What method involves vegetative reproduction by means of underground stems?

The method that involves vegetative reproduction by means of underground stems is tuberous roots. Tuberous roots are specialized storage organs that serve not only to store nutrients but also to propagate new plants through vegetative means. Unlike true roots, they usually arise from the stem and can produce new shoots during the growing season, allowing the plant to reproduce without the need for seeds.

In contrast, layering and runners refer to different forms of vegetative reproduction. Layering involves bending a stem down to the ground and covering it with soil to encourage root development, while runners are horizontal stems that grow along the surface of the ground, producing new plants at nodes along the way. Separation typically involves dividing a plant into sections that each have roots and shoots, rather than relying on underground stems for reproduction. Thus, tuberous roots distinctly highlight the underground stem aspect of vegetative propagation.