General description:- Herbs (non-European genera include trees and shrubs).
Leaves:- Simple, entire; stipules minute or absent.
Flowers:- Hermaphrodite, regular, perigynous, 4- to 6-merous, solitary or in small cymes or clusters in the leaf-axils, rarely in terminal spikes. Hypanthium shallow cup-shaped (pelviform) to cylindrical. Epicalyx often present. Petals free, pink or purple, inserted on lip of hypanthium; sometimes 0. Stamens 2-12, inserted on tube of hypanthium. Ovary superior, 2- or 4-locular; style single; stigma pin-headed (capitate).
Fruit:- A capsule; seeds numerous.
The calyx is here refered to as the hypanthium, and the sepals and epicalyx-segments are referred to respectively as inner and outer calyx-teeth.
LYTHRUM
General description:- Annual or perennial.
Leaves:- Alternate, opposite or whorled.
Flowers:- Usually 6-merous (sometimes 4- or 5-merous), usually solitary in the leaf-axils, sometimes in pairs or in small, whorl-like cymes, which may form terminal spikes; each flower shortly pedicellate, with 2 bracteoles. Hypanthium usually tubular, sometimes short and wide. Epicalyx present. Petals usually present. Stamens 2-12.
Fruit:- Capsule usually septicidally dehiscent with 2 valves, rarely dehiscing irregularly or by 4 apical teeth.
Key features:-
1) Flowers, at least in lower part of plant, in cymes or clusters of 3 or more in each leaf-axil.
2) Epicalyx present.
3) Petals 6-12 mm, conspicuous.
4) Leaves alternate.