Several species of cypress trees, which can grow in wetlands and shallow water, produce unusual structures called knees. Unlike roots, cypress knees grow above the ground. They are strange formations that extend upward from the trees’ roots into the air. While the function of roots is well established, the function of knees is still unclear, but scientists have several hypotheses. Oxygen absorption First, knees may help the trees’ roots absorb or take in oxygen. Plant roots require oxygen for important cellular functions, and the soil in the watery habitats where cypress trees often grow is low in oxygen. Roots of plants growing in low-oxygen soil must often obtain oxygen from another source. For example, mangrove trees, which also grow in soil that is underwater, are known for having above-ground roots that absorb oxygen. Like the roots of mangroves, cypress knees may take in oxygen from the air and deliver it to the deeper roots. Additional stability Second, knees may help cypress trees stand firmly in the soil, making them better able to remain securely in place when exposed to high winds or strong water currents. The fact that the cypress has knees in addition to a network of roots may provide additional stability, allowing the tree to anchor itself well in the soil. Cypress trees may need this additional support due to the softness of the wet soil in which they grow. Methane release Finally, knees grow on trees in areas where methane gas exists in the soil. The knees are in direct contact with the soil infused with methane, which is of no use to trees. Therefore, the function of knees may be to release methane gas from the trees' roots into the air. The fact that measurable amounts of methane have been detected in the air above the knees supports this idea.