How the Callus Tissue is Formed?
Formation of callus tissue is the outcome of cell division and cell exoansion of the cells of explant. During the formation of callus tissue, the explant loses its original characteristics. So under the influence of exogenously supplied hormone. The explant is triggered off a growth sequence in which cell enlargement and cell division predominate to form an unorganised mass of cells. As a result , the explant undergoes an irreversible changes of its shape, size , symmetry, structural organization and cellular integrity.
Depending upon the types of explant viz. leaf, stem segment, root segment etc. either enlargement in size or the swelling followed by rupture of tissue within few days of inoculation take place. This change indicates the response of explant for callus formation and is followed by the appearance of little irregular cellular masses around the cut edges or from the cut edges or from the ruptured surface. It is now explained that initial formation of cellular mass particularly at the cut end may be due to injury during excision. Some endogenous growth substances oozes out through the injured tissue at cut end and stimulates the cell division which is simultaneously induced by the exogenously supplied growth hormones. It is assumed that both endogenous product and exogenous hormones make a threshold level and their interaction causes formation of unorganised cellular growth at cut end. Auxin is required for growth and cytokinin is required for cell division. The type of tissue as a explant plays important role, such as meristematic tissue containing vascular cambium.