Many starchy foods harvested from the soil are collectively labeled as “root vegetables,” blurring the lines between true roots, modified stems, and other plant parts. This distinction is particularly important for the sweet potato, a common food item whose classification often sparks debate among home cooks and gardeners alike.
The Sweet Potato is a Storage Root
The sweet potato (Ipomoea batatas) is definitively not a tuber, but rather a storage root, sometimes called a tuberous root. This edible part develops directly from the plant’s lateral roots, functioning as an enlarged storage area for starches and water. Belonging to the morning glory family, Convolvulaceae, the sweet potato shares its lineage with ornamental flowering vines.
The storage root’s internal structure is derived entirely from root tissue, which is distinctly different from a true stem. The structure of the sweet potato lacks the nodes and internodes that characterize a stem. Instead, it has adventitious buds that can sprout new shoots, typically located at the proximal end near where it attaches to the main stem.
What Defines a True Tuber
A true tuber, known botanically as a stem tuber, is a modified, underground stem, not a root. The common white potato (Solanum tuberosum) serves as the primary example of this classification, belonging to the nightshade family, Solanaceae. Stem tubers develop from specialized underground stems called stolons or rhizomes, which enlarge at their tips to store energy.
The defining feature of a true tuber is the presence of nodes, commonly referred to as “eyes,” which are arranged spirally along the surface. Each of these eyes represents a lateral bud and can sprout both new stems and roots, enabling the plant to reproduce asexually. Furthermore, if a true tuber is exposed to light, it can develop chlorophyll, turning green, which is a characteristic function of stem tissue.
Why Scientific Confusion is Common
The confusion between sweet potatoes and true potatoes persists largely because they are both starchy, underground storage organs used similarly in cooking. The culinary world groups them as “root vegetables” because they are harvested from below the soil line and serve a comparable functional role on a dinner plate. Despite this shared utility, they belong to different botanical families and genera, meaning they are only distantly related.
Further complicating the matter is the long-standing practice of mislabeling sweet potatoes as “yams,” particularly in North America. True yams are monocots in the Dioscoreaceae family and are a distinct group of tubers, separate from both the stem-tuber potato and the root-storage sweet potato.