Which type of stem grows horizontally underground?

The type of stem that grows horizontally underground is a rhizome. Rhizomes serve as a critical part of plant biology because they allow the plant to spread and propagate effectively. These underground stems grow horizontally, producing shoots that emerge from the soil, leading to new plants. This method of growth not only facilitates vegetative reproduction but also enables the plant to survive adverse conditions above ground since the rhizome can store nutrients and energy.

In contrast, stolons, often referred to as "runners," grow above ground and create new plants at their nodes when they touch the soil. Bulbs are a different type of storage organ that develops underground but have a different structure, consisting of layered fleshy leaves or scales and cannot be considered horizontal growth stems. Tubers, such as potatoes, are also storage structures that swell and grow underground, but they differ from rhizomes as they do not have the capability to grow horizontally over distances like rhizomes do. Therefore, rhizomes are uniquely suited for horizontal underground growth, making them the right choice for this question.