Growth in a single-celled organism may be due either to an increase in the size of the cell or to an increased number of cells in one location (population growth). Thus we may speak of a bacterial cell enlarging or that the bacterial colony is in the growth phase--an increase in numbers. In an organism with many cells--vertebrates, insects, and higher plants--growth is an increase in the size of the entire organism or of one of its organs or tissues. This growth is due either to an increase in the size of individual cells (hypertrophy), to increased numbers of cells (hyperplasia), to increased production of materials between cells whether mineral (as in bone), fibers (as in tendon or scar tissue), or the gelatinous ground substance, or any combination of increase in cell size, cell number, or extracellular products. Growth depends on intake of food, which is metabolically converted to suitable chemical building components. Animal cells are not surrounded by the rigid cellulose wall found in plants and thus retain the ability to grow and divide as well as to change position. In plants these properties are restricted to specific growing cells. In general, in animals, growth is chiefly due to hyperplasia (new cells), although the increased muscle mass from exercise is the result of hypertrophy. Plants, however, typically produce very small cells which then massively enlarge, the visible growth thus being due to hypertrophy. Animal and plant growth differ in another striking way: in most animals, the growth period ends with maturity (completed development), whereas plants maintain embryonic tissues, called meristems, throughout their lives, and growth may occur at any time. Cell Growth Growth involving the production of new cells may occur by fission, budding, or filamentous growth. In fission, the parent cell divides into two smaller but equivalent cells. In budding, which is typical of certain fungi such as yeast, a small new cell (the bud) is pinched off from the parent cell and then grows into a duplicate of its parent. Filamentous growth, seen in certain algae, occurs when hairlike filaments of two or more algal cells join end to end; enlargement of the tip of the filament forms new cells. Most animal and plant cells undergo a form of fission termed \Tmitosis\t in which each daughter cell receives at least one copy of each gene present in the parent, plus approximately one-half of the cellular structures (organelles) and materials of the parent. In germ cells that form the egg and sperm, cells undergo a process termed \Tmeiosis\t in which the genes are divided between the two daughter cells. Growth occurs in two phases: immediately after mitosis, the new cell grows to adult size and remains at this size until shortly before it is to undergo mitosis. At this time, it duplicates its \TDNA\t, which contains most of the cell's genetic information, and increases in mass to less than twice normal size. The first period of growth is the most variable phase of the mitotic cycle. The cytoplasm of the cell must have a specific size relative to the size of the nucleus before cell division can occur, and the growth period must be long enough to achieve the proper relative proportions. Cell division can be prevented indefinitely by periodically cutting away portions of the cytoplasm or by starving the cell. Differentiation Growth in animals and plants must produce an unfolding of the shapes and structures characteristic of the organism, as well as increase its size or mass. The development of shapes and structures, called morphogenesis, is brought about by the coordinated action of growth and differentiation, the process by which cells develop into such specialized types as muscle, nerve, skin, leaf, or root cells. In a developing organism, different centers of growth are active at different times and proceed at different rates. This pattern of growth is called differential growth and results in the "sculpturing" of various kinds of body parts. For example, when the rate and amount of growth varies in different directions, new shapes can arise, such as elongations and altering contours; a solid mass can become hollow if the outer layers of the mass grow faster than the core; and a hollow structure can become solid if the inner layers of the rind grow faster than the outer. Differential, or relative, growth in the embryo is responsible for forming the vertebrate eye from a hollow ball of ectoderm (outermost of the three primary tissue layers of an embryo) and for forming the heart from a spongy sheet of mesoderm (middle layer of embryonic tissue). In plants, changes in shape result almost entirely from differential growth; the petiole (stalk) of a leaf, for example, develops at a rate different from that of the blade (see \Tdevelopment\t). Differential growth stops when a certain size, proportional to the whole organism, is reached. In \Lmammal\ls, growth in overall length ends after ossification (conversion to bone) of the epiphyseal plate (an area near the ends of a bone where new bone is made). At the same time, other organs and tissues cease growing, indicating that the total supply of body cells (approximately 100 trillion in adult humans) is limited by a central regulatory system. The constancy of size of organs or whole organisms merely indicates that there is no net gain or loss of cells: the death of cells is balanced by the production of new cells. Some animal tissues are capable of growth throughout life, whereas others lose this capacity after formation in the embryo. In humans, red blood cells die and are replaced by blood-forming tissue in the bone marrow at the rate of 3 million cells per second. Cells in the skin and in the lining of the digestive tract continually die and are renewed by growth. If a portion of the liver is surgically removed, cells of the remaining part multiply until the original mass is restored, and then growth ceases. Most nerve cells are formed before or soon after birth and cease multiplying soon after birth. Organs or tissues forced to function at higher than normal levels may grow to compensate for the increased demands placed on them. For example, when one kidney is lost, the other undergoes compensatory growth and enlarges; constant stress may cause enlargement (hypertrophy) of the adrenal gland, and the heart often enlarges by hypertrophy when the heart must work harder. The factors resulting in balanced growth and morphogenesis have not yet been identified, nor is it known how the different parts of a fully grown adult retain correct proportions and shapes. In a healthy body, no group of cells dominates; an effect called contact inhibition, in addition to other factors, slows or stops the multiplication of cells when they are crowded together and the group has reached its proper size. \TCancer\t is a term for a class of diseases in which growth escapes normal controls limiting cell multiplication; the uncontrolled growth results in tumors (masses of cells). Allometric Growth The proportionate growth of different parts of the animal body is sometimes called allometric growth. The role of allometric growth in evolution was first described by the Scottish biologist d'Arcy W. Thompson in On Growth and Form (1915). Thompson found that the varied body shapes of animals belonging to the same family could be represented by proportionate distortions of a single basic shape, much like the stretching of a figure drawn on a rubber sheet. The distortions, or transformations, could be expressed by simple mathematical formulas. Among the primates, for example, Thompson found that the differing skull shapes of humans, gorillas, chimpanzees, and baboons, if drawn on an imaginary rubber sheet, are simply different proportionate distortions of the same basic skull. This finding indicates, according to Thompson, that the modifications of jaws, braincase, and the regions between are regulated by a center in the body coordinating their relative growth. Regeneration All plants and animals have some ability to survive injury by regenerating lost tissue to heal wounds. Some animals, however, have the extraordinary ability to restore lost body parts. (See \Tregeneration\t.) Certain lizards can regenerate entire limbs and tails, a new hydra can grow from a fragment of its original tissue, and flatworms and starfish can do the same. Plants can repair or replace many lost parts, and sometimes new plants can grow from cuttings. Little is known about how regeneration occurs. Experiments with the salamander and axolotl have shown that regeneration of an amputated limb occurs only if a critical mass of the leg nerve is present in the regenerating stump. Similar experiments with mammals are more difficult; reportedly, however, some regrowth of the amputated hind limb of the opossum is possible under experimental conditions. It is conjectured that such experiments could eventually provide knowledge enabling medical scientists to induce regeneration of lost limbs and other structures in humans. Animal Growth At birth or hatching, the vertebrate is usually far from mature. The final stages of development involve the growth of tissues and organs generated in the embryo. Young mammals, for example, possess the elements of a reproductive system, but the system is not fully developed until the onset of puberty and the accompanying maturation of these existing structures (see \Treproductive system, human\t). Hormonal Control. Animal growth requires the coordinated action of various kinds of \Lhormone\ls. The hormones active in regulating growth in higher animals are produced by glands in the \Tendocrine system\t. Two animal groups are known to possess specialized glands that synthesize and release growth factors--the insects and the vertebrates. A hormone called the growth hormone, secreted by the \Tpituitary gland\t in vertebrates, stimulates mitosis in experimental animals and tissue cultures (tissues grown on a nutrient medium in the laboratory). Presumably by accelerating mitosis, growth hormone effects an increase in body growth. An excess or deficiency of this hormone in humans and other higher animals results in gigantism or dwarfism. Many circus giants or dwarfs are of this "pituitary" type--their growth was accelerated or retarded abnormally during early life. In young animals in which the epiphyseal plates have not yet ossified, growth hormone stimulates growth, and surgical removal of the pituitary gland inhibits growth. If growth hormone reaches excessive concentrations in an adult, characteristic overgrowth (in thickness, not length) of facial bones and bones of the hands and feet occurs, resulting in the coarse features of a disorder called acromegaly. (See \Tendocrine system, diseases of the\t.) Another hormone essential for normal growth and maturation in animals is thyroxine, secreted by the thyroid gland. In children with a deficiency of thyroxine, bone growth is slowed, secretion of growth hormone is suppressed, and cretinism results. Tadpoles never become frogs if their thyroid gland is removed. The hormone \Tinsulin\t, secreted by the \Tpancreas\t, stimulates growth by accelerating the manufacture of protein. Failure to grow can be a symptom of insulin deficiency (\Tdiabetes\t MELLITUS) in children. \Tsex hormones\t, produced in mature ovaries and testes of vertebrates, maintain secondary male-female characteristics, including different patterns of growth, and differences in muscular development and the amount of fat under the skin. Invertebrate Growth. The postembryonic development of invertebrates can be markedly different from that of vertebrates. The young of many insects and echinoderms hatch in a larval form bearing little resemblance to the adults of their species. (This occurs rarely in some vertebrates, such as the frog, which passes through a larval tadpole stage.) In the development of the adult butterfly, for example, the process of \Tmetamorphosis\t converts the larva (caterpillar) to its adult form. The caterpillar grows to many times its original size and then forms a pupa, from which the adult arises. The wings and other structures are developed internally through a series of larval molts (shedding of the hard outer cuticle; see \Tmolting\t). Such development is called complete metamorphosis. Other insects, such as grasshoppers and cockroaches, undergo incomplete metamorphosis: the young resemble the adult except for poorly developed wings and other organs, and development occurs through both relative growth of various parts and a series of molts. Insect metamorphosis has been studied extensively and has been found to be regulated by two hormones: \Tjuvenile hormone\t, secreted by cells behind the brain and so named because it blocks the development of the adult form; and ecdysone, secreted by a gland in the thorax and initiating molting. Insect form is determined by the balance between the levels of the two hormones. Ecdysone appears to be the molting hormone for all other arthropods, as well as insects. Juvenile hormone is more specific; the juvenile hormone active in one insect group may have no effect in other insects. Human Growth At birth, a child weighs about 3.4 kg (7.5 lb), an increase of about 3 million times the weight at conception. Each child has an inherited pattern of skeletal development. In the embryo, most bones begin as connective tissue and gradually change to cartilage, which forms the model for the hard bones of the adult. Deposition of fibers and salts, chiefly a calcium salt similar to the mineral apatite, causes the cartilage to ossify. Ossification begins in spots called ossification centers; the rate of ossification is an index of growth because the maturation of the skeleton is an indicator of the development of the remainder of the body. The "bone age," determined by X rays, therefore indicates the state of the individual's maturation. Growth patterns and the ultimate height attained by an individual are also influenced by hormones, nutrients required for growth, and even the individual's emotional environment. The most important environmental factor affecting growth is diet, which must include adequate protein, essential vitamins and minerals, and calories. Injury and disease stunt growth because they increase protein catabolism (breakdown of body protein in metabolism, as a response to stress). The yearly growth in height, or "height velocity," measured in inches or centimeters per year, is determined by the net effect of hereditary, hormonal, and environmental factors. Height velocity is not evenly spaced over the 12 months of the year, and periods of rapid growth may follow periods of slow growth and vice versa. Following illness in children can be a period of "catch-up growth." Puberty. The number of years during which growth occurs is largely dictated by the age at which puberty occurs, because the hormones initiating puberty also terminate the process of physical growth. These hormones are the sex hormones: androgens, secreted by the testes in males and the adrenal gland in females; and estrogens, secreted by the ovaries and the adrenal gland in females. Androgens initially stimulate growth, but ultimately terminate growth by causing the epiphyses to fuse. A growth spurt occurs at the time of puberty, due largely to stimulation of protein synthesis by androgens. The body changes that develop in girls at puberty, in addition to enlargement of the breasts, uterus, and vagina, are due in part to estrogens and in part to the absence of testicular androgens. Women tend to have narrow shoulders, broad hips, and thighs that converge and arms that diverge ("wide carrying angle"). This body pattern, plus the female distribution of fat in the breasts and buttocks, is seen also in castrated males. The presence of testicular androgens in males is responsible for the typical male body pattern of broad shoulders, enlarged muscles, and lengthened and thickened vocal cords, which deepen the voice. Before puberty, however, most children of the same age grow at the same rate. The child who seems too short or too tall may actually have a normal growth velocity for his or her age. Exceptionally short or tall children may simply represent one extreme or the other of the normal range of sizes. The range of normal heights at each age of childhood has been well established and is the basis for the growth curves known as the Wetzel grid. The grid is a chart indicating the normal range of heights at each age and is used by physicians for predicting the growth of children. The average height of a 2-year-old boy, for example, is about 87.5 cm (35 in), but most children will be between 80 and 97.5 cm (32 and 37 in) tall. From the third year of life until puberty, growth velocity for most children continues at a rate of about 6.25 cm (2.5 in) per year. Growth in height stops only after pubertal changes are complete. By the time of the first menstrual period (usually at the age of 11 or 12), the growth velocity of girls is already slowing down. Puberty starts and finishes about 2 years later in boys. If puberty occurs at an unusually early age, growth will cease earlier than it does with other children the same age and may result in short stature. If puberty is delayed, growth will continue for a longer period of time than is the case with other children of the same age. The bone age, determined by an X ray, may be used to estimate the chronologic age at which puberty will occur and the number of years of growth potential remaining in a particular child. Hormonal Effects. During the prenatal period, thyroxine stimulates the formation of bone from cartilage and affects the contours of the face and proportions of the body. A deficiency of thyroxine (hypothyroidism) results in a dwarfed adult with infantile features (\Tcretinism\t). In adults, thyroxine is necessary to maintain a normal level of growth hormone secretion. African pygmies have been shown to have normal levels of growth hormone, but their tissues appear to be subnormally responsive to this hormone, probably due to a decrease in the number of receptor sites for the hormone on the surface of body cells. Other causes of short stature include--in addition to pituitary insufficiency, hypothyroidism, and sexual precocity--the syndrome of inherited deficiency of growth hormone. Individuals with this syndrome are called sexual ateliotic dwarfs and remain short; however, they mature sexually and women with this disorder can become pregnant, deliver, and lactate (produce breast milk). Alterations in height are also seen in gonadal dysgenesis, a group of disorders in which there is faulty duplication of the sex chromosomes. Those having XO are dwarfed and those with XYY are very tall. Normal females are XX and normal males XY. Various bone and metabolic diseases also cause stunted growth, and in many cases there is no known cause ("constitutional" delayed growth). Size Limits of Animals The possible range of size of animals is determined by such factors as the ability of the animal to support its own weight, the adequacy of blood circulation, metabolic rate, and whether the animal is homothermic (maintains constant body temperature) or poikilothermic (has a changeable body temperature, depending on the surroundings). In the evolution of invertebrates, the development of an adequate circulatory system may have been the first requirement for growth beyond a certain size, except where the body is filamentous or greatly flattened. Circulation, together with branchial (relating to gills) respiration, permitted crustaceans and mollusks to grow to considerable size. Insects are severely limited in their ultimate size, as their respiratory system consists of tiny tubes that carry air from openings in the body wall to every cell. Data compiled for many types of animals indicate that metabolic rate is a major factor limiting the size of animals. Within a given group of animals, the metabolic rate (per gram of body weight) invariably increases with diminishing body size; the higher metabolic rates require correspondingly higher food intake, faster digestion, and increased circulation. Hypothetically, an animal might be so small that it would have to eat so much so quickly and expend so much energy looking for food that it actually could not exist. For example, the smallest mammals, shrews, may weigh as little as 4 g (0.14 oz) and must consume nearly their own body weight in food each day; if deprived of food, a shrew's high metabolic rate consumes body reserves so fast that it starves to death in a day or less. Land animals cannot increase to unlimited size, because their muscles and bones could not support the weight. As size increases, weight increases more rapidly than strength. The largest living land animal, the elephant, weighs about 3,600 kg, whereas the largest of animals, the blue whale, weighing up to 130,000 kg, is aquatic and fully supported by the surrounding water. Plant Growth Plants grow by mitotic cell division from a single original egg cell that has been fertilized by pollination. In this growth, cells multiply and then enlarge and differentiate into specialized types as root, stem, and leaf cells. The formation of flowers, fruits, and seeds is usually considered an aspect of plant reproduction or plant propagation, not plant growth. Plant growth occurs only in tissues called meristems, in which, under favorable conditions, new cells are more or less continually being formed as a result of repeated mitosis. The tip of every shoot or branch has a shoot-apical meristem, and each root tip has an analogous root-apical meristem. The shoot meristem ultimately forms leaves, and root meristems develop the more extensive root system needed for continued growth. Rapid mitosis in the shoot- and root-apical meristems of germinating seeds (see \Tgermination\t) produces the young seedling. These meristems maintain embryonic tissues capable of growth throughout the life of the plant, producing stem and root systems, leaves, and flowers. The growth activity of these meristems is called primary growth. Stems thicken as a result of mitosis in another meristem called cambium, which is present in the stems of gymnosperms (seed plants in which the seed is not enclosed in an ovary) and most dicotyledons (plants having two seed leaves, or cotyledons). Cambium also generates the woody xylem, the tissue through which water and minerals flow upward from the roots to every leaf. Still another portion of cambium generates phloem, the tissue that transports products of photosynthesis from leaves to other parts of the plant. In trees, the so-called cork cambium produces most of the tissue called bark. The growth activity of the cambium producing other tissues is called secondary growth. Embryonic Growth. The initial growth of the fertilized egg proceeds only until the resulting embryo is "packaged" in a seed case. Further growth into an adult occurs after germination. The fertilized egg of a flowering plant, for example, first develops a short filament of cells, one end of which then burgeons to form a small spherical embryo. The remaining portion of the filament, called the suspensor, elongates and pushes the embryo into the surrounding nutrient tissues; because plant egg cells have no yolk, they are normally embedded in a mass of cells that contain nutrients for the developing embryo. Further cell divisions transform the embryo into a torpedo-shaped body. The surrounding tissues meanwhile develop into a seed coat and a store of food. Finally, the embryo is packaged in the coat to await germination. Growth Rings of Trees. It is well known that the age of a tree may be estimated by counting the "annual rings" showing on the face of a cut stump, allowing one ring for each year of age. The annual rings are more accurately called growth rings, because they are not necessarily formed at the rate of one per year. In temperate regions, a growth ring is formed by the difference in size between the wood cells formed in the spring and those formed in the previous summer. In the spring, growth is rapid and the cells formed are large; as spring wears into summer, growth slows and smaller cells are formed. A dormant period occurs in the winter and is followed by a surge of new growth the following spring. In tropical forests, with no unfavorable growth season, trees do not normally show growth rings. Light and Growth. Plants require light for photosynthesis, and so plant growth is attuned to the length of the day. The growth response of plants to changes in the duration of light is termed photoperiodism. Plants grown in temperate regions may be divided into three groups according to the effect of different exposures to daylight. So-called short-day plants develop flowers only if they are illuminated for less than 12 hours daily. Such plants include violets, asters, and some varieties of strawberries and wheat. Long-day plants develop flowers only if the daily photoperiod is longer than 12 hours. Long-day plants include clover, beets, lettuce, and other varieties of wheat. Some plants produce flowers regardless of the length of the daily photoperiod. Examples of these plants are tomatoes, cucumbers, cotton, dandelions, and sunflowers. REVIEWED BY A. D. FREEDMAN Bibliography: Berrill, Norman J., Developmental Biology (1971) and Growth, Development and Pattern (1961); Black, Michael, and Edelman, Jack, Plant Growth (1976); Brookbank, John W., Developmental Biology: Embryos, Plants and Regeneration (1978); Fogg, G. E., Growth of Plants (1975); Graham, C. F., and Wareing, P. F., The Developmental Biology of Plants and Animals (1976); Sinclair, David, Human Growth after Birth, 2d ed. (1973); Sussman, Maurice, Animal Growth and Development, (1964); Smith, David W., Growth and Its Disorders (1977); Tanner, J. M., Fetus into Man (1978).