An adult plant consists of many specialized cell organizations: tissues and organs. Tissues, such as meristem, cortex, phloem and epidermis, consist of cells of uniform shape and specialized function. Several tissues are organized together to form an organ, such as leaves, roots, flowers and the vascular system. The process of initiation and development of an organ is called organogenesis. In plant tissue culture, inducing organogenesis is an important way to regenerate plants from the culture.
Organogenesis in plant tissue culture involves two distinct phases: dedifferentiation and redifferentiation. Dedifferentiation begins shortly after the isolation of the explant tissues with an acceleration of cell division and a consequent formation of a mass of undifferentiated cells (called callus). Redifferentiation, also called budding in plant tissue culture, may begin any time after the first callus cell forms. In this process of tissue called organ primordia is differentiated from a single or a group of callus cells. The organ primordia give rise to small meristems with cells densely filled with protoplasm and strikingly large nuclei. The development (or growth) of an organ is monopolar. Polarity of the longitudinal axis of the organizing growing points of the organs can be seen some time after the formation of meristem tissues. Different types of specialized cells again differentiate. The vascular system is formed connecting the new organs to their parent explants or callus mass.