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Supplements To Biology 101 Cell Unit 1. Fluorescence In A Chlorophyll Solution
2. Simplified Illustration Of A Mitochondrion
3. ATP Structure & Function
The structure of adenosine monophosphate, an RNA nucleotide containing the purine base adenine, is very similar to ATP (adenosine triphosphate), except that ATP has three phosphates (PO4) instead of one. ATP is synthesized in all living cells by the addition of a phosphate to ADP (adenosine diphosphate). ATP is the vital energy molecule of all living systems which is absolutely necessary for key biochemical reactions within the cells. The terminal (3rd) phosphate of ATP is transferred to other molecules in the cell, thereby making them more reactive. For example, the monosaccharide glucose is very stable at ordinary body temperatures and would require a great amount of heat (such as from a flame) to break it down into carbon dioxide and water. After receiving a phosphate from ATP (a process called phosphorylation), glucose becomes glucose-phosphate and can be enzymatically broken down within seconds. Most of the ATP in eukaryotic cells of animals is made inside cellular organelles called mitochondria from the oxidation of glucose, a process called cellular respiration. Glucose combines with oxygen (oxidation), forming carbon dioxide, water and 38 molecules of ATP. During the oxidation process, electrons from glucose are shuttled through an iron-containing cytochrome enzyme system on the inner mitochondrial membranes (called cristae). The actual synthesis of ATP from the coupling of ADP (adenosine diphosphate) with phosphate is very complicated and involves a mechanism called chemiosmosis. The electron flow generates a higher concentration (charge) of positively-charged hydrogen (H+) ions (or protons) on one side of the membrane. When one side of the membrane is sufficiently "charged," these protons recross the membrane through special channels (pores) containing the enzyme ATP synthetase, as molecules of ATP are produced. The detailed, step-by-step breakdown of glucose during cellular respiration is called the Krebs Cycle or Citric Acid Cycle. 4. Simplified Illustration Of A Chloroplast
Light Reactions Of Photosynthesis
CAM Photosynthesis Some plants adapted to hot, arid regions have a different photosynthetic mechanism called CAM photosynthesis. CAM (Crassulacean Acid Metabolism) photosynthesis is found in cacti and succulents, including the crassula family (Crassulaceae). During the hot daylight hours their stomata are tightly closed; however they still carry on vital photosynthesis as carbon dioxide gas is converted into simple sugars. During the cooler hours of darkness their stomata are open and CO2 enters the leaf cells where it combines with PEP (phosphoenolpyruvate) to form 4-carbon organic acids (malic and isocitric acids). The 4-carbon acids are stored in the vacuoles of photosynthetic cells in the leaf. During the daylight hours the 4-carbon acids break down releasing CO2 for the dark reactions (Calvin cycle) of photosynthesis inside the stroma of chloroplasts. The CO2 is converted into glucose through a series of complicated reactions involving ATP (adenosine triphosphate) and NADPH2 (nicotinamide adenine dinucleotide phosphate), the latter two compounds which were synthesized during the light reactions of daylight in the grana of chloroplasts. The adaptive advantage of CAM photosynthesis is that plants in arid regions can keep their stomata closed during the daytime, thereby reducing water loss from the leaves through transpiration; however, they can still carry on photosynthesis with a reserve supply of CO2 that was trapped during the hours of darkness when the stomata were open. The tropical strangler Clusia rosea also has CAM photosynthesis. This unusual tree starts out as an epiphyte on other trees and then completely envelops and shades out its host. In fact, it greatly resembles strangler figs (Ficus) of tropical regions of the world.
C- 4 Photosynthesis Another interesting modification of the photosynthetic pathway is called C-4 Photosynthesis. During C-4 photosynthesis, CO2 combines with phosphoenolpyruvate (PEP) to form a 4-carbon organic acid (oxaloacetic acid) which migrates (diffuses) to the photosynthetic bundle sheath cells surrounding the vascular bundles (veins) of the leaf. PEP essentially shuttles the CO2 to the bundle sheath cells where it is released for the dark reactions (Calvin cycle) of photosynthesis. During hot weather the CO2 level inside leaves is greatly reduced because the leaf stomata are closed. In ordinary C-3 plants which form a 3-carbon compound (PGA) during the initial steps of the dark reactions, photosynthesis in the leaf shuts down without a sufficient supply of CO2. C-4 plants have a competitive advantage during hot summer days because they are able to carry on photosynthesis in the bundle sheaths where CO2 levels are concentrated. Weedy C-4 plants such as Bermuda grass, spurges and purslane grow rapidly during hot summer days, while photosynthesis and growth in C-3 plants shuts down.
ATP Production In Bacteria Similar electron transport systems occur in the membranes of prokaryotic bacteria. Methanogenic bacteria live in marshes, swamps and your gastrointestinal tract. In fact, they are responsible for some intestinal gas, particularly the combustible component of flatulence. They produce methane gas anaerobically (without oxygen) by removing the electrons from hydrogen gas. The electrons and H+ ions from hydrogen gas are used to reduce carbon dioxide to methane. In the reaction, the H+ ions combine with the oxygen from carbon dioxide to form water. During this process, the electrons are shuttled through an anaerobic electron transport system within the bacterial membrane which results in the phosphorylation of ADP (adenosine diphosphate) to form ATP (adenosine triphosphate). This process is much less efficient than aerobic respiration, so only two molecules of ATP (rather than 38) are formed. Desert varnish bacteria make their ATP in a similar fashion, only the electrons are coming from the aerobic oxidation of iron and manganese. A thin coating of iron or manganese oxide is deposited on the surfaces of desert boulders and rocky slopes. During the oxidation process, the electrons are shuttled through an iron-containing cytochrome enzyme system on the inner bacterial membrane. One has only to gaze at the spectacular panoramas of varnish-coated, granitic boulders throughout desert areas of the American southwest to appreciate the magnitude of this bacterial ATP production. The mechanism of ATP synthesis in prokaryotic bacteria is remarkably similar to eukaryotic cells. In addition, the circular DNA molecules of these bacteria are similar to the DNA molecules within some organelles of eukaryotic cells. In fact, some biologists believe that mitochondria and chloroplasts within eukaryotic animal and plant cells may have originated from ancient symbiotic bacteria that were once captured by other cells in the distant geologic past. This fascinating idea is called the "Endosymbiont Theory" (or "Endosymbiont Hypothesis" for those who are more skeptical). 5. Guard Cells & Stomata On Leaves & Stems
Plants carry on gas exchange through minute pores called stomata. Carbon dioxide from the atmosphere enters the stomata and oxygen produced by photosynthesis diffuses out of the stomata. Water molecules also escape through the stomata, especially in hot, dry weather. Water loss through the stomata is known as transpiration. If the plant loses too much water it will wilt and eventually die. To cope with this dilemma, plants have evolved paired guard cells on each side of the stoma. When the guard cells are fully turgid or expanded, they have an elongate opening (stoma) between them. The walls adjacent to the stoma are very thin and flexible, while the outer walls on the opposite sides of the stoma are much thicker and more rigid. This differential thickening causes an opening to develop when the guard cells are inflated by internal water pressure (called turgor pressure). When the guard cells lose water on a hot day, they become deflated and push together, thus closing off the stoma. This cleaver strategy prevents the plants from losing excessive water through transpiration. See the following highly magnified view of the paired guard cells:
All text material & images on these pages copyright © W.P. Armstrong Page 2
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.
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. 4. Cladogram 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:
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)
Limnobium: Floating Aquatic Superficially Resembling Spirodela
5. Controversies Over The Genus Landoltia Many traditional phylogenetic groupings of species within families and genera are not monophyletic and are inconsistent with modern cladistical analyses based on DNA. In other words, the groupings are paraphyletic or polyphyletic, and do not show all species within a group descending from a common ancestor. Monophyly is the natural evolutionary pattern in which all species are descended from a common ancestor. In order to have consistent computer-generated, monophyletic cladograms, it is sometimes necessary to change paraphyletic and polyphyletic groupings by moving species into different genera, and by moving genera into different families. Many of the taxonomic revisions in the Jepson Manual 2nd Edition (2012) were made in order to have consistent monophyletic groupings. This is why Spirodela punctata was placed in the genus Landoltia and why the Lemnaceae was placed in the familiy Araceae.
In 1999, D.H. Les and D.J. Crawford 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. Without this change, the genus Spirodela would be paraphyletc.
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). The Angiosperm Phylogeny Group (APG) has proposed some significant changes in the classification of many traditional angiosperm families, including the placement of all duckweeds in the Araceae rather than the Lemnaceae. Nomenclatural changes are cited under the APG II system (2003) and superceeded by APG III system (2009). These changes are based on computer-generated evolutionary trees or cladograms. Thousands of data characters have been used, including morphology, anatomy, flavonoids, allozymes, and DNA sequences from chloroplast genes and introns. The Jepson Manual Second Edition (2012) essentially follows the changes summarized in the following reference by W.T. Judd, et al. 2008. Since the genus Landoltia was proposed by D.H. Les and D.J. Cawford in 1999, several classic papers on the phylogeny of the duckweed subfamily (Lemnoideae) and other aroids (Araceae) have used the name Landoltia. In my opinion, the name Landoltia is warranted because it is consistent with the objectives of the Jepson Manual 2nd Edition (2012) based on phylogenetic studies using plastid DNA.
Published Names For This Species Lemna punctata G.F.W. Meyer This was Meyer's original name based on the type specimen collected along the Essequibo River, Guyana, South America in 1818. Unfortunately, Meyer's original type specimen was lost. Spirodela punctata (G.F.W. Meyer) Thompson C.H. Thompson placed this species in the genus Spirodela in 1898. Since the type specimen was lost, he based the new name on a specimen from the 1938-1842 Wilkes Expedition, labeled Orange Harbor, Tierra del Fuego. According to Landolt (1986), Thompson neotypified this species in his 1898 publication. Landoltia punctata (G.F.W. Meyer) Les & D.J. Crawford In 1999, D.H. Les and D.J. Crawford placed this species in the genus Landoltia based on DNA evidence. Re-Neotypification Of G.F.W. Meyer's 1818 Type Specimen Of "Lemna punctata" Note: This is a complicated taxonomic subject involving many articles from the International Code of Nomenclature For Algae, Fungi, and Plants (Melbourne Code) 2011: Available on-line at: http://www.iapt-taxon.org/nomen/main.php. An argument for replacing the names Landoltia punctata and Spirodela punctata with the previous name Spirodela oligorrhiza has been made by Daniel B. Ward (2011). In order to make sure we are referring to the same species, Ward has suggested calling this "Lesser Greater Duckweed" to avoid confusing it with the larger species of Spirodela (S. polyrrhiza & S. intermedia) called "Greater Duckweeds." In this article I will simply call it LG Duckweed instead of Lesser Greater Duckweed. Ward's proposal involves the re-neotypification of G.F.W.Meyer's 1818 type specimen named Lemna punctata which was apparently lost. Ward also proposed as the new type a different species that we know today as Spirodela intermedia.
In July 2012, I received an e-mail message from Dr. Thomas Rosatti, editor of the revised Jepson Manual (2nd Edition), asking my opinion on Ward's retypification. Since C.H. Thompson already neotypified this species as Spirodela punctata in 1898, Ward's retypification should really be a "re-neotypification." Since I wrote the section on duckweeds (subfamily Lemnoideae), adopting Ward's re-neotypification would result in changes to several related species. In July 2012, I stated my opposition to Ward's proposal on my on-line Lemnoideae page on Wayne's Word. I also included a two-paragraph e-mail message from Dr. Elias Landolt, Zurich stating his opposition to the proposed re-neotypification (see below). This quotation can be verified on the Internet Archive Wayback Machine dated 8 September 2012. Spirodela punctata (Meyer) Thompson was named by C.H. Thompson in 1898 based on a collection from the 1938-1842 Wilkes Expedition, labeled Orange Harbor, Tierra del Fuego. Whether this collection actually came from the tip of South America is debatable. The parenthetical author G.F.W. Meyer described this species earlier as Lemna punctata from a type specimen collected in Guyana, South America in 1818. Unfortunately, Meyer's original type specimen was lost. According to Ward (2011), LG Duckweed does not occur in the areas where these collections were made: The Tierra del Fuego collection was mislabeled and the Guyana collection was not LG Duckweed. Futhermore, he states that the only native Spirodela in South America is S. intermedia. Since Meyer's type specimen was lost, Ward re-neotypified the species as Lemna punctata G.F.W. Meyer and he designated S. intermedia as the type. Thompson's binomial is still Spirodela punctata (Meyer) Thompson; however, this no longer refers to LG Duckweed. It is now the correct binomial for the South American Spirodela intermedia. The correct name for LG Duckweed now becomes Spirodela oligorrhiza (Kurz) Hegelmaier, a name published by Hegelmeier in 1868. Hegelmeier apparently never saw the South American specimens discussed above, so his name is probably based on the true LG Duckweed. Ward's 2011 neotypification will make Landoltia a synonym of Spirodela and no longer available for the intended LG Duckweed. The restoration of separate generic status for LG Duckweed now known as Spirodela oligorrhiza (Kurz) Hegelm. will require the creation of a new genus name. The binomial Spirodela punctata (Meyer) Thompson will now refer to the South American species known as Spirodela intermedia W. Koch. By neotypification the name Landoltia becomes a synonym of Spirodela intermedia. Quoted E-Mail Message From Dr. Elias Landolt According to E. Landolt (Personal Communication, 2012), the name change proposed by Ward is untenable. This quotation can be verified on the Internet Archive Wayback Machine dated 8 September 2012.
"I can understand that Thompson choose a new type for Lemna punctata. The correctness of his decision is not disputed. I checked the neotype the collection of Wilkes from copies in four different Herbara. It is clearly the species which is now called "punctata". It is not important if the material was collected in Orange Harbor or somewhere else. Because it is not possible and will probably never be possible to decide the identity of Lemna punctata with certainity it is not advisable to change the correctly published neotype of Thompson. If we change the type of L. punctata again we will have a terrible chaos in nomenclature. Therefore I am not following the proposal of Ward."
My objection to Ward's proposed neotypification is based on two primary points. (1) He is re-neotypifying Meyer's lost type specimen with the name Lemna punctata; however, he is using Spirodela intermedia as the type. It is impossible to know with 100% certainty which species Meyer was describing under the name Lemna punctata back in 1818. It could have been the "LG Duckweed" that we know as Landoltia punctata (Spirodela punctata = Spirodela oligorrhiza), or it could have been another species of Spirodela such as S. intermedia. Why complicate this taxonomy based on speculation. (2) Cladistical analysis has clearly shown that Spirodela punctata belongs in a separate genus (Landoltia), otherwise the grouping of Spirodela with 3 species is paraphyletic. The trend in modern floras such as the Jepson Manual Second Edition (2012) is for consistent monophyletic groupings. A Review Of Ward's Proposed Re-Neotypification Ward's re-neotypification of Lemna punctata has been reviewed by J.H. Wiersema of the USDA Agricultural Research Service, National Germplasm Resources Laboratory, Beltsville, Maryland.:
6. An Updated Key To The Duckweed Family
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. 7. Identification Of Morphologically Similar Species Lemna minuta vs. L. valdiviana 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.
Veins (Nerves) and Air Spaces
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:
Using Dorsal Row Of Papules To Separate Lemna turionifera From L. minor Another difficult group of duckweeds is Lemna turionifera and L. minor. L. turionifera has three main veins and is superficially similar to L. minor and nongibbous L. gibba. It differs from L. minor and L. gibba in having a row of 3-7 minute papules along the midline of the dorsal surface. It also differs from L. minor by developing reddish anthocyanin on its underside, starting in the region around the root. What really sets this species apart from other duckweeds is the presence of rootless, overwintering turions in the fall months. These are referred to as "winter buds" in the Jepson Manual of California Plants (1996). L. turionifera appears to be more common than L. minor in San Diego County. It generally replaces L. gibba in the higher elevations. Unfortunately, reddish anthocyanin and turions are not always present, so you must rely on the row of papules along the midline of dorsal surface. This can be difficult to see, especially on dried herbarium specimens. Ideally, herbarium specimens should include field notes on the presence of a dorsal row of papules and reddish anthocyanin on the ventral surface. With some practice, these traits can be observed with a hand lens.
8. 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. With a good 10x hand lens this can be accomplished by holding the plant body up against the bright sky. 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. 9. 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. 10. Axenic Culture Of Duckweeds In Nutrient Agar
11. 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:
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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Plant bodies minute and rootless, with granular or mealy texture when rubbed between fingers of hands; generally globoid to ovoid-ellipsoid or cylindrical (flat-topped in some species); 0.4-1.3 mm long and 0.2-1.0 mm wide, floating on or partially below water surface; veins 0; pale transparent green throughout or with dark green dorsal surface; some species punctate with brown pigment cells in epidermis (visible on dead plants of W. borealis & W. brasiliensis); solitary or with smaller daughter plant attached at basal end; single, funnel-shaped budding pouch at basal end; daughter plants produced in basal budding pouch (in most species, some daughter plants may sink to bottom and function as overwintering turions); parenchyma without druse or raphide crystals of calcium oxalate; one bisexual flower produced inside dorsal floral cavity, consisting of a single pistil and single stamen (some authorities consider this to be an inflorescence with 2 unisexual flowers); pistil situated nearest the basal budding pouch; anther unilocular and apically dehiscent along pigmented line; ovary unilocular with one orthotropous ovule; utricle globose and slightly compressed, bearing 1 globose-ovoid, smooth seed with distinct conical operculum (seed may be slightly reticulate but not longitudinally ribbed); size and shape of plant body important for species identification (ideally under 10-20X magnification); at least 9 spp. worldwide, especially warm temperate and tropical regions; J.F. Wolff, German botanist and physician, 1778-1806; Armstrong, W.P. & R.F. Thorne (1984), Madrono 31: 172-179; Armstrong, W.P. (1989), Madrono 36: 283-285; Armstrong, W.P. (1985), Fremontia 13: 11-14.
All text material & images on these pages copyright © W.P. Armstrong Page 4Images of Lemnaceae in Western North AmericaLemna - Spirodela - Landoltia - Wolffia - Wolffiella - General
All text material & images on these pages copyright © W.P. Armstrong |