the increase in an organism’s size caused by the accretion of cells, cell mass, and noncellular structures. A living system grows because anabolism predominates over catabolism.

In animals. During the development (ontogeny) of animals, growth is closely associated with qualitative changes, or differentiation. Growth and differentiation often do not occur simultaneously, but one does not exclude the other. The usual parameters of growth are changes in the mass (weight) or linear dimensions (length) of an individual or organ.

Growth is usually described by curves that characterize changes in body weight or length during ontogeny, by the absolute and relative increments occurring during a specific period of time, and by the specific growth rate. The growth process may sometimes be described quite accurately by a comparatively simple mathematical equation. There are empirical growth equations that describe factual information and are convenient for making calculations; their constants are not usually of biological significance. There also exist theoretical equations, based on such theoretical considerations as comparison of growth with a monomolecular chemical reaction or growth as the realization of a genetic program. Efforts are made to attach biological significance to the constants of this type of equation.

The formula of simple allometry is ordinarily used to express the quantitative relation between the growth of a particular organ and the growth of the individual as a whole: y = bx^α, where y is the size of the organ, x the size of the organism, α the index of allometry, and b an empirical constant. An organ may grow at the same rate as the organism (α = 1; isometry), more rapidly than the organism (α > 1; positive allometry), or more slowly than the organism (α < 1; negative allometry).

The growth rate is higher in young animals and normally decreases with age. Some animals grow all their lives, although the growth rate may be low during maturity; examples are mollusks, fish, and amphibians. In other animals, growth ceases at a certain age, as in the case of higher insects and birds. In ontogeny, successive phases of development have different growth rates; in many animals, particularly mammals, the embryonal and postembryonal periods and the periods before and after the onset of puberty are distinct. In animals with a hard chitinous integument, such as insects and crustaceans, the body lengthens mainly during the molting periods.

The growth process is rhythmic. Seasonal and 24-hour rhythms have been studied more than others. Seasonal rhythms, related to the succession of seasons, are manifested by the annual layers formed in skeletal structures. Twenty-four-hour rhythms can be detected from the frequency of cell division and from changes in the size of the organism as a whole. There are other growth rhythms with different periodicity, such as the 15-day rhythms of marine mollusks, which are related to low and high tides.

Growth is influenced by both genetic and environmental factors. Genetically it is determined by the combined action of many genes whose individual effect is slight. However, some anomalies of growth, such as dwarfism and shortness of extremities, are caused by the action of individual genes. The most important environmental growth factors are food supply, temperature, humidity (for terrestrial animals), water salinity (for aquatic animals), and population density. An unfavorable environment may inhibit growth to the point where it stops, but after the inhibitory factor ceases to act, growth may resume at a high rate, in which case the animal reaches its normal size. This phenomenon is known as compensatory growth. Growth is regulated chiefly by hormones. In vertebrates, it is regulated by the hormones of the pituitary, thyroid, thymus, and sexual glands.

In plants. Growth in plants is an irreversible increase in the plant’s height and weight caused by the formation of structural elements. The nature of growth depends on the aggregate of metabolic processes occurring in the plant. Total growth consists of the growth of cells, tissues, and organs. In higher plants, there are three stages of growth: embryonal (cell division and formation of the protoplasm’s components), extension (lengthening of the cells and thickening of their walls), and differentiation (formation of the main types of tissue from the meristem).

Plants can continue growing all their lives in local zones (meristems), in which the cells divide quickly. When a meristematic cell passes into the extension phase, numerous vacuoles appear within the cell. These vacuoles coalesce into a single one, the cell walls elongate, and the vacuoles absorb a large quantity of water. The large number of elongating cells increases, after which the specialized cells inherent in the different tissues are formed. Cell differentiation occurs both during extension and after the cessation of growth.

The types of growth that a specific organ manifests are determined by the location of the organ’s growth cone. Stems and roots grow at their apexes (apical growth). Leaves often grow at their base (basal growth). Organ growth, however, is often determined by the species. In grasses, for example, stem growth takes place at the base of the internode (intercalary growth).

The growth process in plants is rhythmic. Some growth rhythms depend both on environmental changes and on endogenous rhythms that are internally controlled and genetically fixed during the course of evolution. The growth processes are interrupted by lengthy periods of inhibition caused in the northern latitudes by the end of summer and approach of winter. Growth sometimes appears to stop, but even at these times morphogenesis may be occurring in the plant.

The various parts of all plants—cells, tissues, and organs—interact during all the stages of ontogeny. A general biological property underlies this interaction, namely, the integrity of the growing organism. This integrity is determined by the organism’s polarity and depends on external factors. The removal of certain organs or parts thereof disturbs a plant’s integrity and usually slows the growth of the other organs. The growth processes in plants are regulated by metabolites of a general type (trophic correlations) and by plant hormones (hormonal correlations). These processes also underlie plant movements (tropism and nastic movements).