By definition, growth simply means increase in mass. This increase in mass is due to the synthesis of cellular protoplasm or intercellular material formed by the cells, exclusive of waste materials or secretion products.
Although the imbibitions of large amounts of water and the intake of food materials certainly bring about an increase in mass, such an increase before digestion, absorption, and metabolism have taken place should not be considered growth.
From a metabolic standpoint, growth constitutes a preponderance of anabolic synthetic processes in an organism over those which are catabolic. When the biologist states that growth is a characteristic of living systems, he does not mean that every organism is adding to its total quantity at all times or that every cell in a complex body is synthesizing more protoplasm.
Although most plants continue to grow as long as they live, it is characteristic of animals to reach a certain size at which point further overall growth is not necessarily demonstrated. At this point, catabolic and anabolic processes are in quantitative equilibrium.
The statement that growth is a characteristic of living systems means that protoplasm exhibits the potentiality of growth, although in cases where cells are sufficiently specialized as to have lost the ability to increase their total mass, this potentiality may be limited to the replacement of degenerating cellular parts.