This section is from the book "The Gardener V1", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
The genus Cotyledon is a small one, comprising plants of no striking ornamental qualities; but they are useful for planting on old walls with a view to covering them, and are easily established in such positions if inserted in crevices where a little soil or decayed lime or stone exists - anything, in short, that will serve to retain them in their place, along with a little moisture, which is all they appear to want in the shape of nutriment. The species selected grows about 6 or 9 inches high. The leaves are thick, fleshy, and round, with a few remote teeth, and attached by their centres to the longish stalks. The flowers are greenish yellow in long racemes, and are produced throughout the summer. Native of Britain and Ireland, and many parts of western Europe.