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..
Lythrum lineare L. Sp. Pl. 447. 1753.
Perennial slender, glabrous, rather pale green, much branched, 2°-4° high. Leaves narrowly linear, sessile, 6"-12" long, 1"-2" wide, nearly all opposite, acutish at the apex or the lowest obtuse, narrowed at the base; flowers solitary in the upper axils, light purple or nearly white, dimorphous, about 1 1/2" broad; stamens of the short-styled flowers exserted; no hypogynous ring; ovary short-stalked; fruiting calyx 1 1/2"-2" long.
Lythrum Vulnerāria Ait., of Mexico, admitted into our first edition as recorded as found at St. Louis, Mo., is not definitely known within our area.
Lythrum Salicaria L. Sp. Pl. 446. 1753.
Perennial, erect, 2°-3° high, glabrous or pubescent, sometimes tomentose above, at length much branched. Leaves opposite or sometimes verticillate in 3's, sessile, lanceolate, cordate or clasping at the base, 2'-3' long, 3"-5" wide; flowers purple, trimorphous, 6"-8" broad, in dense compound terminal interrupted bracted spikes; stamens 8-10, alternately longer and shorter, even the longer ones little exserted; ovary short-stalked; fruiting calyx about 3" long; calyx-lobes shorter than the narrow appendages.
In swamps and wet meadows, Cape Breton Island to Ontario, south to southern New York, Delaware and the District of Columbia. Naturalized from Europe. June-Aug Widely distributed in the Old World. Spiked willow-herb, long purples, soldiers. Purple-grass. Willow- or kill-weed. Sage-willow. Milk or purple willow-herb. Red sally. Rainbow-weed.
Lythrum virgātum L., differing by leaves narrowed at the base, is recorded as established in eastern Massachusetts.