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An organ is a functional structure of multicellular organisms which consists of a group of several different tissues. Many multicellular organisms have individual cells grouped together into tissues, a group of many associated cells with similar function; tissues grouped together into organs, a group of tissues interacting so as to form a functional unit; and organs grouped together into organ systems, a group of closely interacting organs.

Animals, plants, and fungi often have many different organs whose different functions are integrated, enabling multicellular organisms to maintain themselves, grow, and reproduce. The organization of cells into tissues and organs presumably leads to a higher level of physiological integration and efficiency. In general, plants and fungi have fewer organs than animals. Moreover, the bodies of plants and fungi are not always as easily divided into discrete organs as are the bodies of animals.

Some examples of organs in animals are the stomach, heart, and lungs. The structure and function of the stomach are described here as an example. The stomach is an expanded region of the digestive system which is connected to the esophagus at the anterior end, and to the small intestine at the posterior end. The stomach stores and breaks down food before it passes through the pyloric valve, a special valve at the posterior end, and then into the small intestine, another organ. Glands in the stomach wall secrete special chemicals and enzymes which are responsible for the partial digestion of food.

Some examples of organs in plants are the leaf, stem, and root. Leaves are among the most prominent plant organs, so they are described here. A leaf is an out-growth of a stem. Leaves have three main physiological functions: photosynthesis, the biological conversion of light into chemical energy; transpiration, the evaporative movement of water out of the plant; and cellular respiration, the breakdown of foods and synthesis of high energy compounds.

Some examples of organs in fungi are haustoria (absorbing organs) and the sexual organs. The sexual organs of basidiomycete mushrooms are the best known fungal organs, so they are described here. In typical basidiomycete mushrooms, the fungal body develops into a morphologically complex organ called a basidiocarp. The basidiocarp is composed of sterile cells called psueudoparenchyma, and fertile club-shaped cells, called basidia. The basidia arise from the underside of the fleshy gills of a mushroom and each bears four haploid spores. These spores are shed from the gills, germinate, and eventually fuse with another sexually compatible individual.

See also Tissue.

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