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.
Annual, biennial, or perennial herbs, usually erect and glabrous. Leaves simple, entire, opposite or whorled (except in Menyanthes, where they are alternate and trifoliolate; and alternate and floating in Limnanthemum), exstipulate, often strongly nerved. Flowers regular, bisexual, solitary or in dichotomous or trichotomous cymes. Calyx inferior, 4- to 8-lobed; lobes valvate or contorted in bud. Corolla hypogynous, often persistent, rotate, funnel-shaped or campanulate, 4- to 8-lobed; lobes mostly contorted in bud. Stamens 4 to 8, inserted upon the corolla-tube; filaments free. Capsule 1- or partially 2-celled, containing many seeds attached to 2 opposite parietal placentas. Seeds small, albuminous. This order numbers about 60 genera and 450 species, chiefly from temperate and mountainous regions. Several of our native species are very beautiful, and a few of them merit introduction into large gardens. The Bog-Bean, Menyanthes trifoliata, is a hand-some plant for marshy bogs. It has trifoliate leaves and radical scapes of white or pink fringed flowers about a foot high. Limnanthemum nymphaeoldes is a rare aquatic plant with small orbicular floating leaves and bright yellow umbellate flowers about 1 inch in diameter. The Yellow Wort, Chlora perfoliata, is a glaucous annual growing a foot or more high, remarkable on account of the leaves being joined together or connate by their bases. The bright yellow flowers are borne in trichotomous cymes. There is a fine variety in cultivation with flowers about an inch in diameter called gran-difldra. Besides the above we may mention the Centaury, Erythraea Centaurium, a pretty annual with small pink or white flowers; and Gentiana Pneumonanthe, a perennial species from 1 to 2 feet high, bearing large deep blue flowers towards the end of Summer.