The algae (singular is alga) comprise several different groups of living things that produce energy through photosynthesis. They are generally regarded as simple plants, and some are related to the higher plants. Others represent independent lines of evolutionary development, although they all appear to have acquired photosynthesis from cyanobacteria. All algae lack true leaves, roots, flowers, and other tissue structures found in higher plants. They are distinguished from bacteria and protozoa in that they are photoautotrophic. Thus, the algae are no longer considered a natural grouping, but the term is still used for convenience. The study of algae is called phycology or algology.