Water

Would you like to go up into space? If you did, you would see that the Earth is almost a blue planet. The blue colour is due to water. It covers nearly three quarters of the Earth. Most of the water forms the oceans and the seas. As you moved round above the Northern Oceans or the Southern Oceans in springtime you would see a change in the water. Instead of the water being blue, it would be pale green. This colour change is due to billions of tiny plants.

In the springtime, the sunlight over the ocean increases. This makes the tiny plants that live in the ocean start to breed. Each plant has a very simple body without a stem, root, leaves or flowers. In fact, these tiny plants belong to a group of living things called algae. If you were to look at some algae with a microscope you would see that each one is like a box containing a green substance. You would also see that the boxes have many different shapes and some of them have spines sticking out of them. There are algae present in the upper waters of the oceans at all times of year but when the Sun adds extra light and heat to the water surface in spring, the algae breed rapidly. It is the billions of algae bodies that make the water green.

If you were to return to Earth and land close to the ocean shore you would see larger green plants clinging to the rocks. These plants do not have flowers, stems or leaves either and belong to the algae group. We call them seaweeds. A seaweed is made from a flat, slimy strip called a frond. Some fronds have small balloons in them called bladders. The bladders help a frond to float and keep the seaweed in the sunlight. At one end of the frond is a holdfast. It looks like a root of a flowering plant. The holdfast grips the rocks on the shore and holds the seaweed firm so it is not washed away by waves.

Plants need water to make food. The algae in the ocean and the seaweed on the shore are surrounded by it and have no problem getting all the water they need. Other plants, those that grow on land, are not so lucky. Land plants get their water from the rain. It splashes on the ground and sinks into the soil. The wetness of the soil depends on two things. It depends on how often it rains and how well the soil lets the water drain through it. The wetness of the soil affects the plants that can grow in it. If it rains often and the soil is well drained, as in many places in Great Britain, there is always some water present in the soil for plants. This means that a wide range of plants can grow in the soil. In places like deserts, it rarely rains. This means that for much of the year the soil does not contain any water. It does not mean that some plants cannot survive in the desert. A few plants, such as cacti, have long roots to find all the water they can after a shower then store it in their spiky stems.

While too much water is not a problem for algae, it is a problem for many land plants. This is because plant roots need air. In most soils, even wet ones, there are air spaces between the lumps of soil. The roots can find the air they need from these spaces. In soils which drain badly, there is a danger of the air spaces filling up with water. When this happens, water may even form pools on top of the soil and we say that the soil is waterlogged. Many plants can survive a short time in waterlogged soil but if air does not return to the soil soon, they die.

Water is necessary for all plant life and many people think that all their houseplants need is plenty of water to stay healthy. This is fine if you are keeping seaweed or swamp plants but most houseplants grow naturally in well drained soil. Help them stay healthy by keeping their soil damp but not waterlogged.

Why do algae have spines?

The spines help them float high in the upper waters of the oceans. They stop them sinking quickly and this gives them a better chance of catching currents of water that bring them back to the surface. Some algae have drops of oil in their bodies which help them float near the surface.

Why do algae have to stay near the water surface?

This is the place where there is the most light. As light passes onto the ocean the water absorbs it so the deeper you go the darker it gets. Not far below the ocean surface there is too little light for the algae to make food. They have spines and oil to help keep them in the water where there is plenty of light.

What happens to all the billions of algae?

Most of them are eaten by animals. Around the algae are billions of tiny animals. Many are like tiny shrimps. There are the young offspring of crabs and lobsters too. This huge group of algae and tiny animals is called the plankton. It is eaten by larger animals such as krill and fish.

Is rainwater the same as sea water?

No, it isn't. Sea water has a large amount of salt in it. Rainwater does not have any salt in it. The rainwater drains through the soil to form streams, rivers, ponds and lakes. The water that collects in these places is called fresh water.

Can land plants stay healthy in sea water?

Most of them would die if they were watered with only sea water. A few flowering plants, which grow close to the shore, can survive being sprayed with sea water.

Can seaweed survive in fresh water?

No. It would die. The plants that you see living in fresh water belong to the flowering plant group. There are some kinds of tiny algae which can live in fresh water. You sometimes see them making the water green in a badly maintained aquarium tank.

Where do roots take up water from the soil?

They take it up near their tips. There are large numbers of small white hairs near a root tip. They grow between the spaces in the soil and suck up the water from the soil.

Why do the roots need air?

They need it to take up nourishment, from minerals, which are in the soil water.

Do plants breathe?

In a way they do. They take oxygen from the air as we do and give out carbon dioxide as we do. They use the oxygen to get energy from their food so they can stay alive. There are tiny holes on the underside of the leaves and on the sides of the stem which take in air so the plant can use the oxygen in the air.

How do swamp plants survive in waterlogged soil?

There are spaces in their stems, which take air down to their roots to keep the roots alive.