This section is from the book "Commercial Gardening Vol4", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Of the sixty species known, only two or three are commercially valuable. The Common British Buckthorn (R. catharticus), 5-10 ft. high, with ovate serrate leaves; the Black Dogwood (R. Frangula), with entire leaves; and R. Alatemus, from South Europe, with ovate - elliptic serrate leaves, are best known. There are several varieties of the last two named. The plants grow in any garden soil, and are raised from seeds, cuttings, and layers.