Corn - biology.

Corn - biology. I INTRODUCTION Corn or Maize, common name for the cereal grass widely grown for food and livestock fodder. Corn ranks with wheat and rice as one of the world's chief grain crops, and it is the largest crop of the United States (see Crop Farming). II DESCRIPTION The corn plant has an erect, solid stem, rather than the hollow one of most other grasses. It varies widely in height, some dwarf varieties being little more than 60 cm (2 ft) at maturity, whereas other types may reach heights of 6 m (20 ft) or more. The average is 2.4 m (8 ft). The leaves, which grow alternately, are long and narrow. The main stalk terminates in a staminate (male) inflorescence, or tassel. The tassel is made up of many small flowers termed spikelets, and each spikelet bears three small anthers, which produce the pollen grains, or male gametes. The pistillate (female) inflorescence or ear is a unique structure with up to 1,000 seeds borne on a hard core called the cob. The ear is enclosed in modified leaves called husks. The individual silk fibers that protrude from the tip of the ear are the elongated styles, each attached to an individual ovary. Pollen from the tassels is carried by the wind and falls onto the silks, where it germinates and grows down through the silk until it reaches the ovary. Each fertilized ovary grows and develops into a kernel. III HISTORY Corn is native to the Americas and was the staple grain of the region for many centuries before Europeans reached the New World. The origin of corn remains a mystery. Conclusive evidence exists, from archaeological and paleobotanical discoveries, that cultivated corn has existed in the southwestern United States for at least 3,000 years. Wild corn was once thought to have existed in the Tehuacán Valley of southern Mexico 7,000 years ago. More recent evidence puts the appearance of corn in that region at a much later date, about 4,600 years ago. Early wild corn was not much different in fundamental botanical characteristics from the modern corn plant. IV VARIETIES The many varieties of corn show widely differing characteristics. Some varieties mature in 2 months; others take as long as 11 months. The foliage varies in intensity of color from light to dark green, and it may be modified by brown, red, or purple pigments. Mature ears vary in length from less than 7.5 cm (3 in) to as much as 50 cm (20 in). The number of rows of kernels ranges from 8 to 36 or more. Six general groups of varieties are differentiated by the characteristics of the kernel. Dent corn is the leading type of corn grown on U.S. farms. The sides of the kernel consist of hard, so-called horny starch, and the crown contains soft starch. As the grain matures, this soft starch shrinks, forming the characteristic dent. In flint corn, the horny starch extends over the top of the kernel, so that there is no denting. Some varieties of flint corn, which are used for the same purposes as dent corn, are favored in cold climates because of their ability to germinate at low temperatures, or in tropical climates because of their resistance to attack by weevils. Popcorn is a light, highly popular snack throughout the United States, a variant of flint corn with small kernels of great hardness. When heated, the moisture in the kernels expands, causing the kernels to pop open. Flour corn contains a preponderance of soft or less densely packed starch, and it is readily ground into meal. It is grown extensively in the Andean regions of South America that were part of the Inca Empire. Sweet corn is the type commonly grown in the United States for human consumption as a vegetable. The sugar produced by the sweet-corn plant is not converted to starch during growth, as it is in other types. The seeds are characteristically wrinkled when the plant is allowed to mature. Pod corn is seldom used as food but is often grown as a decorative plant; each kernel is enclosed in its own set of diminutive husks. Another decorative corn, commonly called Indian corn, consists of multicolored varieties of flour and flint types. V BREEDING AND CULTIVATION The most important advance in the cultivation of corn was the introduction of hybrids in about 1933. Botanists have developed thousands of hybrids, of which one or more can flourish in almost any combination of soil and climate found in the farming areas of the United States. Hybrids have also been developed to increase corn yields in many other areas of the world. Open-pollinated varieties of corn, which were the normal or standard types used for many years, are inbred (self-pollinated), and plants with desirable characteristics are selected for further breeding. Inbred lines are weak, but when two of these lines are crossed the selected best are much more productive than the original varieties. The canning and freezing industries make use of the single-crossed corn of the type described--that is, the cross of two inbred lines. Most of the hybrid seed used by farmers is double-crossed corn, or the cross product of two single-cross hybrids. Recently farmers have also been using single-cross hybrids because higher yielding plants have been developed. Hybrids do not transmit their increased vigor to their offspring, so that the parent stocks must be crossed each year to produce a new crop of hybrid seed. This is done by seed companies and by some farmers who specialize in the production of hybrid seed. Hybridization adds to the cost of the seed, but the improved yields more than compensate for the added expense. Yield increases of 25 to 50 percent have been attributed to the use of hybrid corn. A major finding in 1978 was the rediscovery in Mexico of a species of perennial wild corn thought to be extinct; this corn could provide the basis for the development of a plant that would not need to be reseeded annually. Corn is an important food staple and animal feed. It is an excellent source of carbohydrates, but since it is low in total protein and the protein is of poor quality, a corn diet must be supplemented with proteinaceous foods for satisfactory growth. Two genetic mutants, known as opaque-2 and floury-2, which cause a change to floury endosperm in normal dent corn in which they are found, have been discovered to result in an increase in tryptophane and lysine, two essential amino acids. These amino acids are in short supply in corn protein. The presence of either mutant gene in corn results in what is called high-lysine corn and renders it equivalent to skim milk in the diet of humans. Swine fed this type of corn will gain weight three times as fast as those fed normal corn. Plant breeders everywhere are now transferring these genes to varieties and parent lines of hybrids. The development is said to equal in importance the discovery of hybrid corn. VI CORN PESTS Corn is attacked by a number of plant parasites and insect pests. An important group of fungi attacks the roots, stalks, and ears, causing rots that reduce yield and damage grain quality. Corn smut is caused by a parasitic fungus that produces a large mycelial mass at various places on the plant (ear, stalk, tassel). As it matures a mass of black spores develops. In some parts of Central and South America, the fresh, unsporulated smut galls, or swellings, are used as food. Corn is also attacked by several fungi that form lesions on the leaves that result in reduction of grain yield. Two important virus diseases, maize dwarf mosaic and stunt, both transmitted by leaf hoppers, attack the corn plant. If the virus is transmitted to corn at an early stage, yield reduction can be serious. The corn earworm, also called cotton bollworm, feeds on the kernels within the husk. The European corn borer mainly attacks the stems of the plants. In recent years the corn rootworm has caused serious losses. These small larvae of the cucumber beetles feed on the root system of the developing plants. VII PRODUCTION AND CONSUMPTION World output of corn at the beginning of the 21st century was about 603 million metric tons annually; in volume of production, corn ranked first, ahead of rice and wheat. A net gain of about 51 percent in production was realized during the last two decades; intensive cultivation with heavy use of fertilizer and herbicides was responsible for the increase. The United States is the leading corn-growing country, with about 40 percent of the world's production. Most of its crop is grown in the midwestern region known as the Corn Belt, comprising Ohio, Indiana, Illinois, Iowa, Missouri, Kansas, and Nebraska. The other leading corn-growing nations are China, Brazil, Mexico, France, and Argentina. Approximately three-fifths of the corn sold by farmers in the United States is used as livestock feed. About half of that amount is fed directly to hogs, cattle, and poultry, and the rest is used in mixed feeds. Another one-fifth of U.S. corn is exported; the remaining one-fifth is sold as food and taken by commercial users for the production of alcohol and distilled spirits, syrups, sugar, cornstarch, and dry-process foods. Corncobs are an important source of furfural, a liquid used in manufacturing nylon fibers and phenol-formaldehyde plastics, refining wood resin, making lubricating oils from petroleum, and purifying butadiene in the production of synthetic rubber. Ground corncobs are used as a soft-grit abrasive. Large, whole cobs from a special type of corn, "cob pipe" corn, are used for pipes for smoking tobacco. Corn oil, extracted from the germ of the corn kernel, is used as a cooking and salad oil and, in solidified form, as margarine; it is also used in the manufacture of paints, soaps, and linoleum. The search for alternate sources of energy has brought attention to corn as a fuel source. High in sugar content, corn is processed to produce alcohol for use with gasoline as gasohol, and the dry stalk is a potentially important fuel biomass. Scientific classification: Corn is classified as Zea mays. The perennial wild corn thought to be extinct and rediscovered in Mexico is classified as Zea diploperennis. Contributed By: Albert Henry Moseman Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.