The term 'underground stem' refers to the various structures that plants use to store energy and reproduce. These structures include rhizomes, tubers, roots, and stolons. Rhizomes are horizontal stems that grow underground and can produce new shoots and roots. Tubers are swollen, fleshy underground stems that store food and can be used to propagate new plants. Roots are the main anchoring structure of a plant and absorb water and nutrients from the soil. Stolons are similar to rhizomes, but they grow above ground and can produce new shoots and roots.