The cells of Spirogyra are longer than their breadth. The cell wall is two layered. The inner wall is made up of cellulose and outer of pectose. When pectose comes in contact with water it gives the filament slimy or slippery. Hence, the algae is called pond silk. Nostoc is a filamentous form of both terrestrial and aquatic habitats. It occurs in symbiotic association with fungi to form lichen. It fixes atmospheric nitrogen. Ulothrix is a common, freshwater, thread like alga. Its cell wall consists of two concentric layers inner cellulosic and outer pectic.