Why and how do things grow? John Stringer examines a characteristic of living
Living things grow; but children seldom recognise growth as a condition of life. Why should they, when animals do much more exciting things like moving and eating, and plant growth may be too slow to notice at all?
In spring, plant growth is seen at its best. As our world wakes from winter, let's explore the growth of plants, animals and children.
As the ground warms up in spring, seeds begin to germinate. They split their seed coats, and put out first roots, and then shoots.
The roots grow down, taking moisture from the soil. The shoots push up to break into the light. Leaves spread to catch the light. A new plant is growing.
If you look inside a seed, you will find a huge food store. The new plant needs this food to grow, before it can make food for itself. You won't find a perfect plant inside the seed; but often you can see the parts that will grow into the root and shoot.
A similar thing happens with mammals. If you were to look inside a fertile hen's egg, you would see a chick beginning to develop. Like the seed, most of the egg is a huge food store. The chick is a tiny scrap and at first, you can't see the head and legs. But as the chick grows, and the food store disappears, head and body, legs and wings, begin to form. The hen's egg and the womb of a mammal are like ponds in which new animals can develop.
Questions for discussion * What is special about the spring? Why do many plants start to grow then?
* Why do many animals start their lives in spring?
* Many animals grow throughout their lives. Do they grow faster in the spring?
Babies in the womb are growing all the time. When newly-born, a child is heavily dependent upon its parents, but later it will develop skills that allow it to cope with life
Living things grow: plants germinate from seeds; eggs hatch; mammals are born. Plants make their own food, with energy from the Sun. Carbon dioxide gas from the air and water in the ground combine to make more plant material. As they add this material, the plants get bigger.
Animals need plant or animal food to grow. Most of the food they eat gives them energy to live; but some is changed and added to their bodies. This new material repairs, renews and may make the animal bigger.
As they grow, animals may be able to do more, or to do things differently. Growing children are learning and developing; they can do more than they could as babies.
Questions for discussion
* Not all plants grow from seeds. Name some other ways that new plants can start their lives.
* Name some animals whose bodies change completely as they grow. Why do they change like this?
NEEDS FOR GROWTH
Animals need the right conditions to grow. Without food, water and oxygen, animals cannot live. Many animals have other needs - shelter and warmth.
Seeds also need the right conditions to germinate. Plants need the right conditions to grow. Without warmth, water and sunlight, there will be no new plants.
Plants and animals grow very fast at first. Later, growth slows down and, for some plants and animals it eventually stops.
Children grow at different speeds. Some grow fast at first, and slow down later. Others may be small at first, then grow quickly later. It is hard to tell how tall someone will be until they are an adult. Children need care - they cannot look after themselves until they have grown up.
Questions for discussion * Why do plants need sunlight to grow? Would a green plant grow in the dark?
* What other animals care for their young until they are able to care for themselves?
Young children find growth a difficult concept to grasp because:
* They may not recognise growth as a characteristic of living things - especially of plants.
* They may think that growth is something that happens once a year - "I get bigger on my birthday".
* They may not understand that some things attain a maximum size, while others go on growing throughout their lives, or that growth is not a continuous process, like expansion.
* They may have difficulty with the idea of growing by adding material, rather than by swelling.
* Spring offers a good opportunity to tackle these issues. Many seeds, bulbs, corms (an underground stem)and tubers respond to the growing warmth of the soil by germinating or sprouting. Many animals - including insects and birds - end their dormant period, and lay or hatch from eggs.
The questions lead to answers like those shown below: Life begins * Seeds respond to the warm weather. This is a good time to germinate, since plants need light. The days are getting longer in spring, and the Sun shines more strongly. New plants have the opportunity to live out a full life cycle - including flowering, pollination and seeding. Growing in the spring and summer gives the best chance of the continuation of the life of the plant.
Getting bigger * Plants grow from bulbs, corms, tubers and rhizomes as well as seeds; some form runners, too. New plants can begin to develop from damage to a parent plant, and we make use of this when we graft new stems onto older plants.
* Some animals change by metamorphosis; butterflies and frogs are the most familiar. Flies, moths, crickets and many other kinds of insects, however, go through larval stages, as do crabs, shrimps and other crustaceans. Metamorphosis enables animals to have a feeding, growing stage usually followed by a mobile, mating stage.
Needs for growth * Green plants are able to manufacture their own food, harnessing the energy of the Sun to combine water and carbon dioxide to produce the materials for growth and renewal, as well as the energy needed to live. This miraculous process - photosynthesis - is essential for life on Earth. It cannot take place in darkness. Green plants can grow in the dark - indeed, seeds almost always germinate in the dark- but the plants grow long and yellow in their search for light, using up their food stores as they do so.
* As a general rule, the more developed the animal, the greater the degree of parental care; many animals simply lay eggs and leave the young to their fate. Birds and mammals, however, have ways of reproducing that result in poorly developed young to look after. A hen's egg or a mammalian womb cannot contain a fully developed adult, so in both cases, there has to be parental care. Another general rule is that mammals born in the open - deer, horses, elephants - have young born well advanced since development has to be swift if they are not to be killed by predators. Mammals born in shelter - rabbits and mice, for example - are less well developed since they will not be facing the dangers of the world so soon. There are special cases of parental care, like the seahorse, where the males care for the young in a pouch to prevent them from being swept away by currents.