Growth at the organismic level is a reflection of growth at the cellular and intracellular levels, since the fundamental unit of mass or structure is the cell.
Therefore, at the organismic level, growth may be the result of an increase in cell number, an increase in cell size, or a combination of the two. Of these possibilities, overall growth is rarely the result of an increase in cell size exclusively, although this is known to ‘occur. In most organisms, growth is the result of a combination of both increases in cell size and in cell number.
In embryonic stages prior to cell specialization or differentiation, the cells increase in number by division. However, as certain cells begin to specialize they fail to cycle back into active mitosis; instead, they proceed along a certain path of differentiation. This specialization accompanied by intracellular synthesis, which results in an increase in cell size.
Nevertheless, certain cells retain the capacity for active division and account for a major part of the growth that occurs from the immature stage to the adult stage. As mentioned previously, most animals reach a point at which overall growth ceases. At this time, those cells winch are actively dividing merely replace cells that are degenerating.
In contrast, muscle cells may increase in size throughout most of the life of the individual, depending upon diet and exercise. In higher plants, certain cells retain the capacity for active division throughout the life of the organism, and instead of replacing cells which degenerate, they merely add to the total mass.
As result, growth continues throughout the life of the plant. Although growth may assume different forms in various organisms, or in different tissues of the same organism, it is a universal characteristic of protoplasmic systems.
The intricate mechanisms controlling growth in organisms are incompletely understood at the present time, but eventually, many of them will undoubtedly be elucidated. When this occurs, it is reasonable to predict that such factors will be discovered at the cellular level. Because of this, research in the areas of cell division and intracellular synthesis continues to be very active.