Leaves entire. Flower heads may contain from 1 to 20 flowers. Corolla remains closed at flowering, hence the name "bottle" or "closed gentian" (clausa standing for "closed"). Bumblebees, which pollinate bottle gentian, have to force-open each flower. Beneath corolla, there are small, spreading calyx lobes. They are green, nearly orbiculate, obovate or elliptic, from less than 1 to 2 cm long.