This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol2", 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..
Leaves in whorls of 3's-5's, or sometimes scattered, the floral ones linear, serrate or pectinate, 21/2"-6" long, gradually passing into the submerged ones which are crowded, pinnately dissected, the divisions few and capillary; spikes 4's' long; petals purplish; stamens 4, very rarely 6; carpels strongly 2-keeled and scabrous on the back, separated by deep grooves, their sides flat; mature fruit about 1" long and somewhat more than 1/2" thick.
In ponds, Massachusetts to Florida, Iowa, Missouri, Nebraska, Louisiana and Texas.
M. Farwellii Morong, Bull. Torr. Club 18: 146. 1891.
Submerged; leaves in whorls of 3's-6's, or scattered, narrow, pinnately parted, the divisions finely capillary in 5-7 opposite or subopposite pairs, with minute black spines in the axils; flowers solitary in the axils of the leaves; petals 4, purplish, oblong; pistillate flowers only seen; styles 4, short; stamens 4, minute, abortive; fruit about 1" long and 1/2" thick; carpels crossed longitudinally by 3 or 4 rough tuberculate or slightly toothed ridges, 2 on the back and commonly 1 on each margin; groove between the carpels shallow.
In still water, Quebec to Michigan, Maine and New York.
Myriophyllum proserpinacoides Gill., the Chilian water-milfoil, or water-feather, with flowers on submersed stems, introduced into aquatic tanks in a nursery at Bordentown, New Jersey, and some years ago observed in a pond at Haddonfield, New Jersey, does not appear to have become established.