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.

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.

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:

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.

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.

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.

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:

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.

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.

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.

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.

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:

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.

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