Explants are pieces of tissue placed in culture. Callus is an unorganized mass of cells that forms from explants. There are three stages of callus culture: induction, proliferation, and differentiation. Cell suspension cultures can be initiated from callus and involve transferring callus pieces into liquid medium with agitation to break up cell aggregates. Organogenesis is the formation of organs like leaves and roots on explants, which can occur directly or indirectly through a callus stage. Somatic embryogenesis involves the formation of embryos from somatic cells and follows a similar development process to zygotic embryogenesis.