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..
Glabrous erect unbranched herbs, with short scarred rootstocks and 3 leaves whorled at the summit of the stem, subtending the sessile or peduncled solitary bractless flower. Solitary long-petioled leaves are sometimes borne on the rootstock. Perianth of 2 distinct series of segments, the outer 3 (sepals) green, persistent, the inner 3 (petals) white, pink, purple or sometimes greenish, deciduous or withering. Stamens 6, hypogynous; filaments short; anthers linear, mostly introrse. Ovary sessile, 3-6-angled or lobed, 3-celled; ovules several or numerous in each cavity; styles 3, stigmatic along the inner side. Berry many-seeded. Seeds horizontal. [Latin, in allusion to the 3-parted flowers and the 3 leaves.]
About 24 species of North America and Asia. Besides the following, some 7 others occur in southern and western North America. Known as Three-leaved Nightshade and Birthroot. Phyllody, i. e., the reversion of petals or sepals to leaves, is occasional, and the floral parts are sometimes in 4's. Type species: Trillium cernuum L.
Leaves sessile; sepals not reflexed.
Flowers purple; petals lanceolate.
Flowers green; petals linear.
Leaves petioled; sepals reflexed.
Leaves oval or ovate, obtuse or obtusish, 1'-2' long.
Leaves broadly ovate or rhombic, acuminate, 2'-7' long.
* Leaves sessile, or narrowed at the base and short-petioled.
Petals obovate or oblanceolate, 1 1/2'-2 1/2' long.
Petals ovate or lanceolate, 1/2'-1 1/2' long.
Peduncle 1 1/4'- 4' long, erect or declined; petals spreading.
Petals brown-purple, rarely white; filaments two-thirds as long as the anthers,
Petals white; filaments not more than half as long as anthers.
Peduncle 1 1/4' long or less, recurved; petals recurved.
** Leaves distinctly petioled, obtuse or rounded at the base.