This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
This is the most common method of increasing plants, whether by cuttings, budding, grafting, inarching, layering, rhizomes, corms, eyes, or runners. Due care must be taken as to the likely places where starch or other reserve material may be stored. For instance, a Dahlia cutting, with a piece of the old tuber, will strike with more certainty than a piece of the young stem alone. The same applies to cuttings of Everlasting Peas, Lychnis chalcedonica flore pleno, L. dioica flore pleno, Gypsophila paniculata flore pleno, Begonia Gloire de Lorraine, and other choice varieties, which it is desirable to keep true to name. They should be cut as near the rootstock as possible. The same reason holds good with cuttings of Roses and many other shrubs, to be cut at a joint, or with a heel of the old wood. Such cuttings are always more solid at a joint than elsewhere, and less liable to damp off, but there is always a greater storage of food in those places than between the joints, because it comes from the leaf, and the bud or young shoot in its axil has to be fed.