Organogenic Differentiation - For the regeneration of a whole plant from a cell or from a callus mass, cytodifferentiation is not enough and there should be differentiation leading to. organogenesis (organ formation = formation of stem, roots, flowers, etc.). This may occur either through shoot bud differentiation or through somatic embryogenesis.

In the former, shoot buds (monopolar structures) are formed while in the latter, somatic embryos (bipolar structures) are formed both leading to regeneration of whole plants. Shoot bud differentiation. Shoot bud differentiation is stimulated in plants by a variety of chemical and physical factors, but these factors vary widely for different plant materials, so that no general recipes can be prescribed.

The requirement of cytokinin and auxin for bud differentiation depends on the endogenous levels of these two classes of substances in the tissues used for culture. Thus not only different plant materials differ in. their ability to undergo shoot bud differentiation but some tissues are incapable of bud formation in cultures due to special requirements, rather than due to lack of inherent genetic ability to do so.

Shoot bud formation is also controlled by the size and source of the explant. The larger the explant­(containing parenchyma, cambium and vascular tissue), the better are the chances of shoot bud formation, irrespective of auxin - cytokinin concentration.

Similarly, depending upon the plant species, leaves or leaf fragments, root sections and inflorescence sections or cotyledons may be used as explants leading to the formation of adventitious buds.