This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Thallus small, globose, ovoid-oblong, subcylindric or irregular, rootless, nerveless and leafless. The vegetative growth is from a cleft near one end of the plant, the branch being mostly sessile and soon detached. The ovary contains one orthotropous ovule. Fruit spherical or short-ovate, smooth. Endosperm in a single layer. [Name in honor of Nath. Matth. v. Wolff, 1724-1784, Polish physician and naturalist.]
A genus of few species, mostly in tropical and subtropical regions. Type species: Lemna hyalina Delile.
Thallus globose to ellipsoid, not punctate.
Thallus flattened above, at least on the margin, brown-punctate.
Apex of thallus acute.
A large conic papule on the upper surface.
Wolffia columbiana Karst. Bot. Unters. 1: 103. 1865-67.
Floating as minute alga-like grains just beneath the surface of the water in stagnant ponds, pools and shallow lakes, Ontario to Connecticut and New Jersey, west to Minnesota and Missouri, south to Louisiana. Also in Mexico and South America. June-July.
W. punctata Griseb. Fl. Br. W. I. 512. 1864.
Thallus oblong, smaller than the last, 1/4"-1/3" long, flattish, densely cellular, with numerous stomata and dark green above, gibbous, more loosely cellular, with fewer stomata and paler beneath; brown-dotted throughout with minute pigment cells.
Floating on the surface of stagnant waters, Ontario to Michigan, south to Tennessee. Jamaica. Has been confused with W. brasiliana Wedd. June-July.
Wolffia papulifera Thompson, Ann. Rep. Mo. Bot. Gard. 9: 20. pl. 4D. 1897.
Thallus slightly unsymmetrical, obliquely broadly ovate, about 1/2 broad, the apex rounded, the upper surface flat at the margin, gradually ascending into a prominent conic papule on the median line, brown-punctate, the under surface strongly gibbous, less punctate; stomata numerous on the upper surface; flower and fruit unknown.
Floating, with the entire upper surface exposed to the air, Kennett and Columbia, Mo.