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.
Perennial or annual herbs with radical or alternate leaves, rarely climbing shrubs with opposite leaves. Stipules absent, or adnate to the petiole. Flowers regular or irregular. Sepals 5 or more, rarely fewer, deciduous, often petaloid, imbricate or valvate in bud. Petals often undeveloped, or 5 or more, sometimes only 3, frequently minute or deformed. Stamens many, hypogynous; anthers adnate, dehiscing laterally. Carpels usually numerous, seldom only 1, usually free, 1-celled; style simple; seeds 1 or more on the ventral suture, anatropous, erect with a ventral, or pendulous with a dorsal raphe. Fruit of 1-seeded achenes or many-seeded follicles. A large order dispersed all over the world, not rare in the tropics. Many species are acrid, and some highly poisonous, especially the Aconites.