Knees are woody projections sent above the normal water level, roughly horizontally from the roots, with a near-right-angle bend taking them roughly vertically downward into soil, usually passing through water first. One rather popular view of their function is that they provide oxygen to the roots that grow in the low dissolved oxygen (DO) waters typical of a swamp (see also mangroves, which have similar adaptations). However, there is little evidence for this; in fact, swamp-dwelling specimens whose knees are removed continue to thrive.
Another more likely function is that of structural 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: