What feature do stalagmites lack that distinguishes them from stalactites?

Stalagmites are distinguished from stalactites primarily by the absence of a central tube. Stalagmites form on the ground of a cave from the dripping of mineral-rich water from above, which allows them to build up from the floor without a hollow center. In contrast, stalactites grow downward from the ceiling and typically have an internal structure that may include a central tube through which water drips.

While stalagmites may have a rounded base and can vary greatly in height and surface texture, it is the central tube that is a defining characteristic of stalactites. This difference highlights the distinct formation processes and structural features of these cave formations, emphasizing the method of mineral deposit accumulation over time.