The answer to this question is the term rhizome. A rhizome is a stem of a plant that is growing underground and is growing horizontally. It is also known as rootstocks. The rhizome's function in the plant is that it is used to store starches and protein that enables the plant to grow underground.
Darwin called the ability of an organism to survive and reproduce in its environment fitness. Some organisms were fit to do all of these things, whereas others were not.
