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.
In general terms roots serve to fix the plant in the soil. The primary, descending root of forest trees is of considerable importance to many of them, like the Oak, Elm, and Ash, in preventing them from being overturned during gales and hurricanes of wind. To gardeners it is of leading importance in the case of such root crops as Carrots, Parsnips, and Beet. Great care is taken in preparing the soil to a considerable depth, and the seeds are sown where the plants are to grow till they reach maturity. No transplanting is permissible. If the primary root or radicle were broken, a shapely taproot would be impossible. All of them could be transplanted with the greatest facility, and, with care, almost every root would grow, but they would be short, stumpy, forked, misshapen, unsaleable, and useless except for cattle. A deeply worked and well-pulverized soil is necessary to enable the radicle to descend perpendicularly without twisting or bending between stones and hard lumps; and if well manured for some previous crop, the radicle and slender, lateral fibres will be well able to forage for the requirements of a large and shapely root. It is quite different in the case of Cabbages, Apple, Pear, and other fruit trees, because transplanting multiplies the number of fibrous, feeding or absorbing roots. The more fibres upon the roots of Cabbages, Onions, and the like, the sooner they get established in their permanent positions when transplanted. Taproots are undesirable in fruit trees, because they often get down into uncongenial subsoils, while plenty of fibrous roots near the surface induces early fruitfulness and permits of feeding.