Identification: Hydrocharis morsus-ranae is a free-floating aquatic plant with leathery, heart-shaped leaves and small white flowers with three petals. The root system is well developed, but does not normally anchor the plant to the substrate. The species spreads by sending out runners, and can form dense, tangled mats (Environment Canada 2003).
The growth form is stoloniferous, the stolons having seasonally dimorphic terminal buds with one root. The leaves are petiolate and floating or, in dense vegetation, emergent. The leaf blade is ovate to orbicular in shape, typically measuring 1.2-6 x 1.3-6.3 cm, with a chordate to reniform base, and entire margin. Veination is palmate with cross-veins. The primary veins form a 75--90° angle with the midvein and are broadly curving. Aerenchyma are confined to the midvein region (not margin to margin as in Limnobium). Individual aerenchyma space, located approximately 1 mm from either side of midvein, typically measures 0.1-0.5 mm across its longest axis (eFloras 2008; Gleason and Cronquist 1991).
The species is dieocious (male and female flowers found on separate plants). Both sexes of flowers have an outer whorl of three greenish-red sepals, and a whorl of three membranous white petals. Staminate (male) flowers occur in cymose inflorescences of 2 to 5 flowers on pedicels up to 4 cm long. The inflorescence is enclosed by a spathe of one or two scale leaves which subtend the first two flowers. The stamens number 9 to 12 and are arranged in 3 trimerous whorls, with an innermost whorl of staminodes. The first and third whorls of stamens are partially united along their filaments, and the second whorl of stamens is fused to the staminodes. The anthers are basifixed and consist of four micro-sporangia, with pollen grains dehiscing through lateral slits. In the center, there is a large pistil-like structure (Scribailo and Polsluzny 1985).
Pistillate (female) flowers are solitary and enveloped in a tubular hypanthium, with pedicels up to 9 cm long. The ovary is inferior, with six dorsiventral styles. Each style is bifurcated at the end into two flat, papillose stigmas. There is a whorl of nectaries that occur as appendages on the three antipetalous styles, and a whorl of filament-like staminodes. he fruit is a berry that dehisces longitudinally, releasing seeds 1 to 1.3 mm in length (Scribailo and Polsluzny 1985).
References: (click for full references)
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This information is preliminary or provisional and is subject to revision. It is being provided to meet the need for timely best science. The information has not received final approval by the U.S. Geological Survey (USGS) and is provided on the condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from the authorized or unauthorized use of the information.