In biology and anatomy, an organ (Latin: organum, "instrument, tool", from Greek ὄργανον, organon, "organ, instrument, tool") is a collection of tissues joined in structural unit to serve a common function.
Usually there is a main tissue and sporadic tissues. The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are the nerves, blood, connective etc.. Functionally related organs often cooperate to form whole organ systems. Organs exist in all higher biological organisms, in particular they are not restricted to animals, but can also be identified in plants. An example of this is the bladder. In single-cell organisms like bacteria, the functional analogues of organs are called organelles.
A hollow organ is a visceral organ that is a hollow tube or pouch (as the stomach or intestine) or that includes a cavity (as of the heart or urinary bladder).
The functions of organ systems often share significant overlap. For instance, the nervous and endocrine system both operate via a shared organ, the hypothalamus. For this reason, the two systems are combined and studied as the neuroendocrine system. The same is true for the musculoskeletal system, which involves the relationship between the muscular and skeletal systems.
Organs of plants can be divided into vegetative and reproductive. Vegetative plant organs are root, stem and leaf. The reproductive organs are variable. In angiosperms, they are represented with the flower, seed and fruit. In conifers, the organ that bears the reproductive structures is called a cone. In other divisions of plants, the reproductive organs are called strobili (in Lycopodiophyta) or simply gametophores (in mosses).
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