Cypress trees have the tendency to become hollow as the tree dies over time. The outer layers of buttressing at the base of the trunk continue to support a healthy tree, while the center can erode and eventually become partially or fully hollow. This gives other species the opportunity to take advantage and many times you will see other trees, like the red maple, growing inside  cypress. The upright protrusions near the base of cypress trees are their knees. The knees help stabilize the cypress that typically grow in soggy, and most of the time, submerged soil. At one point the knees were also thought to serve as a mechanism for oxygen uptake, however recent research indicates that is not the case or at least this is a negligible function. Knees range in height from a couple of inches to up to ten feet! Often times the knees take on human characteristics, giving the impression gnomes are watching as you paddle by.