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.
The primary and secondary roots of the Dahlia become greatly swollen and spindle-shaped (fig. 17). The thickened portion is intended for the storage of reserve material with which to make a good start the following season in the production of the flower stem. The material stored is inulin. The base of the stem and the upper part of the root of the Turnip becomes greatly thickened and tuber-like, storing starch for the requirements of the flower stem in the second season. In the case of the fleshy, thickened taproots already mentioned, the Carrot and Parsnip store starch for the same purpose as the Turnip, and, all being good for food, they are cultivated for this special purpose by man. The same applies to Beet, which stores a sugar very like cane sugar.
Fig. 17. - Dahlia - Tuberous Root.