Home Keys To Genera List Of Species Search Lemna Spirodela Wolffia Wolffiella Wayne's Word   Recent DNA Comparisons Show That Duckweeds Belong To The Araceae   Identifying Dried Duckweeds Herbarium specimens of duckweeds should be soaked (hydrated) in a small container of water. This allows the tissues to soak up (imbibe) water so that some characteristics are discernible under a dissecting microscope.  It is preferable to examine them with background lighting or a white background in order to observe the number of veins and other diagnostic features. In the case of difficult intermediate duckweeds, such as Lemna minor and nongibbous Lemna gibba, it is preferable to grow live specimens and observe them over a period of several weeks. Tangled masses of pressed duckweeds typically appear much more distinct when they are soaked in water. In fact, they often contain more than one species. The larger duckweed may be Lemna minor. It does not have a dorsal row of papules as in L. turionifera. The smaller duckweed in the above image (right side) is Lemna minuta, formerly named L. minuscula. Very Old Pressed Duckweeds Lemna gibba (SD27797) collected by Reid Moran in the Sierra Juárez of Baja California (14 July 1979). This view shows the gibbous ventral side with large air spaces.   Explanation Of Aerenchyma Tissue   More Images Of Lemna gibba Lemna (SD79566) collected by Reid Moran and Robert F. Thorne at La Encantada in the Sierra San Pedro Mártir, Baja California (19 Aug. 1967). It was originally labeled L. minor and later determined by E. Landolt to be L. gibba. This collection apparently has intermediate forms that resemble L. minor and nongibbous L. gibba. The lack of a row of papules along the midline of dorsal side probably rules out L. turionifera, although the latter species would be expected at this elevation. The air spaces seem too small for typical L. gibba; however, the pattern and shape of air spaces resembles L. gibba. Nongibbous forms of the latter species have air spaces similar in size to L. minor. In addition, Lemna minor sometimes has a slightly more elongate plant body with a length/width ratio greater than 3/2. Plants of L. gibba often have a more orbicular body with an asymmetrical apex. The upper and lower surface may be suffused with reddish anthocyanin, a color trait that is typically not shared by L. minor. The problem is that all of these morphological characteristics, including size, shape, gibbosity and pigmentation, are influenced by environmental conditions. Lemna gibba often grows in high nutrient ponds, including stagnant water with rich, black mud. In San Diego County, I typically find this species in the coastal areas. Without air spaces that are greater than 0.3 mm in diameter or another trait such as reddish anthocyanin, I'm afraid that I am unable to separate borderline duckweeds intermediate between L. minor and nongibbous L. gibba with 100% certainty. The following quote was taken from page 362 of "Morphological Differentiation and Geographical Distribution of the Lemna gibba-Lemna minor Group" by Elias Landolt (Aquatic Botany 1: 345-363, 1975). "... we have to admit, even for specialists, it is often very difficult to determine plants belonging to the group of L. gibba-L. minor. As in other critical species groups, we are able to recognize a given species only if the plants are in a suitable developmental stage and if the range of variation is verified experimentally." This is precisely why I prefer to work with live specimens that I can grow and observe carefully over a period of several weeks.   See Live Population Of Nongibbous Lemna gibba   Lemna valdiviana (JEPS089224) collected by Brandegee in the Sierra de San Francisquito (1 Oct. 1899). I assume this is from Baja California; however, this collection is on the same page as L. valdiviana from the San Bernardino Mts. of California. The single midvein is not discernible, but the image does show the typical shape and thin, transparent plant body, including the oblique, laterally curved base.   See Midvein Of Lemna valdiviana   More Images Of Lemna valdiviana Lemna aequinoctialis (SD138445) collected by Jon Rebman in the Sierra San Francisco of Baja California (12 April 1994). This view shows a one-seeded fruit (utricle) protruding from a lateral budding pouch. Unlike other duckweeds, this species commonly produces flowers and fruits. I am reasonably certain this ID is L. aequinoctialis rather than L. minuta. Lemna aequinoctialis (SD138445) collected by Jon Rebman in the Sierra San Francisco of Baja California (12 April 1994). This view shows a one-seeded fruit (utricle) protruding from a lateral budding pouch. Unlike other duckweeds, this species commonly produces flowers and fruits. I am reasonably certain this ID is L. aequinoctialis rather than L. minuta. Lemna aequinoctialis (SD138445) collected by Jon Rebman in the Sierra San Francisco of Baja California (12 April 1994). This view shows the basal root sheath with two lateral winged appendages. This is the only Western North American species with a winged root sheath. The root sheath is not distinct in this image, but am reasonably certain this ID is L. aequinoctialis rather than L. minuta.   Better Image Of Winged Root Sheath   Lemna aequinoctialis (SD138445) collected by Jon Rebman in the Sierra San Francisco of Baja California (12 April 1994). This view shows the basal root sheath with two lateral wings and a one-seeded fruit (utricle) protruding from a lateral budding pouch. The number of longitudinal ribs on the seed (faintly visible through the transparent pericarp) indicates that this species is L. aequinoctialis and not L. perpusilla. The head of an ordinary straight pin is used for a size relationship. It is 1.5 mm in diameter. See More Images Of Lemna aequinoctialis   Straight Pin Used In Wayne's Word Articles    San Diego County Plant Atlas Project Lemna valdiviana (GRA #44). This species is characterized by very thin, transparent-green plant bodies connected in clonal clusters of 2-4 or more, a single vein that extends at least 3/4 of the distance from node (point of root attachment) to apex, generally beyond the region of air spaces. The only other species with one nerve is L. minuta; however, it is typically smaller with an obscure vein that extends to only about 1/2 the distance from node and apex. Lemna minuta appears to be much more common in San Diego County than L. valdiviana.   See Midvein Of Lemna valdiviana   More Images Of Lemna valdiviana Lemna minuta (JR #10671). This species is characterized by thin, transparent-green plant bodies typically connected in clonal clusters of 2, a single vein that extends about 1/2 or 2/3 of the distance from node (point of root attachment) to apex. The individual plant body is usually only 1-2.5 mm long, with an obscure vein that is often not discernible. Lemna minuta appears to be much more common in San Diego County than L. valdiviana. Lemna minuta (MRM #1285). This species is characterized by thin, transparent-green plant bodies typically connected in clonal clusters of 2, a single vein that extends about 1/2 or 2/3 of the distance from node (point of root attachment) to apex. The individual plant body is usually only 1-2.5 mm long, with an obscure vein that is often not discernible. Lemna minuta appears to be much more common in San Diego County than L. valdiviana.   See Midvein Of Lemna valdiviana   More Images Of Lemna minuta Lemna turionifera (BK #587). This species has 3 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. 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 (1996). Lemna turionifera appears to be more common than L. minor in San Diego County. It generally replaces L. gibba in the higher elevations. Lemna turionifera (BK #587). Overwintering turions are produced in the lateral budding pouches. They are essentially small, dark-colored, rootless, starch-filled daughter plants. They are called "winter buds" in the Jepson Manual (1996). Because starch has a specific gravity of 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.   See Turions Of Lemna turionifera   Lemna turionifera (CAM #331). View of dorsal surface showing row of minute papules along the midline. This row of papules is absent in L. minor and L. gibba; however, L. gibba may have an apical papule and slight dorsal ridge along the midline.   See Images Of Lemna turionifera   Dorsal View Of Lemna turionifera Lemna minor? (MRM #1286) growing in mud. Soaking the herbarium specimen in water actually revived the green color. In dorsal view there is no row of minute papules along the midline. This essentially rules out L. turionifera. The air spaces are generally too small for L. gibba; however, L. minor and nongibbous L. gibba are sometimes exceedingly difficult to separate morphologically. The shape and asymmetrical apex resembles L. gibba. I am not 100% certain about this ID. Lemna minor? (MRM #1286). In dorsal view there is no row of minute papules along the midline. This essentially rules out L. turionifera. The air spaces are generally too small for L. gibba; however, L. minor and nongibbous L. gibba are sometimes exceedingly difficult to separate morphologically. The shape and asymmetrical apex resembles L. gibba. I am not 100% certain about this ID.   See Images Of Lemna minor   Lemna minor (RBZ #136). There is no row of minute papules along the midline of dorsal surface. This essentially rules out L. turionifera. The air spaces are generally too small for L. gibba; however, L. minor and nongibbous L. gibba are sometimes exceedingly difficult to separate morphologically. This tangled sample also contained smaller, uninerved L. minuta.   See Images Of Lemna minor   Return To The Lemnaceae Home Page Go To Keys To The Genera Of Lemnaceae Go To The List Of Species Of Lemnaceae Go To List Of .GIF Images Of Lemnaceae Go To The WAYNE'S WORD Home Page All text material & images on these pages copyright © W.P. Armstrong