A rhizome is a specialized, horizontal growth structure that plants use to survive and propagate beneath the soil surface. This subterranean adaptation allows plants to manage environmental stresses, such as cold temperatures, drought, or fire, by retreating below ground. The rhizome serves as a biological bank, storing the energy required for the plant to regenerate and spread.
Defining the Rhizome: Structure and Growth
A rhizome is anatomically classified as a modified subterranean stem, a classification that distinguishes it from a true root. The defining characteristic of a stem, even an underground one, is the presence of nodes and internodes. Nodes are points on the stem where structures like leaves, roots, and buds emerge, while the internodes are the segments between these nodes.
This stem grows horizontally, often just beneath or at the soil surface. At each node, the rhizome produces adventitious roots that anchor the plant and draw nutrients from the soil, while buds develop into new shoots that grow upward into the air. This horizontal growth pattern allows the parent plant to continuously colonize new soil adjacent to its existing location.
Rhizomes vs. Other Underground Structures
The term “underground plant part” can be confusing because several different structures perform similar functions, yet they are anatomically distinct. The difference between a rhizome and other subterranean organs lies in their botanical origin and structure. A rhizome is definitively a stem, characterized by its nodes and internodes, whereas a true root lacks these features and is terminated by a root cap.
Tubers, such as potatoes, are thickened storage portions of either a rhizome or a stolon, meaning they are stem tissue enlarged for nutrient storage. Bulbs, like onions, are modified leaves; they consist of a reduced, flattened stem base surrounded by fleshy, overlapping storage leaves. Stolons, often called runners, are similar to rhizomes in that they are horizontal stems with nodes and internodes, but they grow above ground or along the soil surface.
Biological Roles: Storage and Reproduction
The primary biological purpose of the rhizome is to function as a storage organ and a mechanism for vegetative reproduction. Internally, the fleshy tissue is packed with stored carbohydrates, primarily starches, and proteins. This reserve energy allows the plant to survive adverse conditions, such as winter cold, drought, or periods of low light, by entering a state of dormancy.
The ability of the rhizome to sprout new plants from its nodes is a highly effective form of asexual reproduction. If the rhizome is physically broken or divided, each segment containing at least one node has the capacity to grow into a new, genetically identical plant. This vegetative propagation allows a single plant to rapidly spread a large area, which is why many rhizomatous plants, such as certain grasses, are considered aggressive or invasive.
Common Plants That Use Rhizomes
Many familiar plants, both edible and ornamental, rely on rhizomes for their survival and spread. The spices ginger and turmeric are two of the most recognizable examples, as the fleshy, edible portion consumed is the actual underground rhizome.
Other common examples include the iris, which uses its rhizomes to form clumps and survive year after year. Bamboo, whose aggressive, spreading rhizomes (known as leptomorphs) can travel significant distances underground, allows the plant to colonize vast groves rapidly. Various turf grasses, like Bermuda grass and Johnson grass, also use rhizomes to form dense, interconnected mats, which makes them resilient to damage and difficult to eradicate.