A plant grows by increasing its cell population while the cells specialize their functions. Increasing cell population is done by cell division (also called mitosis). Before a mother cell divides into two daughter cells, it makes an exact copy of its genome first. As a result, the two daughter cells usually have exactly the same genetic makeup as their mother cell. Therefore, every living cell of a plant should contain all the genes the plant has and thus has the capacity to grow back to a full plant. This is called cell totipotency .
The process of specializing cells’ functions is called cell differentiation . It is accompanied by morphogenesis, the change of the cells’ morphology. Differentiation is done by turning on certain genes and turning off some others at a certain time. Therefore, for a highly differentiated cell to grow into a full plant, the differentiation process has to be reversed (called de-differentiation ) and repeated again ( called re-differentiation). Theoretically, all living cells can revert to an undifferential status through this process. However, the more differentiated a cell has been, the more difficult it will be to induce its de-differentiation. Practically, the younger or the less differentiated a cell is, the easier to culture it into a full plant. The ease of fulfilling the cell totipotency also varies tissue by tissue, genotype by genotype and species by species. Genotype dependency is often the bottle-neck in plant tissue culture and also in plant genetic engineering.