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, often viscid; branches terete. Leaves opposite or verticillate, ovate or lanceolate, entire. Peduncles from between the petioles, 1- or more flowered. Flowers scarlet, purple, or white. Calyx-tube elongated, ribbed, coloured, produced below in a spur or protuberance with 6 primary teeth, and often 6 secondary smaller ones. Petals 6, small, the 2 upper usually larger, rarely none. Stamens 11, the upper one being deficient; filaments alternately long and short, inserted upon the throat of the calyx-tube. Capsule free, 2-celled, included in the calyx. There are nearly one hundred species, natives of tropical and sub-tropical America. Name from curved, referring to the form of the calyx and seed-vessel in some species.
1. C. ignea, syn. C. platy centra of gardens. This is the species commonly grown, having nearly glabrous lanceolate leaves and apetalous bright scarlet flowers with a black and white little-expanded limb. It is a perennial species from Mexico.
C. viscosissima is a viscid annual with dull purple petals and a green calyx-tube; C. silenoides with the general appearance of a Silene has purple unequal petals and calyx; C. strigillosa is a perennial with hairy cordate leaves, orange calyx-tube, and 2 small purple petals; C. Jorullensis, syn. C. eminens, is a very handsome species with glabrous linear-lanceolate leaves and large apetalous orange and red flowers.