Grapes
Grapes
Native grapes of North America
Grapes are various species of woody vines in the genus Vitis, family Vitaceae. This family contains about 700 species, most of which occur in tropical and subtropical climates, although some occur in temperate habitats. The genus Vitis has about 50 species. Grapes are ecologically important as food for wildlife,
and are also cultivated by humans in large quantities, mostly for the production of table grapes, raisins, and wines.
Biology of grapes
Grapes are perennial woody vines. They often form thickets along rivers and other naturally open habitats, and often drape trees in open forests or at forest edges.
Grape leaves often have three distinct lobes. The leaves are alternately arranged along the stem. Opposite most leaves are structures known as tendrils that grow in a spiral fashion and are important in anchoring the vine to its supporting structure.
Grapes have small inconspicuous flowers arranged in clusters. The flowers have associated nectaries that are important in attracting the insects that pollinate grapes. The fruit is an edible two-seeded berry, usually purple in color. Grapes are avidly eaten by birds and mammals. The grape seed passes intact through the gut of these animals and is deposited into the ground with feces. The edible fruit of grapes is an adaptation for dispersal by animal vectors.
Cultivated varieties (cultivars) of grapes are usually propagated by grafting shoots of the desired type onto the root of a relatively hardy plant. In this way, the desirable traits of the cultivar will be displayed by the grafted shoot, while the grape grower can also take advantage of the adaptation of the rootstock to the local environment.
Native grapes of North America
Various species of grapes are native to North America. Some of the more widespread species are
KEY TERMS
Cultivar —A distinct variety of a plant that has been bred for particular, agricultural or culinary attributes. Cultivars are not sufficiently distinct in the genetic sense to be considered to be subspecies.
Fermentation —This is a metabolic process during which organic compounds are partially metabolized, often producing a bubbling effervescence. During the fermentation of sugar, this compound is split into carbon dioxide and an alcohol.
Grafting —This is a method by which woody plants can be propagated. A shoot, known as a scion, is taken from one plant, and then inserted into a root-stock of another plant and kept wrapped until a callus develops. The genetically based, desirable attributes of the scion are preserved, and large numbers of plants with these characteristics can be quickly and easily propagated by grafting.
Raisin —A grape that has been preserved by drying.
Riparian —A moist habitat that occurs in the vicinity of streams, rivers, ponds, and lakes.
Sultana —A raisin produced by drying a seedless grape.
Tendril —A spirally winding, clinging organ that is used by climbing plants to attach to their supporting substrate.
Vine —A plant, usually woody, that is long and slender and creeps along the ground or climbs upon other plants.
the muscadine grape (Vitus rotundifolia ), the fox grape (V. labrusca ), the summer grape (V. aestivalis ), the forest grape (V. vulpina ), and the river bank grape (V. riparia ). Most of these are species of moist sites, often growing luxuriantly along forest edges and in riparian habitats.
Wild grapes provide a nutritious and seasonally important food for many species of birds and mammals. They also contribute to the pleasing aesthetic of some habitats, for example, when they luxuriously drape the edges of forests beside rivers and lakes.
Agricultural grapes
By far the most common species of cultivated grape is the wine grape (Vitis vinifera ), probably native to southwest Asia, possibly in the vicinity of the Black Sea. This species may have been cultivated for as long as 7,000 years. The wine grape now occurs in hundreds of cultivated varieties and is planted in temperate climates in all parts of the world. The fruits of this species can be blue, yellow, or green in color, and they contain one to four seeds. Ripe wine grapes typically contain 70% of their weight as juice and 20-24% as sugar. This species is widely grown in warm temperate regions of Europe, especially in France and Italy, and to a lesser degree in Germany, Spain, and elsewhere. Other notable centers for the cultivation of wine grapes are California, Chile, Australia, Portugal, Russia, Algeria, and South Africa.
Two North American grape species are also cultivated for the production of wine. These are the fox grape (V. labrusca ) and, less commonly, the summer grape (V. aestivalis ). The skin of the fruits of the fox grape separate quite easily from the interior pulp, which makes it easy to distinguish agricultural varieties of this species from the wine grape.
Grapes are often eaten fresh as a tasty and nutritious table fruit. They can also be crushed to manufacture a highly flavorful juice, or preserved by drying, usually in the sun. Most dried grapes are called raisins, but dried seedless grapes are also known as sultanas.
Wine is an alcoholic beverage that is produced by a careful fermentation of grape juice. The fermentation is carried out by the wine yeast (Saccharomyces ellipsoides ), a microscopic fungus that occurs naturally on the surface of grapes. However, specially prepared strains of the wine yeast are generally used by commercial vintners to help ensure a constant, predictable fermentation and final product.
Wine yeast ferments the sugar content of the juice of pressed grapes into carbon dioxide and ethanol, a type of alcohol. The alcohol yield is about 1% for every 2% of sugar in the juice, but the final alcohol concentration cannot exceed 12%, because this is the upper limit of tolerance of yeast to alcohol in its growth medium. (Actually, there are wines with an alcohol concentration greater than 12%, but these are prepared by adding pure ethanol, a process known as fortifying. ) The initial grape juice is prepared by pressing ripe grapes. Originally, this was done by barefoot people stomping about in large wooden tubs. Today, however, the grapes are usually pressed using large machines. Red wines are obtained when the skins of blue grapes are left in with the fermenting juice. White wines are obtained when the skins are removed prior to the fermentation, even if the juice was pressed from red grapes.
The quality of the resulting wine is influenced by many factors. The variety, sugar content, and other aspects of the grapes are all important, as is the strain of wine yeast used. Soil conditions and climate of the growing region are also highly influential. The incubation temperature during the fermentation is important as is the sort of container that is used during this process. In addition, once the fermentation is stopped, the period of time during which the wine is stored can be important. However, a storage that is too long can be detrimental because the alcohol in the wine may be spoiled by a further metabolism of the ethanol into acetic acid, or vinegar.
Grapes in horticulture
Some grape species are occasionally used in horticulture. The desired utilization is generally as a wall covering and sometimes for the visual aesthetics of the foliage in the autumn. Species commonly grown for these horticultural purposes are Vitis vinifera and V. coignetiae. The Virginia creeper (Parthenocissus quin-quefolia ) is a closely related native species that is also often used for these purposes as is the introduced Boston ivy (P. tricuspidata ).
See also Graft.
Resources
BOOKS
Judd, Walter S., Christopher Campbell, Elizabeth A. Kellogg, Michael J. Donoghue, and Peter Stevens. Plant Systematics: A Phylogenetic Approach. 2nd ed. with CD-ROM. Suderland, MD: Sinauer, 2002.
Klein, R. M. The Green World. An Introduction to Plants and People. New York: Harper and Row, 1987.
Raven, Peter, R.F. Evert, and Susan Eichhorn. Biology of Plants. 6th ed. New York: Worth Publishers Inc., 1998.
OTHER
Michigan State University Extension—Horticulture. “Grapes” <http://www.canr.msu.edu/vanburen/grapeweb.htm> (accessed November 25, 2006).
University of Minnesota Extension Service. “Growing Grapes for Home Use” <http://www.extension.umn.edu/distribution/horticulture/DG1103.html> (accessed November 25, 2006).
Bill Freedman