The word Phycology is formed from the Greek words phycos meaning seaweed and logos meaning special knowledge. Algae refers to seaweed or algae. Avascular plants that contain chlorophyll and cannot be divided into roots, stems and leaves and whose cell walls are composed of cellulose are called algae. All floating algae are called phytoplankton. There are about 30,000 species of algae in the world. The scientist Carolus Linnaeus first used the term Algae.