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FLOWERS (Part 2 of 2)   Leave a comment

Avoiding Self-Pollination

Self-pollination, transfer of pollen from the stamen of a flower to the pistil of the same flower, as distinguished from cross-pollination.

A few kinds of flowers are self-pollinating; that is, they can be fertilized with their own pollen. In most cases, however, nature takes great care to prevent self-pollination. Cross-pollination usually produces more vigorous plants. This requires the transfer of pollen from one plant to the stigma of another plant of the same species.

Flowers avoid self-pollination in several ways. In some cases the stamens and pistils mature at different times. In other flowers the stamens are shorter than the pistils and hence do not deposit pollen on their own stigma. Wind-pollinated flowers usually bear the stamens and pistils in separate flowers. Alders, birches, walnuts, and hickories bear catkins with pistillate flowers on some branches and catkins with staminate flowers on other branches. Corn has the pistils and stamens on different parts of the same plant. The tassel bears the staminate flowers; the ear bears the pistillate flowers. These are known as monoecious (of the same household) plants. A few trees, such as cottonwoods and willows, carry the separation even further, with the staminate flowers on one tree and the pistillate on another. These are known as dioecious (of two households) plants.

How Fruit Develops

After fertilization of the ovule has taken place the petals, sepals, stamens, and usually the upper part of the pistil fall off. Now, as the ovules grow into seeds (embryo plants), the ovary, or seed case, also changes. In some plants it turns into a fleshy covering, called fruit. The ovary wall separates into two layers. The inner layer becomes a hard shell, called a stone, or pit, which encloses the seed. The outer layer forms the pulpy portion of the fruit. The peach, plum, cherry, and apricot are examples. In the case of berries the entire ovary becomes a fleshy mass in which the seeds are embedded. In the apple, pear, and quince, the ovary and its seeds become the core of the fruit. The pulpy part, which is eaten, is the modified calyx.

The ovaries of many plants develop into so-called dry fruits capsules, pods, nuts, and acorns. Like the fruits and berries, they protect the seeds and help scatter them when they are mature. Another kind of dry fruit is the achene. In this case the ovary wall becomes a coating of the single seed. It does not open at maturity, as the pods and capsules do, to release the seed. Achenes are developed by flowers that produce but one ovule, such as the individual flowers of the composites. The style of the pistil sometimes remains attached to the achene as a long, feathery tail that carries the seed away on the wind. The most common flower with seeds that are readily scattered by the wind is the dandelion, regarded by most people as a weed.

The Origin of Flowers

At least 250,000 species of flowering plants are known. All of them descend from a primitive ancestor that no longer exists. The most primitive modern flowers are the members of the buttercup order, Ranales. A step higher is the rose order, Rosales.

The simplest flowers are the least skilful in making seed. Many stamens mean a great deal of pollen is wasted. A large number of pistils means that many will fail to become pollinated and produce seed. All members of the buttercup order, which includes the little buttercup itself and the splendid magnolia and water lilies, and all the roses have many pistils and stamens. The most highly specialized and most successful flowers are the composites.

Two Kinds of Flowering Plants

Angiosperms (or Angiospermae), class of flowering, vascular plants of the division Magnoliophyta having seeds in an enclosed ovary.

Flowering plants belong to the phylum Tracheophyta, or vascular plants. Thus far the flowers and seed making up only one group of this phylum, the angiosperms, have been described. These are flowers that enclose their seeds within an ovary.

Another group of flowering plants, called gymnosperms, has naked, or exposed, seeds. These plants include the conifers, or cone-bearing trees, such as the pine, fir, spruce, cypress, and cedar. Cones take the place of flowers.

Cones are of two kinds staminate and pistillate. They are usually borne on different branches of the same tree. The staminate, pollen-producing cones are small and last only a few weeks in the spring of the year. The pistillate cones are the large familiar ones. The ovules, usually two in number, are located on the upper surface of each scale. The ovule consists of an embryo sac surrounded by a covering that later becomes the seed coat. In the covering is a tiny opening called the micropyle (little gate).

In late spring the pistillate cones stand upright with the scales opened wide to catch the windblown pollen. When pollen lodges between the scales, they close. Thus protected within the closed cone, the pollen sends out a pollen tube that enters the ovule through the micropyle. When the seeds in the cone are fully grown, it again opens, releasing the matured seed. All gymnosperms are wind-pollinated.

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Posted 2012/03/17 by Stelios in Education

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CONSERVATION (Part 2 of 5)   Leave a comment

WATER CONSERVATION

Polluted water in one part of the world affects water sources everywhere. This happens because water moves through what is called the hydro logic cycle.

In the hydro logic cycle

1) water moves into the air and clouds as it evaporates from oceans, lakes, and rivers.

2) the winds that blow around the Earth blow the water vapour around until enough builds up inside a cloud to cause rain to fall.

3) wherever that rain falls, sooner or later it will flow into another body of water.

In this way, a water molecule that left the ocean near California may fall as rain in India. Unfortunately, products that pollute the water can also be carried to other parts of the world along with the water molecules.

Water is an essential natural resource. Everyone uses it. It is needed in homes for drinking, cooking, and washing. Communities must have it for fire protection and recreational activities such as swimming, boating, and fishing. Industries use it to produce electricity and to perform a large number of manufacturing processes.

Watersheds and Their Importance

Watershed (or drainage basin), area of land, of any size, from which all precipitation flows to a single stream or set of streams.

A watershed is the area drained by a river or a stream in a region. Such an area slopes toward a common land trough. Some rain runs off, or drains, over the ground surface. Run-off water forms small streams, which flow into larger ones. These eventually join to form rivers.

A natural watershed conserves water. It has clear streams and an ample cover of trees, grasses, and other plants. Plants help contribute to form a part of the topsoil called humus. Humus consists of decaying leaves and wood, bacteria, dead insects, and other plant and animal remains. It provides some of the nutrients for new plant life. Together with a network of roots, it acts as a blotter that soaks up rain. Plants break the force of falling rain and scatter the drops over leaves and branches. Some of the water returns to the air by evaporation. Part of the water used by plants is passed through their leaves into the air again by transpiration. The rest of the water sinks into the earth through countless tiny channels. Some of the spaces in the soil through which water percolates are caused by natural features of the geology or soil itself. Others are made by plant roots and burrowing animals like earthworms, insects, and moles.

The level at which the earth is permanently saturated is known as the water table. This vast underground water supply fluctuates with the seasons and the amount of rainfall. During long, heavy rains the soil may not be able to soak up all the water. Some of it runs off the surface, but in a forested watershed it moves slowly. Deep snow that melts slowly allows water to soak into the soil gradually.

When all the trees in an area have been cut down or burned off due to poor forestry practices, or grasses and other plants have been stripped off by fire, overgrazing, or poor farming practices, the watershed suffers. The water from rainfall flows over the ground’s surface instead of being absorbed by the vegetation and organic materials that would be present on a natural forest floor.

When there are no leaves and branches or grasses to break the force of falling rain and the blotter of roots and humus is gone, mud closes the channels through which water sinks into the soil. If the land is level, the water stands in stagnant pools; if it slopes, the water runs downhill into the rivers. Streams in a mismanaged watershed become brown with silt, or suspended soil, because the racing water carries soil along with it.

A mismanaged watershed can result in destructive floods in the spring because heavy rains and melting snows overflow the riverbanks. In the summer, streams, springs, and wells can dry up because little or no water has sunk into the underground reservoirs.

Water Pollution

Water can be polluted by many things. One of these is the topsoil or silt that washes into streams and rivers. This silt washes into streams and rivers from land that has been badly managed.

When silt washes into streams and rivers, two harmful things may happen.

1) Silt that floats in the water limits the amount of air in the water. Fish need air to breathe. When silt limits the air in the water, the fish die.

2) As the movement of water slows down, silt drops to the bottom of the stream beds

There are ways of controlling erosion of silt from land into streams and reservoirs. Conservationists try to make sure that the right steps are taken to prevent the silting of streams.

The silting of streams is one kind of water pollution. A heavy load of silt kills fish indirectly by reducing the amount of oxygen in the water. Then, as the flowing water slows, silt is deposited on stream beds Reservoirs behind dams also fill with silt unless erosion is stopped in watersheds above.

The main problem with our waterways is that they have been used as a garbage can for every kind of human waste that you might imagine.

Raw, untreated sewage contains:

  • garbage from individuals and businesses

  • waste products from industry

  • run-off from sewers

Raw sewage is unhealthy and can cause outbreaks of disease. It also severely pollutes the environment.

Raw sewage can be treated in special ways to make it less harmful to the environment. For example, poisonous metals and objects that take a long time to break down can be removed from the waste so it can break down faster.

Other kinds of water pollution have created other problems. Many waterways are used as dumps for household and industrial wastes. Some communities dump untreated sewage and garbage into the nearest streams. Industries contaminate the waterways when they discharge acids, chemicals, greases, oils, and organic matter into them. Such materials foul drinking water and endanger public health. They destroy commercial fisheries. They also make waterways unusable for recreational purposes. Leaks and spills from offshore oil wells and wrecked or damaged oil tankers have caused the widespread destruction of marine life.

A food chain is made up of plants and animals linked together like a chain. Each creature depends on the other creatures in the chain for food.

It takes many creatures at the bottom of a food chain to feed just one animal at the top of the chain.

If one link becomes weak, it affects all others in the chain. Here’s an example:

For a period of time humans sprayed a chemical pesticide called DDT on plants to kill bugs.

The DDT washed off the plants and into rivers, lakes, and streams. Fish ate the poison. Many fish died, but many others survived with traces of the poison in their bodies.

Some animals ate the poisoned fish. Still others ate poisoned insects. Finally, other creatures ate the animals that had eaten the poisoned fish and insects.

Since higher animals in the chain eat a large amount of the lower animals, each link was getting more and more DDT.

While large doses of DDT can kill, smaller doses do damage as well. For example, DDT causes the shells of bird eggs to be too thin. Many kinds of birds, such as the American bald eagle, poisoned by DDT, were unable to hatch young, and their numbers became smaller and smaller.

When conservationists and others saw the harm caused by DDT to various creatures such as the bald eagle, they reasoned that other creatures, including humans, were being harmed by the pesticide. Although the spraying of DDT was stopped by passing a law in the United States, it continues in other parts of the world.

The large-scale use of organic insecticides, herbicides, toxic metals, and pesticides, particularly DDT, has polluted streams and destroyed wildlife. Some pesticides tend to concentrate in the tissues of plants and animals in nature’s food chains. Thus organisms at the ends of these chains, including humans, may take in harmful amounts of pesticides deposited in their food supply.

SOIL CONSERVATION

Whenever land is stripped of its plant cover, soil is inevitably lost by erosion, the so-called silent thief. A single rainstorm can wash away centuries-old accumulations of soil from neglected or badly managed fields. Topsoil is an extremely valuable natural resource. Under this thin blanket of rich dirt and humus, in which plants grow best, is a less fertile material called subsoil. If the surface layer of topsoil is blown or washed away, the remaining subsoil cannot support plant life. The submarginal farms must eventually be abandoned.

Types of Soil Erosion

More than 700 million acres (283 million hectares) of agricultural land in the United States are subject to erosion. Some 230 million acres (93 million hectares) of crop land require constant supervision to control erosion caused by wind and water.

Dust storms are the evidence of wind erosion. Soil unprotected by plant cover simply blows away. During the 1930s millions of acres of farmlands were badly damaged by wind. Many fields lost from 2 to 12 inches (5 to 30 centimetres) of vital topsoil during this period. As a result, the entire southern Great Plains area was called the Dust Bowl.

One of the several kinds of water erosion is sheet erosion the wasting away of level land in thin layers. The deterioration may go on for years without being noticed, though the land yields successively smaller crops. A patch of subsoil showing through on some slight rise of ground may be the first sign that the land is nearly finished as a food producer.

Splash erosion is the washing away of soil by the direct battering of rain. Small channels dug in the soil by run-off are called rill erosion. The little rills run together, form a network of larger rills, and then develop into gullies. When gully erosion occurs, the land can become a desert.

Conservationists also recognize that livestock can overgraze a plot of land until severe soil erosion occurs. About 4 million acres (1.6 million hectares) of topsoil are lost every year through erosion, and about 85 percent of this is the result of overgrazing by livestock.

Although major losses of productive agricultural lands occurred in the first half of the 20th century due to erosion, a major concern today is the loss of natural habitats as a result of commercial development. Large tracts of productive land an estimated 1 million acres (400,000 hectares) each year are lost through road building, suburban housing and industrial site developments, and airport expansion. New dams often flood some of the most productive agricultural land and natural forest habitats.

Posted 2011/12/21 by Stelios in Education

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