Pithophora Wittrock: Cotton-ball algae, horsehair algae. Cladophoraceae. Green algae. Free-floating algae that can coalesce to form larger units. Made up of coarse filaments that branch irregularly at right angles; these branches may divide once again. Individual filaments are visible without magnification. Under high magnification, barrel-shaped cells can be seen among the more common cylindrical ones. Algae float to the surface and are usually seen unattached to substrates; they form floating clumps up to 1 ft (30 cm) in diameter; these can be so numerous that they eventually cover acres of water. The color of the floating clumps ranges from bright to dark green; they often look brown or reddish due to the presence of dissolved iron. A mass of algae held in the hand is described as resembling a wet ball of cotton or a clump of horsehair; the branching, criss-crossing filaments are coarse and lack a mucilaginous coating, making the algae springy and resilient, and preventing the mass from being pulled apart easily. Since filaments do not align in parallel they do not form strands when handled. These algae can produce large infestations, covering areas of shallow water such as ponds, particularly in hard water that has high levels of phosphorous and nitrogen.