The innumerable cells which constitute the body of a higher plant or animal and containing identical genetic material can be traced to a single cell-the zygote. During development cells undergo diverse structural and functional specialisation depending upon their position in the body. Leaf cells bear chloroplasts and act as the site of photosynthesis. The colorless root hairs perform the function of absorbing nutrients and water from the soil and some other cells become part of the colorful petals. Normally fully differentiated cells do not revert back to a meristematic: state, which suggests that the cells have undergone a permanent change. Earlier you know that the regenerative capacity is retained by all living cells of a plant. Several horticultural plants regenerate whole plant from root, leaf and stem cuttings. Highly differentiated and mature cells such as those of pith and cortex and highly specialised cells as those of microspores and endosperm retain full potential to give rise to full plants under suitable culture conditions. G. Haberlandt was the first to test this idea experimentally. This endowment called "cellular Totipotency" is unique to plants. Animal cells possibly because of their higher degree of specialisation do not exhibit Totipotency. Whole plant regeneration from cultured cells may occur in one of the two pathways:
1) Shoot bud differentiation, (organogenesis) and
2) Embryo formation (Embryogenesis).
The Embryos are bipolar structures with no organic connection with the parent tissue and can germinate directly into a complete plant. On the other hand, shoots are monopolar. They need to be removed from the parent tissue and rooted to establish a plantlet. Often the same tissue can be induced to form shoots or embryos by manipulating the components of the culture conditions.