The growth of higher plants depends on the organised allocation of functions to organs which in consequence become differentiated, that is to say, modified and specialised to enable them undertake their essential roles. Unorganised growth is seldom found in nature, but occurs fairly frequently when pieces of whole plants are cultured in vitro. The cell aggregates, which are then formed, typically lack any recognisable structure and contain only a limited number of the many kinds of specialised and differentiated cells found in an intact plant. A differentiated cell is one that has developed a specialised form (morphology) and/or function (physiology). A differentiated tissue (e.g. xylem or epidermis) is an aggregation of differentiated cells. So far, the formation of differentiated cell types can only be controlled to a limited extent in culture. It is not possible, for example, to maintain and multiply a culture composed entirely of epidermal cells. By contrast, unorganised tissues can be increased in volume by subculture and can be maintained on semisolid or liquid media for long periods. They can often also be used to commence cell suspension cultures. Differentiation is also used botanically to describe the formation of distinct organs through morphogenesis.
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