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..
Glabrous low perennial herbs, with simple slender erect stems, and lanceolate ovate or oblong leaves mostly clustered in a verticil at the summit. Flowers few or solitary, terminal, slender-peduncled, small, white or pink, deeply 5-9- (mostly 7-) parted. Sepals narrow, persistent, spreading. Corolla rotate, its tube almost none, its segments convolute in the bud, acute or acuminate, entire; filaments united into a narrow ring at the base; anthers linear-oblong, recurved after anthesis. Staminodia none. Ovary globose; ovules numerous; style filiform. Capsule globose, 5-valved, many-seeded. Seeds trigonous or spherical. [Latin, one-third of a foot, referring to the height of the plant.]
Four species, of the northern hemisphere. Two others occur in northwestern America, the typical T. europeaea L. in Europe and Asia.
Rootstock horizontal or creeping, sending up simple stem-like branches 3'-9' high, which are naked or scaly below, the leaves all in a verticil of 5-10 at the summit, long stolons sometimes developed in their axils. Leaves membranous, lanceolate or oblong-lanceolate, acuminate at both ends, sessile or short-petioled, minutely crenulate, 1 1/2'-4' long, 4"-15" wide; pedicels filiform, erect, 1'-2' long; sepals narrowly lanceolate or subulate, cuspidate, about one-half as long as the oblong or somewhat obovate corolla-segments; flowers 4"-6" broad; capsule shorter than the sepals.