Wǒmen píngshí chī de jiāng, qíshí shì jiāng de gēnjīng.
The ginger we cook with is actually the rhizome of the ginger plant.
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In botany it is the rhizome, a stem that creeps sideways underground, as in ginger, turmeric and lotus root, 藕. In everyday writing it just means the roots and stems of a plant together.