Characteristic of Callus
The most important characteristic of callus from a functional view point is that abnormal growth has the potential to develop normal roots, shoots and embryoid that can form plants. Callus cultures become brown and necrotic if allowed to grow on the same medium for an extended period because of the depletion of essential nutrients, gradual desiccation of the agar due to water loss and accumulation of toxic metabolites in the medium the tissue may eventually die. The callus is cut into two or more pieces and transferred to a fresh medium. Such cultures are referred to as "sub-cultures". After a few subcultures some tissue systems do not require exogenous auxin for callus proliferation. Such cultures are said to have become 'habituated to auxin". However, this phenomenon is not a genetic change but a selective gene expression as habituated cultures are known to revert to auxin requirement with time.