Salvinia is a free-floating fern that forms dense mats on water. It consists of many branched horizontal stems, 1–2 mm in diameter, which float just below the water surface. At each node, or joint, on the stem is a pair of floating, green, oval-shaped hairy fronds. A brown frond, consisting of many hairy filaments, also occurs at each node and trails in the water, looking and acting like a root.