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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.

Posted 2012/03/17 by Stelios in Education

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CORN (Part 2 of 2)   Leave a comment

How Experimenters Developed Hybrid Corn

In 1905 George H. Shull and Edward M. East began developing new kinds of corn by placing pollen from one desirable strain of corn onto the silks of another strain. The process produced cross-bred strains called hybrid corn. After World War I, Henry A. Wallace (who became secretary of agriculture in 1933) and Lester Pfister began hybridizing experiments. By 1926 they had made hybrid pollinization completely workable.

The hybrid plants are remarkable growers. They commonly grow to be 18 or 20 feet tall; some have grown as high as 28 feet. A more important factor is that they have added millions of dollars to the income of corn farmers.

Before farmers had hybrid corn, an average acre of corn yielded 30 bushels. But farmers had to spend the money they received for 25 bushels to pay their costs for each acre planted, leaving only 5 bushels an acre for profit. Hybrid corn has raised the national average to more then 95 bushels an acre. Some states average more than 130 bushels an acre.

A hybridizer produces hybrid seed by first inbreeding. This fixes desirable qualities in the seed. He covers the ears of selected plants to keep airborne pollen from the silk. Later, he takes pollen from the tassels of a plant and dusts it on the silks of the same plant. After inbreeding each strain for several generations, he starts cross-breeding He takes pollen from the tassel of a plant having one desirable strain and dusts it on the silk of a plant with some other strain. The cross-bred product, or hybrid, has the qualities of each parent strain.

Next comes double-crossing. The experimenter dusts pollen from one hybrid onto a hybrid with two other strains. The seed from this cross produces a super corn with four strains bred in. This corn is sold to farmers as seed. Their crop cannot be used as seed next year because hybrid corn is not self-perpetuating. Farmers must buy new seed each year. Great use of hybrid corn threatens the supply of corn pollinated naturally. This loss would restrict improving hybrid strains and prevent developing new ones. To preserve seed of native varieties, the federal government stores seed in corn banks.

Planting and Cultivating

A strong, full crop of corn comes from fertile soil, good seed, thorough cultivation, and clean culture. The soil should be easily worked, well drained, and rich in plant food. The dark loam of the Midwestern United States is particularly well adapted for corn. The farmer chooses the seed to suit conditions on his land. In dry regions he may plant corn in deep furrows. If rainfall is plentiful he puts the seed down in hills or in drills. Once the plant starts to grow, cultivation must never be deep, or the tender, grass like roots will be injured.

Corn draws heavily on the plant food in the soil. Production is higher when corn crops are rotated on a three-year cycle. The first year a legume, such as alfalfa or sweet clover, builds up the soil with nitrogen and humus. The next year corn grows tall on these, its favourite foods. The third year a small grain is planted. Then the cycle is renewed with a legume.

Different Ways of Harvesting

If the farmer wants to store the whole plant in a silo, he cuts the corn while it is still green. If the corn is to be used for grain, it is not harvested until it is fairly dry. The ears may be picked by hand from the standing corn and husked and thrown into a wagon. On most farms mechanical corn pickers are used.

Some farmers turn cattle in to feed on the corn stalks after the ears are picked. Others cut the stalks, tie them into shocks, and let the ears get dry before husking. Many livestock raisers turn hogs into the ripe fields to feed and fatten on the corn. This method is called hogging down.

Fighting the Enemies of Corn

Corn ear worm (also called tomato fruit worm, or tobacco bud worm, or cotton boll worm), larva of a moth (Heliothis armiger); names vary depending on the various plants it infests; larvae on corn first eat the leaves, then the ears; pupation occurs in the ground; winter ploughing in North kills many pupae.

More than 350 insect pests attack the grain. The most destructive are the corn ear worm, the European corn borer, and the corn root worm Fungus growths, such as smut and various rots, are costly foes. In many cases insecticides are too expensive to be practical. Therefore the farmer uses the less expensive methods of clean culture and crop rotation. Clean culture means harvesting or destroying every part of the plant. Careful farmers either burn or plough under the stubble. This rids the cornfield of pests that live above the ground. Crop rotation suppresses root pests that live on corn by depriving them of food for one or two years.

Composition of a Corn Kernel

A kernel of corn is wrapped in a tough, fibrous outer hull (bran). Inside is the germ, or embryo, from which the new plant develops. Around the germ is a food supply called endosperm. This is chiefly starch. When the kernel germinates it draws its nourishment from the endosperm until it can put forth roots and leaves and obtain food from the soil and the air.

The moisture content of a kernel varies from 10 to 25 percent, depending upon weather and other conditions under which it was grown. Of the dry portion, about 70 percent is starch (carbohydrates). About 10 percent is gluten (protein), found in a shallow layer just under the hull. The remainder is fat or oil in the germ (4.5 percent), fibre in the hull, and minerals.

A Great Variety of Corn Products

All the parts of a corn kernel can be used to make products. From the whole kernels manufacturers make cornmeal, breakfast foods, and hominy. Some people make hominy at home by removing the hull with lye and cooking the whole grain. When the kernel is crushed it forms hominy grits. Distillers make alcohol and whiskey from whole corn kernels.

Since corn became so dominant a grain in American agriculture, it has naturally found its way to Europe and Asia. There, whether imported or grown locally, it is used mostly for animal feed, as it is in the United States. For humans, corn is less desirable nutritionally than for livestock. The protein value is of low quality, and corn is devoid of niacin one of the B-vitamins that is essential to humans. People who rely heavily on corn in their diets are subject to such niacin-deficiency diseases as pellagra. Corn cannot be used to make leavened bread, although it is much used in Latin America to make dough for such flat breads as tortillas.

The corn products refining, or wet-milling, industry makes a great variety of products from different parts of the corn kernel. Wet milling is so called because the kernels are steeped in tanks of water to soften them, and water is used in the processes that separate germ, gluten, and starch.

First to be separated from the kernel is the germ. Refined and crude corn oil have many uses as human and animal food and in industry. When oil is pressed from the germ a hard cake is left. It is ground into stock feed. One of the proteins in gluten is zein. A synthetic fibre is made from it. It is also used in lacquer, plastics, textile colours, and printing inks.

The final product of the wet-milling separation process is starch. The housewife, food manufacturers, and laundries have many uses for cornstarch. Paper manufacturers use more starch than any other industry to toughen and size (glaze) paper. Textile manufacturers are second. Cotton and synthetic yarns and fabrics are sized with starch.

Glucose (or dextrose, or grape sugar, or corn sugar), simple (monosaccharide) sugar found in fruits and other foods and in the blood of animals; fuels the energy needs for most living organisms.

A huge amount of starch is converted into corn syrups (glucose), sugars, and dextrose by cooking and chemical treatment. These too have countless uses in cooking and in various industrial processes. Even the steep water in which the kernels are soaked is important. Evaporated to a thick, soupy liquid, it is used as a food for the moulds that produce penicillin and other wonder drugs.

Corncobs are ground for a coarse livestock feed. They are used also in a polishing powder, insulation, and a form of sandblasting. Furfural, an oily liquid extracted from corncobs, goes into man-made fibres, drugs, and solvents. Some specially grown cobs are made into pipes for smoking.

Millions of tons of cornstalks are made into a rubber substitute, maizolith. A large quantity is used for making paper and wall board Even the gases from fermenting corn are used to make methyl alcohol.

American Indians had many kinds of corn, and there are now more than 1,000 named varieties. The smallest is the golden thumb popcorn plant, about 18 inches (46 centimetres) high. Some varieties have only eight rows of kernels; others, as many as 48 rows. Colours include white and shades of yellow, red, and blue.

The chief types of corn are pod, soft, sweet, pop, flint, and dent corn. Pod corn has each kernel enclosed in a pod or husk. Soft corn is used for corn flour and for roasting ears. Sweet corn has the smallest amount of starch; popcorn, the highest. Flint and dent corns lead all other varieties on the grain markets and for livestock feeding. The scientific name of corn is Zea mays.

Posted 2012/03/04 by Stelios in Education

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