Any organism in the kingdom Plantae, consisting of multicellular, eukaryotic life forms (seeeukaryote) with six fundamental characteristics: photosynthesis as the almost exclusive mode of nutrition, essentially unlimited growth at meristems, cells that contain cellulose in their walls and are therefore somewhat rigid, the absence of organs of movement, the absence of sensory and nervous systems, and life histories that show alternation of generations. No definition of the kingdom completely excludes all nonplant organisms or even includes all plants. Many plants, for example, are not green and thus do not produce their own food by photosynthesis, being instead parasitic on other living plants (seeparasitism). Others obtain their food from dead organic matter. Many animals possess plantlike characteristics, such as a lack of mobility (e.g., sponges) or the presence of a plantlike growth form (e.g., some corals and bryozoans), but in general such animals lack other plant characteristics. Some past classification systems (seetaxonomy) placed difficult groups such as protozoans, bacteria, algae, slime molds, and fungi (seefungus) in the plant kingdom, but structural and functional differences between these organisms and plants have convinced most scientists to classify them elsewhere.