Photosynthetic, almost exclusively aquatic, plants of a large and diverse division (Algae) of the thallophytes, including seaweeds and their fresh-water allies. It ranges in size from simple unicellular forms to giant kelps several meters long, and displays extremely varied life-cycles and physiological processes, with, e.g., different complexes of photosynthetic pigments. Algae range from the Precambrian. An individual plant is called an alga.