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..
Herbs or shrubs, with 4-angled stems, opposite alternate or rarely verticillate entire leaves, and solitary cymose-paniculate or spicate and terminal often dimorphous or trimor-phous flowers. Calyx-tube cylindric, 8-12-ribbed, straight, not gibbous at the base, with 4-6 primary teeth and an equal number of appendages in the sinuses. Petals 4-6, usually obovate, rarely wanting. Stamens 4-12, inserted on the calyx-tube, included or exserted. Ovary oblong, sessile, 2-celled; style filiform; stigma mostly capitate; ovules numerous. Capsules enclosed by the calyx, membranous, 2-celled, 2-valved, or bursting irregularly. Seeds flat or angular. [Greek, gore, from the purple color of the flowers.]
About 30 species, of wide geographic distribution. Besides the following, about 5 others occur in the southern and western United States. Type species: Lythrum Salicaria L. Flowers axillary, solitary; stamens not more numerous than petals.
Leaves mostly alternate.
Leaves obtuse; stamens all included; annual.
Leaves acute; stamens of short-styled flowers exserted; perennial.
Leaves mostly opposite, narrowly linear, narrowed at the base.
Flowers in soicate nanicles terminal: stamens twice as many as petals.
Lythrum Hyssopifolia L. Sp. Pl. 447. 1753.
Annual, glabrous, pale green, erect or assurgent, at length widely branched, 6-24' high. Leaves sessile, mainly alternate, the lowest sometimes opposite, oblong or linear-oblong, obtuse at the apex, rounded at the base, 4"-10" long, 1"-4" wide, those of the main stem larger than those of the branches; flowers solitary and sessile in the axils, not dimorphous, pink-purple, about i" broad; petals nearly erect; stamens all included; calyx 2 1/2"-3" long in fruit.
Lythrum alatum Pursh, Fl. Am. Sept. 334. 1814.
Perennial, erect, glabrous, usually much branched, dark green, 1°-4° high, the stem angled and often slightly winged. Leaves sessile, alternate or the lowest opposite, lanceolate or oblong, acute or acutish at the apex, rounded or cordate at the base, 6"-15" long, 2"-5" wide; flowers solitary in the upper axils, short-pedicelled, deep purple, 3"-5" broad, dimorphous; petals erect-spreading; stamens of the short-styled flowers exserted; hypogynous ring fleshy; ovary nearly sessile; calyx 2 1/2"-3" long in fruit.
In low grounds, southern Ontario to Massachusetts, Kentucky, South Dakota, Utah, Kansas and Arkansas. The closely related L. lanceolatum E11. of the southern states has the leaves narrowed or cuneate at the base. Milk willow-herb. June-Aug.