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A plant or plant part that consists of two or more genetically different types of cell. In a graft hybrid the distinct cell types are derived from two different species. Such chimaeras are formed when a bud grows from the region of a graft union and contains tissues from both scion and stock (see periclinal chimaera). Chimaeras can also arise by a mutation in a meristematic region. This may give rise either to a sector of different cells (sectorial chimaera) or to a complete layer of different cells, as might occur if the mutation is in the top layer of the tunica, which subsequently gives rise to the epidermis. Chimaeras consisting of cells of different chromosome number may occur in cytologically unstable tissues. This may result from wounding. It is also seen in trisomic plants in which some cells lose the extra chromosome to revert to the normal chromosome number. In such cases both normal shoots and shoots showing the morphological changes associated with trisomy can occur.
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