Introduction Formation of Adventitious Meristems and Organs Preexisting and Adventitious Meristematic Tissues Organogenesis and Regeneration in Animals Regeneration of Roots and Shoots is a Process Consisting of Distinct Steps Factors Influencing Adventitious Organogenesis Explant Effects General Remarks about Plant Hormones Auxin, Cytokinin, and Regeneration Other Hormones and Hormone-Like Factors Nutritional and Physical Factors Biochemical and Molecular Aspects Concluding Remarks Bibliography