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..
Low herbs, erect or prostrate, with opposite mostly stipulate entire leaves, and small, perfect flowers in cymes. Sepals 4 or 5, distinct, or partly united, white or greenish, persistent. Petals none. Stamens 1-10, usually 4 or 5, borne at the base of the ovary, or rarely on the calyx-tube (hypanthium); filaments slender; anthers 2-celled, short. Ovary sessile, I-celled; styles mostly 2, more or less united, often short; ovule solitary, amphitropous. Fruit an achene or utricle, I-seeded. Endosperm nearly enclosing the embryo.
About 18 genera and 100 species of wide geographic distribution. Leaves stipulate.
Sepals awn-tipped; calyx sessile.
Sepals not awned; calyx pedicelled.
Styles long; sepals cuspidate; radicle ascending.
Styles very short or wanting; sepals mucronate; radicle descending.
Leaves not stipulate.