Bald cypress trees growing in swamps have a peculiarity of growth called cypress knees. These are woody projections from the root system project above the ground or water. Their function was once thought to be to provide oxygen to the roots, which grow in the low dissolved oxygen waters typical of a swamp (as in mangroves). However, evidence for this is scant; in fact, roots of swamp-dwelling specimens whose knees are removed do not decrease in oxygen content and the trees continue to thrive. Another more likely function is structural support and stabilization. Bald cypress trees growing on flood-prone sites tend to form buttressed bases, but trees grown on drier sites may lack this feature. Buttressed bases and a strong, intertwined root system allow them to resist very strong winds; even hurricanes rarely overturn them.