This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
Herbs with linear or lanceolate flat usually sheathing leaves and solitary spicate or umbellate flowers. Perianth-segments 6, the outer 3 herbaceous, inner petaloid. Stamens 6 or fewer. Fruit a 2- or 3-celled few-seeded dehiscent capsule. There are about 20 genera and 250 species, with the exception of those enumerated below, nearly all tropical.