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.
Many plants may be propagated by cutting up the roots into short lengths and inserting them as cuttings, and in the absence of seeds they may be rapidly increased in this way. The roots of the Gean (Prunus Avium), Plum stocks for varieties of garden Plums, Poplars, and English Elm naturally develop suckers on their roots and may be propagated in this way. The roots of Bouvardias, Senecio puleher, Anemone japonica, Ailanthusglandulosa, Seakale, Horse Radish, and many others may be cut into short pieces and inserted as cuttings, when they give rise to buds which grow into plants. Some of these roots are fleshy, but in any case they can only give rise to buds owing to the presence of formative matter or reserve food in them, usually starch. The underground parts of Mints, Solomon's Seal, Lily of the Valley, and many others used for propagation are not roots but rhizomes or underground stems, and branches in the case of the Potato and Jerusalem Artichoke.