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Economic Plant Photographs #8B
The Peanut: Amazing Geocarpic Legume
© W.P. Armstrong updated 2 January 2020
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, a process known as geocarpy.

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.

Sessile peanut flowers develop at the base of plant in the leaf axils.

  Read About Nitrogen Fixation In Plants  

  Subfamilies of the Legume Family (Fabaceae)  

The peanut fruit is a dehiscent legume that is harvested from below the soil. The legume was originally formed above ground following pollination. After fertilization, the developing pod is forced down into the ground by the proliferation and elongation of a special structure called "peg" from cells below the ovary (called intercalary meristem by some authorities). 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.

  Read About Aflatoxins In Moldy Peanuts  

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

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 Plant In Full Bloom: Mechanism Of Geocarpy

Note: My initial on-line description of the peanut flower was oversimplified and inaccurate. Special thanks to Dr. Christopher Grefen of Ruhr-University Bochum, Germany for bringing this to my attention and correcting my page.

What I initially referred to as the flower stalk (pedicel) in my images was the hypanthium, sometimes called a floral tube or calyx tube. In fact, the peanut flower is sessile (or nearly so) in the leaf axil at base of plant. See: Rao V.R., Murty U.R. (1994) Botany morphology and anatomy. In: Smartt J. (eds) The Groundnut Crop. World Crop Series. Springer, Dordrecht See This Reference On-Line. The long, tubular hypanthium extends above the ovary with petals at the tip.

Like other members of the subfamily Papilionoideae, the peanut flower (Arachis hypogaea) is papilionaceous, typical of a pea blossom. The flower is produced 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).

A Spanish peanut plant with sessile flower at the base. The mature peanut flower has an elongate, tubular hypanthium (calyx tube). This is not a pedicel. The pollen tube has to grow through this tube to reach the two ovules in the ovary at the bottom of flower, directly at the axillary bud. After self-pollination the petals, including the hypanthium, dehisce and remain on the tip of a developing "peg" at base of ovary (called a gynophore by Darwin). The peg enlarges and grows down and away from the plant forming a small stem-like structure (with peanut embryo at its tip) that penetrates the soil. Peanut growers call this elongation of the peg "pegging." 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.

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 peg (stalk) of the peanut curves downward and the developing fruit (legume) is forced into the ground. 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 (peanut pods). After pollination and fertilization above ground the developing pegs grow downward (toward gravity), penetrating the soil, each peg with an immature peanut pod (ripening ovary) at its tip. The peanut pods complete their growth underground. 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.

The mechanism for peanut hypersensitivity is a complex cell-mediated immune resonse involving white blood cells of the body's defense system against invading organisms. This is beyond the scope of my peanut page; however, I included a link to my poison oak page to show the amazing complexity of a cell-mediated immune response. It involves antigen-presenting dendritic cells similar to Langerhan's cells in the poison oak immune resonse. Other cells involved in the response include killer T-cells and memory cells. In the case of peanuts, the peanut protein allergen is recognized as a foreign pathogen by the body's immune system. I am quite certain that I have oversimplified this very serious immune response to peanuts.

  Cell Mediated Immune Response In Poison Oak  

The topic of aflatoxins in peanuts is another complex and controversial subject. In fact, I know of one microbiology professor who would not eat peanut butter because of this potential danger. Please see following link on this subject, especially if you need reassurance for eating peanut butter.

  Read About Aflatoxins In Moldy Peanuts  

Why Are Peanuts An Important Seed Crop?

  1.   Peanuts are one of the most sustainable and environmentally-friendly food sources.

  2.   Peanuts are self pollinated. They do not require outside aid, such as bees, other insects or wind to carry pollen.

  3.   Peanuts can grow in geographical areas where pollinators are scarce or absent.

  4.   Peanuts (and peanut butter) are a highly nutritious food source for people. They have been major factors in the elimination of malnutrition in some African countries. Peanuts provide important dietary fiber, vitamins, minerals and other healthful compounds (including antioxidants).


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