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 or shrubs often of twining or prostrate habit; sap usually milky. Leaves simple, opposite or whorled, rarely scattered. In habit, and to a certain extent in structure, the members of this group come very near the Apocyneae, but the lobes of the corolla are commonly valvate, and the anthers and stigmas are consolidated, forming a column, and the pollen coheres in wax-like masses. This character is common to this order and the Orchids alone. The fruit is composed of two erect or divergent follicles, occasionally reduced to one by abortion; and the seeds are almost invariably plumose. There are about 150 genera and nearly 1,000 species belonging to this group. They are chiefly tropical or sub-tropical, and especially numerous in South Africa, where there are many highly curious succulent species. A few extend to the temperate regions in the North.