A taproot is the one big main root that grows straight down from a plant, thicker than everything else, with the smaller roots branching off it. A carrot is a taproot you can eat; so is a radish, and so is the long white 蘿蔔. Trees like oaks have one too, deep in the ground, which is part of why they are hard to blow over and very hard to dig up. Not every plant works this way — grass, for example, has a spreading mat of thin roots instead, with no single main one. Biology class is where students meet this word most. But people also use it in a figurative way, for the one deep cause underneath a problem or a tradition: 'poverty is the taproot of the whole problem' means it is not just one cause among many, it is the deep one everything else grows from. That use is a bit literary; you will see it in writing more than hear it in speech.

主根（植物學用語）；比喻用法可說「根源、深層原因」