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.
A highly curious assemblage of plants, in nearly all of which the leaves are undeveloped or reduced to spines or scales, and the stems fleshy, abounding in the most remarkable and ungainly forms. The flowers are often large and brilliant, the calyx-lobes as well as the petals being coloured, and the stamens numerous. Style long, often terminated by a radiating multipartite stigma. Fruit inferior, baccate; seeds numerous, on parietal placentas. With the exception of the genus Rhipsalis, all are natives of America, from Chili to Canada in 50° north latitude, but chiefly from Mexico. A few species are hardy in dry situations or on rockwork.
Opuntia Rafinesquiana, 0. vulgaris,and two or three species of Mammillaria are amongst the hardiest, but they are rarely cultivated, except by curious amateurs.