Corn
From Encyclopædia
Corn or maize, Zea mays, is an annual
plant of the
grass family, Gramineae. The largest of the cereals, it can reach heights of 4.5 m (15 ft). The stem, or cornstalk, is hard and jointed, resembling bamboo. The
plant possesses both male and female
flowers; the male
flowers are borne in the tassel at the top of the stalk, and the female is a cluster, called a cob, at a joint of the stalk.The corn
silks hanging from the husk of each cob are the
pollen receptors; each
thread of
silk must receive a grain of
pollen in order for its fruit, or kernel, to develop. A fertilized cob, or ear, contains eight or more rows of kernels. One to three or more cobs grow on each stalk.Among the
world's four most important crops (the others are wheat,
rice, and potatoes), corn is one of the few economic
plants native to
America. The
United States produces over 40 percent of the
world output. Worldwide, more than 132 million ha (327 million acres) of land are planted in corn each year, with a grain production of more than 477 million metric tons (more than 526 million U.S. tons). Countries with large areas devoted to corn include the
United States, China, Brazil,
Russia,
Mexico, Argentina, Romania, France, India, and South
Africa.ORIGIN OF CORNArchaeological and geological excavations, and measures of radioactive decay of old ears of corn found in
caves, indicate that a type of primitive corn was used as a food in
Mexico at least 7,000 years ago. No wild forms of corn have been found, and the origin of domesticated corn is a speculative and controversial issue.In its present form, the corn
plant is highly specialized, and is unsuited for efficient, natural
reproduction. Although the ear is specially constructed for producing high seed yields, the
plant has no mechanism for broadcasting its seeds without human
intervention. The processes of
mutation, natural selection, and
Mass selection by the American Indians gradually transformed certain varieties of wild corn into the cultivated
plant called maize--a cultigen, or a product of human culture.Corn was unknown outside the New
world before 1492, but the
plant was extensively cultivated, in all its present forms, by the Indians of North and South
America. Seed grains of Indian maize, brought to
Europe and
Africa by 16th-century
Explorers, were planted and eventually thrived throughout most of the
world. Since the 1930s, the development of hybrid varieties of corn has resulted in greatly increased yields and improved quality. Today, corn is considerably larger in cob size and in the
number and weight of the kernels than the corn grown by the Indians.CULTIVATIONCorn grows best in well-aerated, deep, warm loams containing an abundance of organic matter,
nitrogen, phosphorus, and
potassium. The crop thrives in areas with moderately high summer temperatures, warm nights, and adequate but not excessive rainfall that is well distributed during the growing season. Length of the growing season and length of day also influence production. The Corn Belt of the midwestern
United States is ideally suited to these conditions.In areas of intensive corn cultivation--especially in the Corn Belt where farms are large and the topography is relatively flat--mechanization has resulted in higher corn yields and greater crop-labor productivity. Many specialized machines are used throughout the Corn Belt. Among the most important are
wheel-track planters, which dig the furrows,
plant the seed, and cover it in one operation; mechanical corn pickers, which harvest only the ears (some machines
shell the ears as well); and field choppers, which cut and chop the stalks into silage.Pest
Control is accomplished through the use of pest-resistant hybrid varieties, proper cultivation, and the judicious use of herbicides and pesticides (see
HERBICIDE;
PESTICIDES AND PEST CONTROL). Viral and other diseases have caused heavy crop losses during the past 2 decades, and
plant geneticists are doing research in types of corn that are resistant to specific diseases and insects.Yields from desirable corn hybrids are greatest when
plants are grown in
soils well supplied with balanced amounts of
plant food and considerable
soil moisture, especially in areas of low rainfall. Corn grown continuously for 100 years on the experimental Morrow Plots at the
university of Illinois now ranges in yield from about 3,136 kg/ha (50 bu/acre) to 10,035 kg/ha (160 bu/acre), the disparity depending entirely on the treatment of the
soil.Use of adapted hybrids, increased
nitrogen application, and heavy
plant populations contributed to an annual increase of 157 kg/ha (2.5 bu/acre) of corn during each of the past 25 years in the Corn Belt of the
United States. Other contributing factors were favorable
soil and climatic conditions, efficient mechanization, effective pest
Control, modern cultural practices, intelligent water management, and greater crop-labor productivity.Before 1930, more than 30 minutes of labor were required to produce a bushel (25.4 kg) of corn in the
United States; less than 3 minutes is now required.TYPES AND CHARACTERISTICSCorn may be divided into several groups, each characterized by a difference in the character of the seeds.Dent CornBy far the most widely grown type, dent corn (so named because the seed has a depression, or dent, in the crown) furnishes millions of tons of grain for human and industrial use and for livestock feed. The soft starch extends to the summit of the seed, and the sides have a corneous starch. The characteristic denting is caused by rapid drying and shrinkage of the soft starch. Corn Belt dents of the
United States originated from mixing
New England flints and gourdseed, an older variety grown by the Indians of southeastern North
America.Flint CornThe kernels of flint corn are hard and smooth and contain little soft starch. Columbus and those who followed him landed in countries where flint strains were widely grown. Consequently, flint corn was probably the type first seen by Europeans. Flint corn is more widely grown in
Europe,
Asia,
Central America, and South
America than in the
United States. In temperate zones, flint corn often matures earlier, germinates better, and has earlier
plant vigor than dent strains.PopcornPopcorn, an extreme form of flint, contains only a small proportion of soft starch. A minor crop, it is grown primarily for human consumption as freshly popped corn and popcorn confections. The ability to pop seems to be conditioned by the quality of the horny endosperm, a tough, elastic material that resists the steam
pressure generated within the heated kernel until it reaches explosive force.Flour CornThe kernels of flour corn are composed largely of soft starch and have little or no dent. Although it is now of little importance, flour corn has been widely grown in the drier sections of the
United States and in the Andean region of South
America. An old type of corn, it is frequently found in graves of the ancient Aztecs and Incas. Because of the softness of the kernels, American Indians were able to grind them for flour.Sweet CornThe kernels of sweet corn have a translucent, horny appearance when immature and are wrinkled ?hen dry. The ears are eaten fresh or are canned. Sweet corn differs from dent corn by only one recessive gene, which prevents some
sugar from being converted into starch. A considerable quantity is grown as a winter crop in the southern part of the
United States.Waxy CornThe kernels of this corn are waxy in appearance. Its starch differs chemically from common corn starch. Corn having the recessive waxy gene was developed in China. Waxy
mutations have since occurred in American dent strains. Waxy hybrids are grown on a small scale to produce a starch similar to tapioca starch.PodcornAlthough not grown commercially, podcorn is of
interest in studying the origin of corn, since it is thought to resemble varieties of primitive corn. Each kernel is enclosed in a pod or husk and the entire ear is also enclosed in a husk (as are the ears of other types of corn).DEVELOPMENT OF HYBRIDSMore is known about the genetics of corn than about any other economic
plant or animal. The
plant is well suited for genetic research, since it is easily grown and is adaptable to a wide range of environmental conditions. It has a large
number of distinct hereditary variations. Inbreeding, or crossing, is simple and rapid, and hundreds of kernels may be obtained on one ear from a single pollination. Rates of
mutation for specific genes can be measured, and the comparatively small
number of relatively large chromosomes within the germ
cell facilitates cytological studies.Beginning in the 1920s, a method for breeding hybrid corn was developed, using crossbreeding between inbred lines. Normally, corn is
cross-pollinated:
pollen is carried from one
plant to another by wind or insects. Inbred corn is self-fertilized; that is,
pollen from the tassel is used to fertilize the
plant's
silks, which are protected from chance pollination by other
plants. Inbreeding is often continued through several generations, using only those progeny that show desirable characteristics. Producing
cross-strains, or hybrids, by
cross-breeding different varieties of inbred corn involves complex techniques. Since the 1930s, the results from breeding and cultivating hybrid corn have been spectacular.Corn and such other monocot grains as
rice and wheat proved less amenable to the standard
genetic engineering techniques than such broad-leaved dicots as tobacco and tomatoes. Scientists successfully introduced a foreign gene into corn for the first time in 1988, and researchers believe that this genetic manipulation will
lead to the development of improved varieties of corn and other cereals.
