They are mostly aquatic, abounding in salt and fresh water

They also frequent moist ground and damp rocks, the bark of trees, the ti ue of higher plants, and unite with fungi in the formation of lichens. Some forms are cosmopolitan; but, as with land plants, the various species are restricted in their distribution, being controlled by climatic conditions, substratum and depth of water. In structure the algæ are widely various, ranging from microscopic forms to great seaweeds that ramify like trees. The plant body may be unicellular or multicellular or coenocytic, the cells being arranged in series to form filaments, or they may be united to form cell surfaces or complex structures. Many forms are free, others attached by disks or rhizoidal growths.