Mountain-tops are usually bare of trees because growing conditions are not suitable for them. The soil is usually very thin on mountain-tops, and the location is too cold and exposed for trees to grow without damage. Some trees can grow there, but they are usually dwarfed and flattened by the wind. Sometimes, although they are trees, they look more like mosses.
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The oldest-known trees are bristlecone pine trees. They grow in the White Mountains in California, in the USA. Although they are quite small, some of these gnarled trees are more than 4,500 years old. The oldest tree is named Methuselah. It is thought to be more than 4,600 years old.
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Every year as a tree grows it deposits a new layer of cells on the outside of its trunk, beneath the bark. This new layer is called an annual ring.
By counting the annual rings it is simple to work out the exact age of a tree, Even in the case of a living tree, it is possible to drill into the trunk and remove a sample in order to date the tree without causing any permanent damage.
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Some trees that grow in soft, wet ground or very thin soil sometimes develop buttress roots, which stop them from blowing over inn strong winds. These roots are like very large, flattened plates that stiffen the base of the tree. Some trees which grow in very stony ground produce roots that actually run over the surface of the ground in search of softer soil .In this way they grip the rocky surface to stabilize the tree against the effects of winds.
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Trees are the largest living organisms that live on Earth. The biggest tree is the Californian giant redwood, which grows nearly 100 m high and can have a trunk that is 11 m thick. The total weight of one of these trees is more than 2,000 tonnes, which is more than 10 times heavier than a blue whale.
These ancient giant trees have very few branches and leaves, and are often scarred by fire and lightning strikes.
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Conifers produce their seeds without the aid of flowers, whereas deciduous trees are actually flowering plants. Conifers produce cones, which are whorls of papery scales packed tightly together to protect the seeds developing inside them. Male and female cones are produced separately, and pollen shed from the male cones fertilizes the egg cells and leads to the production of seeds. The seeds, which are usually winged, are released in dry weather when the scales of the cone open up and release them. Most conifers keep their needle-like leaves throughout the winter, although a few types are deciduous.
What are deciduous trees?
Tree that lose their leaves in winter are ailed deciduous trees.
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A non-woody plant is held upright by the pressure of water in its cells. These cells are tightly expanded like balloons. If too little water reaches the cells, as happens in a drought or if the stems are damaged by disease, the cells soften and the plant sags or wilts. Usually it can stiffen up again when water is available, provided the plant has not dried out too much. Woody plants can also wilt at the tips of their shoots.
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Plants that cannot survive the winter are called annuals. They grow very quickly when their seeds germinate in the spring, because they have to complete their entire lifecycle in a short time.
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Plants that are about to lose their leaves in the autumn conserve their food supplies by withdrawing all the nutrients from the leaves. Chlorophyll is broken down in the leaves, causing their colour to change. Eventually all the nutrients are removed from the leaves and they wither and turn brown.
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Perennial plants survive from one year to the next. They usually grow quite slowly, and can afford to build up their strength before they need to produce seeds. Another group of plants, called biennials, grow in one year and flower in the following year before dying.
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Fertilizers contain some of the essential chemicals that plants need to stay healthy. Nitrogen is one of the most important chemicals, and it is in constant demand from the plant. Nitrogen cannot easily be obtained from the air, except by plants that have organs containing nitrogen-fixing bacteria. Fertilizers used on crops and garden plants usually contain nitrogen, phosphorus, iron, potassium and several other minerals that can boost the plant’s rate of growth.
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Water reaches the leaves by several methods. Transpiration is the evaporation of water through the leaf’s surface. As the water evaporates, more water is drawn up the plant’s stem. The other method consists of water and dissolved minerals being pumped up the stem under pressure from the roots. A tree trunk ‘bleeds’ when it is cut because the sap is pumped out under pressure until the cut dries over.
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Hormones called auxins are responsible for a plant growing towards the light. Auxins cause the cells of a shoot to lengthen. They are also destroyed by light, so if the plant leans over, auxins collect on the shady underside. They cause the cells on the underside to grow longer, pushing the shoot upright, usually towards the light. Plant shoots are able to grow away from the pull of gravity.
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Several outside factors influence the way in which roots grow. They grow away from light and are pulled downwards by gravity, as you can see in simple experiments with large seeds such as beans or peas. This downward growth naturally takes the roots towards water in the soil. Some roots seek water more actively, for example trees such as willows. They send out roots that extend well beyond the spread of their branches; these roots may enter the joints of water pipes and sewers in search of water.
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The embryo in a seed contains everything that the seed needs to grow. The seed will not start to germinate until conditions are right. This usually means that the seed must be warm and moist. The embryo starts to grow, using its stored food supply, and soon bursts the seed coat. Usually the root emerges first, growing downwards in search of water, and the shoot follows. The first leaves to emerge are often fleshy organs that are part of the food store, and the leaves that appear later do not resemble them.
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