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Definition: A member of the Kingdom Plantae. All plants carry out Photosynthesis, with the exception of a few parasitic species that have lost this ability. As photosynthesis relies on the green pigment Chlorophyll, all non-parasitic plants are green, unless this color is masked by some other pigment. Plants are non-motile, unlike most animals, and this allows them to have a far more plastic growth-form – they generally have no definite size or shape to which all members of the species conform. The cells of plants are Eukaryotic, but with certain features not seen in other eukaryotes (animals and fungi): Chloroplasts and other Plastids, a large central vacuole, and Cellulose cell walls. Other characteristic plant features include a waterproof Cuticle, openings known as Stomata through which gas exchange takes place, and areas where growth occurs, known as Meristems. In higher plants, the plant body can be divided into Roots, Stems, Leaves and Buds. Among simpler forms the plant body consists of a Thallus, sometimes with root-like Rhizoids. Growth and other processes are controlled by plant Hormones, though these do not act in exactly the same way as animal hormones. Almost all plants show at least two forms of reproduction: the asexual production of Spores, and sexual reproduction involving gametes; in all terrestrial plants and in most algae an Alternation of generations is seen, in which the sexual and asexual forms alternate in the life cycle. Many plants also produce offspring by a variety of other asexual methods, generally known as Vegetative reproduction. Exactly which organisms are to be included in the plant kingdom is still a matter of debate among taxonomists. All flowering plants (Angiosperms), conifers and their allies (Gymnosperms), ferns, clubmosses and horsetails (Pteridophytes), and mosses and liverworts (Bryophytes) are included. (These are often referred to as the “terrestrial plants”; though some, such as the waterlilies and duckweeds, have returned to an aquatic way of life.) Most taxonomists also include the Algae – the seaweeds, freshwater weeds, and unicellular algae such as Chlamydomonas. Other taxonomists place the algae in the Kingdom Protoctista on the basis of their simpler level of organization. In reality there is probably a strong evolutionary link between unicellular green algae, multicellular green algae (such as Spirogyra and the sea-lettuce Ulva) and the simplest of terrestrial plants, the liverworts and mosses; this makes the separation of the algae from the terrestrial plants rather artificial. Plants are extremely important in the Ecosystem as they are Autotrophs, upon which other organisms (Heterotrophs) all ultimately depend for their food. The role of plants in the Carbon cycle is crucial.
Source: Illustrated Dictionary of Science, Andromeda. London: Andromeda, 1988. Credo Reference. Web. 10 August 2012.