the common (non-taxonomic) name for a relatively simple type of eukaryotic plant which is never differentiated into root, stem, and leaves, contains chlorophyll a as the primary photosynthetic pigment, has no true vascular (water conducting) system, and in which there is no sterile layer of cells surrounding the reproductive organs. The algae range in form from eukaryotic single cells to plants many metres in length. Algae can be found in most habitats on Earth, although the majority occur in freshwater or marine environments.