tap·root /-ˌrut, ˌrʊt/
主根,直根
Tap·root n. Bot. The root of a plant which penetrates the earth directly downward to a considerable depth without dividing.
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taproot
n 1: (botany) main root of a plant growing straight downward from
the stem
2: something that provides an important central source for
growth or development; "the taproot of his resentment";
"genius and insanity spring from the same taproot"