Peanut

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Economic Plant Photographs #8B
The Peanut: A Subterranean Legume
© W.P. Armstrong 15 March 2009
The legume family (Fabaceae) is the third largest family of flowering plants with more than 18,000 described species. It is surpassed in size only by the orchid family (Orchidaceae) with about 20,000 species and the sunflower family (Asteraceae) with about 24,000 species. The family includes herbs, shrubs, trees and vines distributed throughout the world, especially the tropical rain forest. The fruit is technically called a legume or pod. It is composed of a single seed-bearing carpel that splits open along two seams. Legume fruits come in an enormous variety of shapes and sizes, including indehiscent pods that do not split open. Of all the legumes, the peanut is especially fascinating because it develops below the ground.

The world's largest legume fruits (bean pods) are produced by the tropical liana Entada. The longest pods of the Central American E. gigas may be up to 5 feet long (1.5 m). This gigantic woody vine is truly like Jack's fabulous bean stalk. In Costa Rica it is called "monkey ladder" or "escalera de mono." The woody seeds of E. gigas are called "sea hearts" and are often washed down streams to the sea where they drift across the ocean to distant continents. Familiar edible legume pods in the background include green beans, peanuts, soybeans and snow peas.

  Diversity of Flowering Plants  
  The World's Longest Bean Pod  

The peanut (Arachis hypogaea) is native to central South America. By the time Columbus reached the New World, peanuts were already cultivated throughout warmer regions of the Americas. Peanuts have been introduced into Africa and Asian countries where they have become an important food crop. Like clover and alfalfa, the peanut root system contains nodules of nitrogen-fixing bacteria that convert inert atmospheric nitrogen into ammonia. Other bacteria convert the ammonia into nitrites and nitrates that enrich the soil with a usable form of nitrogen.

  Read About Nitrogen Fixation In Plants  

Like other members of the subfamily Papilionoideae, the peanut flower is papilionaceous, typical of a pea blossom. The peanut flower is produced on a slender stalk (pedicel) near the base of the plant. Each flower consists of five petals: a large banner, two lateral wings, and a keel formed by two fused petals. The keel petals enclose the 9 stamens (androecium) and pistil (gynoecium).

  Subfamilies of the Legume Family (Fabaceae)  

The peanut (Arachis hypogaea) is a dehiscent legume that is harvested from below the soil. The legume was originally formed above ground following pollination. After fertilization, the flower stalk of the peanut curves downward, and the developing pod is forced into the ground by the proliferation and elongation of cells under the ovary. The pod typically contains two seeds, each with a papery seed coat. Peanut seeds are eaten raw, salted and roasted. Peanuts are ground into peanut butter and Thai peanut sauce, and the expressed oil is used in cooking. Peanuts are also used in cookies, peanut brittle and candy bars.

Question:   Considering double fertilization in angiosperms, how many
sperm are involved in the formation of a mature 2-seeded peanut pod?

After self-pollination and fertilization inside the flower, the pedicel curves downward. Cells beneath the ovary begin to divide, producing a "peg" that forces the ovary into the ground. As the peg elongates, a cap of cells forms next to the withered style. This cap protects the ovary as it is pushed into the soil. This is similar in function to the root cap at the tip of a root. After the developing ovary has pushed a few centimeters into the soil, downward elongation of the peg ceases. The ripening ovary becomes oriented parallel with the ground surface where it completes its development.

Two germinated Spanish peanut seeds showing the embryonic root (radical) and root cap. The root cap protects the delicate meristematic root tip as it pushes into the soil.

A tiny embryonic plant within the two fleshy halves (cotyledons) of a Virginia peanut seed. The cotyledons provide carbohydrates and protein for the developing embryo until it develops into a seedling with functional roots and photosynthetic leaves.


Spanish Peanut Plants In Full Bloom

A Spanish peanut plant with a flower at the base. The flower is produced on a slender stalk (pedicel) that curves downward and pushes into the soil following pollination and fertilization.

The self-pollinated flower of a peanut plant is produced on a slender stalk (pedicel) near the base of the plant. The flower is papilionaceous, typical of the subfamily Papilionoideae. Following pollination and fertilization, the flower stalk curves downward and pushes into the soil. Below ground, the ovary develops into a 2-seeded pod. Unlike other members of the legume family, the pod develops underground.

The peanut flower is produced on a slender pedicel near the base of the plant.

The peanut flower is produced on a slender stalk near the base of the plant.

The peanut flower is produced on a slender stalk near the base of the plant.

The peanut flower is produced on a slender pedicel near the base of the plant.


Subterranean Production Of A Peanut

A peanut plant (Arachis hypogaea) that has been pulled out of the ground to show the subterranean, seed-bearing, dry fruit (called a pod). After fertilization, the flower stalk (pedicel) of the peanut curves downward, and the developing fruit (legume) is forced into the ground by the proliferation and elongation of cells under the ovary. The peanut pod subsequently develops underground. As in other members of the enormous legume family (Fabaceae), the roots bear nodules containing nitrogen-fixing bacteria.

A Spanish peanut plant (Arachis hypogaea) that has been pulled out of the ground to show the subterranean, seed-bearing, dry fruits (called pods) attached to their original flower stalks. Pedicels of the undeveloped pods were also curved downward into the ground, but the immature seeds (ovules) were apparently not fertilized. This plant was grown in the sand of a horseshoe pit.


Some Tasty Uses For Peanuts

An assortment of tasty, high caloric snacks made from peanuts, including peanut butter, chocolate-covered peanut clusters, cookies, roasted peanuts, honey-roasted peanuts, toffee peanuts, and various candy bars made of peanuts embedded in chocolate and caramel.

Another major use for peanuts is the moderately saturated peanut oil. It has an iodine value of 84-100 compared with saturated coconut oil (7-10) and unsaturated safflower oil (140-150). Moderately saturated (monounsaturated) olive oil has an iodine value of 78-88.

  Read About Fats & oils  


Severe Allergic Reactions To Peanuts

It should be noted here that some people have severe allergic reactions to peanuts. In a recent case in Quebec, a girl reportedly died after a seemingly harmless kiss from her boyfriend. In this case the girl was extremely allergic to certain proteins in peanuts, and the boy had just eaten a peanut butter sandwich. In severe reactions, the larynx swells shut and the victim is unable to breath, a condition known as laryngospasm. Another potentially life-threatening reaction from peanut hypersensitivity is anaphalactic shock. People with severe allergies to legumes and other kinds of nuts must read all food labels very carefully, especially if their child has this extreme hypersensitivity.

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