growth (biology)

In biology, increase in size and mass during the development of an organism over a period of time. Growth is often measured as an increase in biomass (mass of organic material, excluding water) and is associated with cell division by mitosis, subsequent increases in cell size, and with the differentiation of cells to perform specific functions, for example red blood cells in mammals and root cells in plants. All organisms grow, although the rate of growth varies over a lifetime. Typically, growth in an organism follows an S-shaped curve, in which growth is at first slow, then fast, then, towards the end of life, non-existent. Growth may even be negative during the period before death, with decay occurring faster than cellular replacement. In humans, there is a short period of rapid growth at puberty. Growth is affected by genetic factors, which dictate the eventual size and appearance of an organism. It is dependent upon an adequate supply of water and mineral salts (diet), and, particularly in plants, appropriate conditions of light and temperature, which determine the rate of photosynthesis.

In plants, growth directed by a specific stimulus is called tropism; for example, phototropism involves growing towards the light, while gravitropism is directed by gravitation.

Increase of size by expansion, as when a cell enlarges through taking in water, is not usually considered as biological growth because this process does not involve any increase in dry weight.