Duckweeds are readily recognized as a genus, but the distinctions between species are difficult. Landolt has found only the species keyed below in annotating our MICH specimens. Several others approach Michigan from the south, however, so additional species may be found with assiduous collecting. See Landolt (1986) for detailed information. When keying, a generous sample is helpful, keeping in mind that mixed populations are normal. Particular care should be taken when collecting Lemna to note color of the top and bottom of the plant when fresh, and the distribution of papules. As with Wolffia, living plants are much easier to determine.
1. Plant bodies ca. 6–15 mm long (rarely as short as 3 mm), including a conspicuous narrow stalk by which fully grown lateral plant bodies are attached to parents, oblong to broadly lanceolate in outline, often floating below surface of water in large masses.
1. Plant bodies less than 6 mm long, without stalks (or these extremely short), rounded, obovate, or narrowly oblong in outline, usually floating on surface of water.
2. Plant bodies less than 1 mm wide, narrowly oblong with ± parallel sides and rounded ends.
2. Plant bodies averaging more than 1 mm wide, rounded on all sides.
3. Plant bodies with a row of uniform sized tiny raised bumps visible in fresh material at 30× on the median line of the upper surface; faint (to prominent) purplish tinting usually present on the underside.
3. Plant bodies lacking raised bumps (or apparently if rarely ±present, the one at the tip larger than the rest); purplish tinting of the underside absent or very faint.
All species found in Lemna
MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. December 11, 2017. http://www.michiganflora.net/genus.aspx?id=Lemna.