Have you grown in the past year? One way of finding out is to measure your height. But has your body grown wider as well as taller, and heavier too? Body proportions change during growth so that adults have longer legs in proportion to their body and arms. And some body parts grow much faster than others. Your hair probably grows about 3 millimetres a week. If your whole body grew at this rate, then as an adult, you could be 4 metres tall. Growth is more complicated than it seems.
DIFFERENT KINDS OF GROWTH
Growth means getting bigger. So ponds grow in the rainy season, crystals grow slowly as they form in rocks and families grow as parents have more children. The growth of living things, such as trees, animals, and people, is known as biological growth. It is a special form of growth, for several reasons.
INSTRUCTIONS FOR GROWTH
Each animal or plant grows in the same way as others of its kind. For example, all golden eagles hatch from eggs and grow into adult golden eagles of similar shapes and sizes. A golden eagle egg cannot hatch and grow into a sparrow or a snake. This is because the instructions for growth are contained inside each living thing, in the form of genetic material, called DNA (deoxyribonucleic acid). All golden eagles have very similar genetic material, which is different from the genetic material of all other birds, and indeed, all other living things. This genetic material is passed on, or inherited, to children from their parents. This is why the offspring of each kind of living thing grow up to look similar to their parents.
The growth of living things is rarely a simple increase in size. It also involves changes in shape and inner parts. This is known as development. An apple seed, called a pip, does not grow into a massive pip. It develops into an apple tree with parts such as roots, a trunk, branches, buds, leaves and flowers. It also develops many new parts inside, such as sap and a hard woody centre.
Sometimes development involves a quick change to a very different body shape. This happens when a caterpillar stops crawling and forms a hard outer casing called a chrysalis (pupa). It happens again when the chrysalis splits open and an adult butterfly clambers out. This rapid change to a very different body shape happens in several kinds of animals. The change is called metamorphosis.
HOW GROWTH HAPPENS
All living things, including our own bodies, are made of tiny building blocks called cells. Growth happens in three main ways. One is the creation of more cells. One cell divides or splits into two, which then divide to make four, and so on. This makes hundreds of cells, then thousands and millions. This is how a baby begins to develop inside the womb of its mother, during the time known as pregnancy.
A second way for growth to happen is for the individual cells to get bigger. This happens when a thin, weak person becomes more active and starts to exercise. The body’s muscles grow larger and more powerful because the individual muscle cells become larger.
A third way is to add more substances around and between cells. Bones grow mainly by this method. The bone cells make increased amounts of substances called collagen proteins and mineral crystals. These increase in an amount of the cells to form the bulk of the bone.
CONTROL OF GROWTH
The genetic material of a living thing has instructions for the overall pattern of growth. Inside the various body parts, substances called hormones control increases in size and development. In plants, they are called auxins. An example is growth hormone. In humans, it is made in a tiny part below the front of the brain called the pituitary gland.
RAW MATERIALS FOR GROWTH
Any kind of growth needs a supply of raw materials, called nutrients, to build up body size. Plants get nutrients from the soil, and animals obtain them from their food.
Plants such as trees and animals such as fish, crocodiles, and tortoises, can vary the speed of their growth. If nutrients are scarce, growth might slow down or even stop. If nutrients are plentiful, growth continues. However, its rate slows down greatly in very old age.
Other living things, such as many flowers and insects, as well as birds and human beings, have a particular pattern and speed of growth. If they do not get enough nutrients, they may become ill and perhaps die. Once the adult size is reached, growth stops.