova, 1981. ("On The Production Ecology of Duckweeds," Intern. Workshop on Aquatic Macrophytes, Illmitz, Austria), one hectare of Lemnaceae cover is sufficient to produce protein for 480 ducks during the warm season. The utilization of duckweeds as food for animals is summarized by E. Landolt and R. Kandeler, pages 382-389 in Veroff. Geobot. Inst. ETH, Stiftung Rubel 95 "The Family of Lemnaceae: A Monographic Study" Vol. 2, 1987. An extensive bibliography of Lemnaceae is also given on pages 414-580. The following 3 classic papers discuss duckweed use in aquaculture:

  1. Culley, D.D., Jr. et al. 1981. "Production, Chemical Quality and Use of Duckweeds (Lemnaceae) in Aquaculture, Waste Management, and Animal Feeds." J. World Maricult. Soc. 12 (2): 27-49.

  2. Hillman, W.S. and D.D. Culley, Jr. 1978. "The Uses of Duckweed." American Scientist 66: 442-451.

  3. Rusoff, L.L., E.W. Blakeney and D.D. Culley, Jr. 1980. "Duckweeds (Lemnaceae): A Potential Source of Protein and Amino Acids." J. Agricult. Food Chem. 28: 848-850.

Stopping the inflow of nutrients and the repetitive removal of the duckweed layer will greatly reduce the growth of duckweeds. Since water fowl and most fish feed on the duckweeds, they can help control the exponential population growth of these plants. In addition, Lemnaceae have a positive effect in eutrophic water because they remove ammonia which is toxic to fish in high concentrations.

In general, Lemnaceae are very sensitive to herbicides. In fact, duckweeds are often used to test the toxicity of herbicides and to detect the presence of herbicides in water. According to Professor Dr. E. Landolt (pages 161-170 in Veroff. Geobot. Inst. ETH, Stiftung Rubel 95 "The Family of Lemnaceae: A Monographic Study" Vol. 2, 1987), heterocyclic compounds (e.g. 6-methylpurin), urea derivatives, and quaternary ammonium compounds (e.g. diquat and paraquat) are the most toxic substances for Lemnaceae. Some algicides, including PH 40:62 are extremely toxic to some species of Lemna. Some of these products are available from agricultural supply companies depending on federal, state or local regulations. They should be used with extreme caution and under very careful supervision. It would be advisable to consult with your city or county weed/mosquito abatement department before attempting any large herbicidal control project.

Biological control using ducks, fish, turtles and crustaceans (water shrimp, crayfish, ostracods, freshwater prawns, daphnia, amphipods, etc.) may also help to control duckweed populations. There are a number of species of freshwater fish that eat duckweeds to supplement their diets, including grass carp (Ctenopharyngodon idella), channel catfish (Ictalurus punctatus), common carp (Cyprinus carpio), common mullet (Mugil cephalis), goldfish (Carassius auratus), and Tilapia (Sarotherodon), including S. mossambicus, S. hornorum, and S. nilotica. Duckweeds are also eaten by Pacu (Colossoma bidens), a freshwater fish native to the Amazon River. Some of these fish species may be available through aquafarm distributors or local county and state agencies. One aquaculture company in southern California was raising tilapia for local seafood restaurants.

More Information About Duckweeds For Wastewater Treatment:

LEMNA Corporation
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St. Paul, Minnesota 55120, USA
Phone: (612) 688-0836
FAX: (612) 688-8813

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