Final answer:
An underground horizontal stem is called a rhizome, which is used by plants like ginger and ferns for asexual reproduction and food storage.
Step-by-step explanation:
The underground horizontal stem that the question refers to is a rhizome. Rhizomes are specialized for vegetative asexual reproduction, and unlike corms, which are more fleshy and rounded, or stolons and runners that may run above ground or just below the surface,
rhizomes grow horizontally underground and can give rise to new plants from the buds that form along them. Common examples of plants with rhizomes include ginger and ferns. This type of plant structure allows species to thrive in various environments by enabling them to store food and reproduce vegetatively.