taproot

The main root of a plant, which penetrates the earth directly downward to a considerable depth; a root in which the descending radicle maintains its superiority in thickne and importance to the rootlets which spring from it on The American Dictionary and Cyclopedia all sides. Example, the carrot, parsnip, or turnip. A tap-root may be fusiform, napiform, premorse, filiform, or cylindrical. "Some put under the trees raised of seed, about four inches below the place where they sow their seeds, a small piece of tile to stop the running down of the tap root , which occasions it to branch when it comes to the tile."-Mortimer: Husbandry.