Final answer:
Rhizomes are horizontal underground stems that facilitate vegetative asexual reproduction and help in plant mobility by growing new shoots from axillary buds.
Step-by-step explanation:
Horizontal underground shoots that grow new shoots from axillary buds at nodes are known as rhizomes. These are special types of plant stems that grow horizontally below the soil surface, producing only rudimentary leaves. Rhizomes, alongside stolons and axillary buds, play a crucial role in vegetative asexual reproduction and plant mobility, as they can lead to the growth of new vertical, photosynthetic stems. Rhizomes differ from stolons in that they are subterranean, while stolons grow above ground. Both can be considered means of reproduction or as a way for plants to spread to new areas.