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Chemicals are used to mark territorial boundaries and to identify individuals in a group. The chevreuil, a type of deer, marks its territory by rubbing a gland near its eye on branches. In this way the male warns other males that they are in his territory and tells the females of his presence. Cheetah use urine and bears use body odours to mark their territories.
Chemical stimuli are very important to social animals. The body of a queen bee produces a chemical called queen substance. This chemical is licked from her body by the attending workers. It is then passed to other workers by mouth to mouth contact, in the saliva. Queen substance has a physiological effect on the worker bees; it stops the development of their ovaries, keeping the workers sterile.
Many animals rely on chemicals to find their mates. Snails and slugs which live on land find their partners by following a slime trail. They keep tasting the trail to make sure that they stay on the right track. Likewise, the male wandering spider finds a female by following the scent of her silken dragline and the male snake tracks down a female by following her skin odour. The male bumble-bee marks leaves with his scent to attract female bumble-bees to him. The aquatic salamander produces chemicals which "persuade" the female to mate with him. To make absolutely sure that these chemicals reach the female, he moves his tail from side to side in the water so that the chemicals are sent towards her.
Filter feeding is a method of feeding used by invertebrates. Some types of jellyfish, molluscs, starfish and arthropods are filter feeders. Some vertebrates also filter their food. Certain fish, birds and mammals feed in this way.
Two of the largest types of shark, the basking shark and the whale shark, eat plankton which they filter out of the water.
The fossil fuels we use today all have their origins in photosynthesis. Coal was formed around 370 million years ago when large trees fell into swamp land. Over millions of years these trees were pressed, at high temperatures and pressures, by the earth on top of them and they gradually changed into coal. The trees in these ancient swamp forests photosynthesized and made organic matter. It is the organic matter they made which we now burn as coal to provide us with energy.
We also need to go back millions of years to discover the origins of oil and natural gas. In those times the Earth was not the same as it is now. Parts of the Middle East and the United States were covered with sea water. When the tiny animals and plants which lived in these seas died, their remains fell to the sea beds. Over millions of years the positions of the seas changed as the land moved. The remains of the animals and plants decayed and were covered by land. Under high pressures and temperatures, these remains were changed into oil and natural gas. We now drill through the earth to reach the ancient sea beds and extract the oil and gas held above them. The original tiny plants photosynthesized and the tiny animals fed on the plants. Oil and natural gas are what is left of their organic matter.
The fungi are well known for their mushrooms but, in fact, most types of fungi are microscopic. The microscopic forms, such as moulds, become visible only when hundreds of thousands of individuals are seen spoiling a piece of fruit. Examples of fungi are yeasts, slime moulds, puffballs and Penicillium.
Many fungi do produce mushrooms. Some of these are good to eat but others, called toadstools, are poisonous. Some moulds are used in making cheeses, such as Camembert and Roquefort.
A large number of fungi feed upon the dead bodies of animals and plants. They digest this food and so cause it to decompose. Then they absorb the digested food. These fungi are very important since they decompose dead, organic material and release minerals into the soil, making the soil more fertile.
Some yeasts feed on the sugars made by green plants. We use these yeasts to make wine, beer and bread. Fungi are also used to make medicines. The mould Penicillium produces a chemical, penicillin, which is an antibiotic and helps us fight disease.
Unfortunately, some fungi decompose things that we want to preserve, such as food, paper in books or wood in our houses.
The essential elements which plants require are called macronutrients. These are nitrogen, phosphorus, sulphur, magnesium, potassium, calcium and iron, as well as oxygen, hydrogen and carbon. Plants also need very small quantities of other elements called micronutrients. These include molybdenum, zinc, boron and copper. Land plants take most of these nutrients from the soil and water plants absorb them from the water which surrounds them.
Van Helmont's Experiments on Plant Growth
The question of how plants feed was investigated in the 17th century by a Dutch scientist called Van Helmont. He took a small willow tree and planted it in a large pot of soil. Before he did this he carefully found the mass of the dry soil and the mass of the tree. He covered the soil with a lid so that nothing could fall onto the surface of the soil and add to its mass. There were holes in the lid so that the tree could grow out of the soil and so that air and water could reach the roots.
Van Helmont left the tree for five years, giving it only rain water to drink. At the end of the five years he took the mass of the tree and the mass of the dry soil for a second time. The results of this experiment are shown below:
The tree had grown a lot, its mass had increased by just over 74kg. The soil, however, had only lost about 60 grammes. Van Helmont thought that the tree must have grown from the rain water only. He did not believe that the soil had given any food to the plant.
Scientists have been studying photosynthesis for a long time. Van Helmont carried out his experiments in 1668. By 1771, other scientists had discovered that plants produce a gas which supports combustion when they photosynthesise. Eight years later, in 1778, it was discovered that light is necessary for photosynthesis and by 1782, scientists had realised that carbon dioxide is necessary for photosynthesis to take place.