Growth is an increase in the size or mass of an organism. It happens in different ways in animals and in plants.
Animals. An animal embryo grows by cell division (mitosis), so the number of cells increases. As the cells divide, many of them differentiate, which means they change to become specialised for a particular job, such as nerve, muscle or blood cells. In animals most cells differentiate early in life, and growth by cell division mostly stops when the animal reaches adult size. After that, cell division is used mainly for repair.
Plants. Plants also grow by cell division, but this happens mainly in regions at the tips of roots and shoots called meristems. The new cells then grow longer by elongation: they take in water, their vacuole swells and the cell wall stretches. Elongation makes a large contribution to the increase in length of a root or shoot. Finally the cells differentiate, for example into xylem. Plants can keep growing throughout their lives.