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..
Lemna minima Philippi, Linnaea 33: 239. 1844.
Thallus oblong to elliptic, 1"-2" long, obscurely i-nerved, or nerveless, with a row of papules along the nerve, the lower surface flat, or slightly convex, the apex rounded; rootcap usually short, a little curved, rarely perfectly straight, cylindric, blunt; spathe open; pistil short-clavate; stigma concave; ovule solitary, obliquely orthotropous; seed oblong, pointed, about 16-ribbed, with many transverse striations.
Georgia and Florida to Kansas, Wyoming and California.
Lemna perpusilla Torr. Fl. N. Y. 2: 245. 1843.
L.-paucicostata Hegelm. in A. Gray, Man. Ed. 5, 681. 1868.
L. perpusilla trinervis Austin, in A. Gray, Man. Ed. 5, 479. 1867.
Thallus small, 1"-1 1/2" long, oblong to obovate, often purplish-tinged beneath, unsymmetrical and abruptly narrowed to a very short stalk, provided throughout with numerous stomata, more or less 3-nerved; rootcap pointed; fruit ovoid, unsymmetrical; seed 12-16-ribbed, and transversely striated.
In ponds, rivers, springs and lakes, New York and New Jersey to Minnesota, Nebraska and Missouri. June-July.
Lemna minor L. Sp. PI. 970. 1753.
Thallus obovate or subcircular, 1"-3" long, thickish, rarely reddish or purplish-tinged, short-stalked when young, provided throughout with stomata, obscurely 3-nerved, very rarely 4-5-nerved. Rootcap obtuse or subtruncatc; fruit symmetrical, sub-turbinate, wingless; seed with a prominent protruding hilum, deeply and unequally 12-15-ribbed.
In ponds, lakes and stagnant waters, throughout continental North America except the extreme north, and in the Bahamas. Also in Europe, Asia, Africa and Australia. Duck's-meat. Toad-spit. Water lentils. Mardling. Summer.
Lemna gibba L. Sp. PI. 970. 1753.
Thallus slightly unsymmetrical, obovate or short-obovate, 1 1/2-3" long, thickish or more or less strongly gibbous beneath, short-stalked when young, soon separating, provided with stomata which are sparse beneath, obscurely 3-5-nerved; rootcap mostly short-pointed, rarely long-pointed or obtuse; fruit symmetrical, winged; seed thick, deeply and unequally ribbed.
In ponds and rivers, Nebraska, Texas, Arizona and California. Also in Mexico, Europe, Asia, Africa and Australia. June-July.