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plant, any organism of the plant kingdom, as opposed to one of the animal kingdom or of the kingdoms Fungi, Protista, or Monera in the five-kingdom system of classification. (A more recent system, suggested by genetic sequencing studies, places plants with animals and some other forms in an overarching group, the eukarya, to distinguish them from the prokaryotic bacteria and archaea, or ancient bacteria.) A plant may be microscopic in size and simple in structure, as are certain one-celled algae, or a gigantic, many-celled complex system, such as a tree.
Plants are generally distinguished from animals in that they possess chlorophyll, are usually fixed in one place, have no nervous system or sensory organs and hence respond slowly to stimuli, and have rigid supporting cell walls containing cellulose. In addition, plants grow continually throughout life and have no maximum size or characteristic form in the adult, as do animals. In higher plants the meristem tissues in the root and stem tips, in the buds, and in the cambium are areas of active growth. Plants also differ from animals in the internal structure of the cell and in certain details of reproduction (see mitosis).
There are exceptions to these basic differences: some unicellular plants (e.g., Euglena) and plant reproductive cells are motile; certain plants (e.g., Mimosa pudica, the sensitive plant) respond quickly to stimuli; and some lower plants do not have cellulose cell walls, while the animal tunicates (e.g., the sea squirt) do produce a celluloselike substance.
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