An underground stem is....

The correct answer is: Tuber

• A tuber is an enlarged part of a plant that swells with starch and is used as energy storage. Tubers can either be root or stem tubers.

• Tubers are actually swollen portions of underground stems or rhizomes that usually lie horizontally or run laterally beneath the soil surface or at the soil level. These swollen structures store nutrients for the plant to use during dormancy and promote new healthy growth in spring.
• An underground stem is tuber.