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 disc-shaped, usually provided with a central nerve and with or without two or four lateral nerves. Each thallus produces a single rootlet, which is devoid of vascular tissue and is commonly provided with a thin blunt or pointed rootcap. The ovary contains from one to six orthotropous, amphitropous or anatropous ovules. Fruit ovoid, more or less ribbed. Endosperm in one or three layers. [Greek, in allusion to the growth of these small plants in swamps.]
About 8 species, in temperate and tropical regions. Besides the following, Lemna angolensis is a native of Lower Guinea. Type species: Lemna trisulca L.
Thalli short-stipitate or sessile.
Thalli 1-nerved or nerveless.
Thalli thin, without papules; rootcap strongly curved, tapering.
Thalli thick, with a row of papules along the nerves; rootcap little curved, cylindric.
Thalli 3-nerved; rootcap cylindric.
Thalli green or purplish beneath; fruit not winged.
Thalli pale beneath, usually strongly gibbous; fruit winged.
Lemna trisulca L. Sp. PI. 970. 1753.
Thallus lanceolate, submerged and devoid of stomata in the primary aquatic form, ovate. to oblong-lanceolate, 6"-9' long, floating and provided with stomata in the later flowering stage. The later and more common form is narrowed at the base to a slender stipe, thin, denticulate, with or without rootlets, and slightly 3-nerved. Several individuals often remain connected, so as to form a chain-like series. Root-cap pointed; fruit symmetrical; seed deeply 12-15-ribbed.
In ditches, springs, ponds, shallow lakes and sloughs, Nova Scotia to British Columbia, south to New Jersey, New Mexico and California. Also in Europe, Asia, Africa and Australia. July-Aug.
Thallus oblong-elliptical, 1"- 1 1/2" long, thin, subfalcate-and shortly stalked at the base, provided with numerous stomata, except on the borders, nerveless; rootcap short-tapering, curved; spathe reniform; fruit ovoid-oblong, unsymmetrical; seed prominently 12-29-ribbed.
In pools and rivers, Massachusetts to Florida, west to Illinois, Wyoming and California. Also in Jamaica and in South America. June-July.