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A cypress knee is a term used in the biology of trees to describe the distinctive structures forming above the roots of a cypress tree of any of various species of the subfamily Taxodioideae. Their function is unknown, but they are generally seen on trees growing in swamps. Some scientists have thought they may help in oxygenation to the tree's roots or assist in anchoring the tree in the soft, muddy soil.
Knees are woody projections sent above the normal water level, roughly vertically from the roots, with a near-right-angle bend taking them vertically upward through water. One early assumption of their function was that they provided oxygen to the roots that grow in the low dissolved oxygen (DO) waters typical of a swamp, acting as pneumatophores: mangroves have similar adaptations. There is little actual evidence for this assertion; in fact, swamp-dwelling specimens whose knees are removed continue to thrive, and laboratory tests demonstrate that the knees are not effective at depleting oxygen in a sealed chamber. Despite the fact that there is no expert consensus on their role, the supposition that they are pneumatophores is repeated without note in several introductory botany textbooks.
Another more likely function is that of structural buttressed support and stabilization. Lowland or swamp-grown cypresses found in flooded or flood-prone areas tend to be buttressed and "kneed," as opposed to cypresses grown on higher ground, which may grow with very little taper.
Trees that develop these "knees" include the following: