Decussate things cross over each other, forming an X shape. In botany, decussate leaves are arranged along a stem in a way that causes pairs of them to overlap each other.
You can use decussate as an adjective or a verb — either way, it's a technical term most commonly used by scientists. In anatomy, nerve fibers that cross from one side of the brain to the other are said to decussate, and when rods of tooth enamel decussate, they cross each other in an X shape. Biologists also describe animal and plant markings this way: "I saw the decussate lines on its back and immediately identified the insect!"