central. vertical root growing directly below the main stem or trunk that may or may not persist into plant maturity.

tap root (Wikipedia)
The two types of root systems in plants. The fibrous root system (A) is characterized by many roots with similar sizes. In contrast, plants that use the taproot system (B) grow a main root with smaller roots branching off of the taproot. The letters mark the beginning of the roots.

A taproot is a large, central, and dominant root from which other roots sprout laterally. Typically a taproot is somewhat straight and very thick, is tapering in shape, and grows directly downward. In some plants, such as the carrot, the taproot is a storage organ so well developed that it has been cultivated as a vegetable.

The taproot system contrasts with the adventitious or fibrous root system of plants with many branched roots, but many plants that grow a taproot during germination go on to develop branching root structures, although some that rely on the main root for storage may retain the dominant taproot for centuries, for example Welwitschia.

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