Growth Patterns

When size is plotted against time, the shape of the curve shows the growth pattern of an organism.Sigmoid (S-shaped) growth is the most common pattern, seen in annual plants and many animals. There is a slow lag phase at first, when there are few cells; then a rapid phase in which the growth rate is greatest; then growth slows and the curve levels off at a plateau as the organism reaches its mature size. In old age there may be a decline in size (negative growth).Humans show a modified sigmoid curve with two periods of rapid growth: in infancy and during adolescence (the adolescent growth spurt). Different parts also grow at different rates: at birth the head is about a quarter of body length, but in an adult it is about an eighth. This is allometric growth. Growth in which all parts grow at the same rate is isometric growth.Intermittent growth occurs in arthropods such as insects and crustaceans. Their hard exoskeleton cannot stretch, so body length increases only in steps, just after each moult (ecdysis), while the new cuticle is still soft. Between moults the length stays constant, giving a stepped curve, although dry mass keeps increasing steadily.Limited (determinate) growth stops once adult size is reached, as in mammals and birds. Unlimited (indeterminate) growth continues throughout life, as in trees and many fish.