Plant growth is unique because plants retain the capacity for unlimited growth throughout their life. This ability at the plants is due to the presence of meristems at certain locations in their body. The cells of such meristems have the capacity to divide and self-perpetuate.
The product, however, soon loses the capacity to divide and such cells make up the plant body. This form of growth wherein new cells are always being added to the plant body by the activity of the meristem is called the open form of growth. Yes, this statement holds true for sponges also.