Growth is an increase in the number or size of a living thing’s cells. All living things grow. A seed can grow into a plant. The giant redwood grows from a seed 1/16 inch (1.6 millimeters) in diameter to become a tree often more than 300 feet (91 meters) tall. A microscope must be used to see a whale egg, but a full-grown whale may be up to 100 feet (30 meters) long and weigh about 160,000 pounds (72,600 kilograms). All living things do not grow to the same extent. As a result, species differ in size. Adult guinea pigs are only five times heavier than baby guinea pigs. But adult elephants are usually 40 times heavier than baby elephants.
How growth takes place
In living things.
Every living organism consists of cells. Each individual begins life as a single cell. The cell takes in materials and converts them into the building blocks that it needs to grow. Thus, the single cell grows from within. This cell can multiply and divide to form other cells. The process of building, multiplying, and dividing is growth. It continues until the organism is fully developed.
As the cells grow, they also change in character. Some grow into the tissues that form skin. Others grow into muscle tissues. Still others form body organs such as the heart, lungs, and liver. This growth and organization of the cells into specific structures is called differentiation. The process of differentiation follows definite rules. The seed of a redwood always grows into a redwood tree, not into a pansy or an oak tree. A dog’s tail always grows at the base of the spine, not between the ears. The process is controlled by the cell’s hereditary traits. Heredity influences certain chemical and physical processes in the cell to make it grow into an organism having the traits of its parents. See Heredity.
Other kinds of regulators are also produced by the cells. As growth proceeds, certain cells develop that produce specific substances that influence and regulate the further development of the organism. These substances are called hormones, and are found both in plants and in animals. See Hormone.
In nonliving things.
Some substances that are not alive also exhibit a kind of growth. Objects like rocks can increase in size if they are in a proper environment. But nonliving things do not consist of cells and therefore do not grow from within. The growth of a rock or mineral is growth by accretion. For example, water dripping from a cave ceiling leaves behind exceedingly tiny mineral particles. These particles cling together until, after hundreds of years, they form great “icicles” of stone, which are called stalactites.
Human growth
Early development.
In its earliest stage of development, the human organism is called an embryo. The embryo develops slowly. When the embryo is about 2 months old, it is only about 11/2 inches (3.8 centimeters) long, but it has the form of a human being. All the parts of the body are well formed. However, the head is quite large compared with the trunk and limbs. At this time, the fully developed embryo becomes known as a fetus.
At 7 months, the fetus weighs about 2 pounds (0.9 kilogram) and is about 15 inches (38 centimeters) long. Two months later, or just before birth, the fetus usually weighs about 6 to 8 pounds (2.7 to 3.6 kilograms) and may be from 19 to 21 inches (48 to 53 centimeters) long. Thus, by the time babies are born, they are growing at an extremely rapid rate.
Growth rate.
The rate of growth depends on the balance between the rate of synthesis (formation) and the rate of breakdown of body materials. The rapid rate of growth during the first part of a baby’s life occurs because the rate of synthesis in the baby’s body far exceeds the rate of breakdown. Boys and girls grow most rapidly during the first two years of their lives. Then, their growth rate becomes gradually slower until they reach adolescence.
For about two years during their adolescent period, boys and girls suddenly grow rapidly. This is what some people call the “adolescent spurt” in height growth. Although all adolescents develop this spurt in height, the age at which it begins and the length of time it continues vary with the individual.
Adolescent boys usually exhibit this spurt of growth between the ages of 12 and 14 years. Their gain in height often ranges from 4 to 12 inches (10 to 30 centimeters). But the fastest rate of growth in boys may take place any time between the ages of 12 and 17.
The adolescent growth spurt in girls usually begins about 2 years earlier than in boys, and their maximum growth rate is somewhat less than that of boys. Women are generally smaller than men, chiefly because of this difference in the adolescent growth spurt. Until that period is reached, the average heights of boys and girls are almost the same.
Comparative rates of growth.
Some children are fast growers, and others are slow growers. Some individuals have large, heavy bones and weigh more than individuals with small, light bones. Thus, height-weight-age tables are not necessarily a good indication of what a child should weigh.
Studies indicate that people have increased in size over the course of time. Suits of armor worn by knights of the Middle Ages are too small for an average man today. From 1876 to 1976, the average height of 18-year-old males in the United States increased by about 4 inches (10 centimeters). However, statistics indicate that Americans probably have reached their maximum growth potential, and that the trend of increasing height has ended in the United States.
Skeletal age.
Because children grow and develop at different rates, scientists often assign developmental ages rather than chronological, or calendar, ages to growing children. Doctors often evaluate growth on the basis of the growth of the bones of the skeleton. If children are developing slowly, they could actually be 6 years old, but their bone structure might only have reached that of a 5-year-old. The skeletal age of people who mature late could run below their actual age until they become adults. Certain diseases may slow down bone growth. Doctors are able to determine the extent of such retardation, or slowing down, by determining their skeletal age.
When people stop growing.
Most healthy human beings stop growing some time between the ages of 18 and 30. But their weight may continue to increase until they are well into their 40’s. Some time after people stop growing in height, they actually begin to shrink. This decrease in height is exceedingly slow and usually does not become noticeable until a person reaches old age. It is caused by a thinning of the pads of cartilage that grow between the bones of the vertebral column, or backbone. The curves in the vertebral column in elderly people also tend to become greater. This results in the bent appearance of many old people, as well as a further reduction in height.
Factors affecting human growth.
Tall parents usually have tall children, and short parents have short children. Therefore, the tendency to be tall or short seems to be largely hereditary. However, tallness appears to be recessive. This means that short parents may have tall children. When one parent is tall and the other short, the children may be tall or short or even midway between. This occurs because the hereditary pattern is complex, and other factors are often involved.
Hormones produced by glands in the body modify growth. Secretions from the pancreas, thyroid gland, and pituitary gland particularly affect growth (see Gland). These secretions—insulin, thyroxin, and growth hormone—greatly influence cell size and cell number. Too little of any of these hormones can slow growth. In hormone-deficient patients, treatment with the particular hormone increases growth.
Foods are also a factor in growth. Certain foods but not others contain materials essential for growth. To promote proper growth, a person should eat a balanced diet that contains proteins, minerals, and vitamins, as well as sugars, fats, and starches. See Nutrition.
Other factors can affect growth. Exercise may promote muscle growth and inactivity may slow it. Communicable diseases or infections interrupt growth. Stress and emotional disturbances can also interfere with it.
Influence of growth.
During spurts of growth, children use energy in the process of growth itself. They need additional food and rest. It is more difficult to study, to pay attention, and to withstand strain during this time. Children should not worry if they seem to be growing much more slowly or rapidly than their friends. They should realize that everyone grows at a different rate.
