General description:- Glabrous bitter-tasting herbs.
Leaves:- Opposite (very rarely some alternate), entire, exstipulate, usually sessile and often connate at the base.
Flowers:- 4- to 5(to 12)-merous, actinomorphic. Calyx more or less deeply lobed. Corolla-lobes contorted in bud. Stamens inserted on the corolla-tube. Ovary superior, 1-locular, with 2 parietal placentae, rarely almost 2-locular owing to the intrusive placentae; ovules numerous.
Fruit:- A septicidal capsule.
BLACKSTONIA
General description:- Annual herbs.
Flowers:- Calyx deeply divided into 6-12 lobes. Corolla rotate, yellow; tube short; lobes 6-12. Anthers basifixed, oblong to linear, not or scarcely twisting spirally after dehiscence. Style thread-like (filiform), falling off early (caducous); stigma 2-lobed.
Key features:-
1) Flowers 6- to 12-merous.
CENTAURIUM
Flowers:- Cymes with lateral branches on both sides of the main axis (dichasial), with the lower branches longer than the upper so that all end up at the same level (corymbiform), pin-headed (capitate) or spike-shaped (spiciform). Calyx deeply divided into (4)5, keeled, linear, acute lobes. Corolla pinkish-purple, rarely white or yellow, funnel-shaped (infundibuliform). Stamens usually inserted at apex of corolla-tube; anthers linear or oblong-linear, twisting spirally after splitting open to release the seeds (dehiscence). Ovary with a short, thread-like (filiform), persistent style with 2 stigmas that fall early (caducous). Nectaries at the base of the ovary absent.
1) Corolla-lobes without spurs.
2) Style slender, distinct, often caducous.
3) Flowers usually 5-merous.
4) Calyx-lobes keeled.
5) Anthers twisting spirally after dehiscence.
The lengths of the calyx and corolla-tube refer to the flowers at anthesis.
Sect. CENTAURIUM
Flowers:- Pink, rarely white. Cyme corymbiform, capitate or dichasial. stigmas broadly ovate, wider than long.
Sect. SPICARIA
Flowers:- Pinkish-purple, rarely white, sessile or subsessile in a spike-like cyme. Stigmas subcapitate, wider than long.
Sect. XANTHAEA
Flowers:- Yellow, in a few-flowered, corymbiform cyme. stigmas oblong, longer than wide.