Warmth
How do you know when you are too cold? You start to shiver and you rub your hands. Both these actions start to warm you up and make you feel more comfortable. We belong to an usual group of living things: the animals that can keep their bodies at a certain temperature. There are two kinds of animals in this group ? furry animals called mammals and the birds. We have hair, which is a kind of fur and so we are a kind of mammal. None of the other animals and none of the plants can do what we do. The temperature of their bodies rises and falls with the changes in the weather.

We could not survive like a plant because all the changes in our body have to take place at a certain temperature ? 37°C. If we get a little too cold or a little too warm our lives are in danger. A plant does not have our problem. It can survive over a wide range of temperatures without having to shiver or sweat but its life does come in danger when it is cold enough to freeze.

Plants have large amounts of water in their body. The water is sucked up by the roots and travels through tiny pipes in the root and stem to the leaves. Once the water is in the leaf part of it is used to make food. When the weather becomes very cold, water freezes. It changes from a liquid into a solid called ice. When most liquids freeze they shrink a little but water does the opposite. It increases in size or expands. The ice that forms from a drop of water takes up more space. If ice forms inside a plant it takes up more space than the water and crushes the insides of the plant. When the weather becomes warmer the ice melts and the newly formed water takes up less space. The damaged insides collapse and die. If large parts of the plant have been damaged the whole plant may die. If only a small part is damaged that part may die but the plant may live on. In either case the plant suffers so it tries to avoid freezing temperatures by keeping warm.

The cold temperatures in a plant's surroundings come from the air. This means that a plant tries to hide away from cold air temperatures in winter. Many trees do this by losing their leaves. They have a thick coating of bark on their stems, which stops the water in the stem from freezing. Many garden plants have stems and leaves which die back in the winter before the frost can kill them. The plants survive under the soil as roots or bulbs or corms. The soil helps to keep away the cold temperatures of the air and stops the water in their bodies from freezing. Some plants survive the winter as seeds. The tiny plant in a seed has a double wrapping to keep it warm. Around the outside of the seed is a thick seed coat and beyond that is the soil in which the seed has fallen. Soil and seed coat keep the tiny plant from freezing.

If you go to a garden centre and look at some alpine plants you will find some which seem to be covered in fur. The fur is actually fibres made by the plant but the fibres do the same task as fur: they keep the plant warm in cold conditions.

Wild plants and many garden plants can look after themselves in winter. You may think that houseplants are safe but some could be at risk. If a plant spends its life on a windowsill it could become too cold in the autumn and winter. Closing the curtains shuts out warm air from the room and leaves the plant close to a cold window. If you have some houseplants on windowsills you could perhaps move them further into the room on the coldest of nights.

Can any plants live in the snow at the North and South Poles?
Yes. These plants are called algae and they are very small. They are so small, in fact, that they fit in between the snowflakes. In the winter the snow falls and forms a layer over the algae. In summer the algae move up through the snow into the sunlight. Their bodies make the snow pink.

Do the algae grow up through the snow?
No. On their bodies they have tiny hairs, which they can wave about. They use the hairs to push themselves up through the snow.

Are the algae pink?
Yes, they are. They have a pink substance, which protects then from ultraviolet light. This is a dangerous form of light, which can pass to the Earth if there is not enough ozone in the atmosphere. Over most of the Earth there is a layer of ozone, which gives living things some protection from ultraviolet light. At the Poles there are holes in the ozone layer so the ultraviolet light can reach down to the snow. The pink substance protects the algae from this light. The algae also have a green substance for making food but this cannot be seen because the pink substance is stronger.

Do any flowering plants grow in the Arctic?
Yes. The arctic poppy. It has large petals in its flower and moves its flower to follow the Sun across the sky. By moving the flower in this way the poppy collects extra heat to help it make its seeds.

What is an alpine plant?
It is a plant which grows high on mountains, near the mountaintops or in lands around the Poles. The edelweiss is an example of an alpine plant, which makes a furry coat to keep itself warm.

Some plants like moss grow very close together and form a cushion. Can this help to keep them warm?
Yes. Many plants in cool climates form cushions. Some of the cushions can be very large as in New Zealand and in the Andes the cushion plants can be the size of sheep and are sometimes called 'vegetable sheep'.

What may happen to a leaf that has been damaged by frost?
The parts that have been damaged will turn brown.

What happens if a whole plant is damaged by frost?
It will turn black and die.

How does a greenhouse help to keep plants warm?
When the Sun sends rays of light to the Earth it sends rays of heat too. These rays of heat are strong enough to pass through glass. When they reach the inside of the greenhouse they pass into the shelves, floor and plants. All these objects then send out some heat rays of their own. These heat rays are very weak and cannot pass through the glass so they stay in the greenhouse and warm the air. Over the course of a day a large number of weak heat rays form and make the air warm.

Why do plants need to be warm?
They need to be warm so that the water in them does not freeze but they also need warmth in order to make food and grow. On a cold winter's day there may be some sunshine but unless a plant's leaves are warm it will not be able to make food. Growth is also affected by warmth. If the temperature of a plant is increased it'will grow faster. However if a plant becomes too hot it will die.

Can any plants survive fire?
Yes. There are plants in Australia which can survive a fire passing through their surroundings. One plant, called Banksia, has cones, which must be scorched by fire before they will open to release their seeds. The presence of the fire tells the cones that the ground below them has been cleared of other plants. This means that when the seeds are released they will not have other plants competing with them for water, nourishment and light so when the seedlings sprout they will be able to grow healthily.