The Peculiar Peanut

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Did you know that peanuts are not nuts at all? More accurately, they are a type of legume. What are you are actually eating are the cotyledons and embryo of the next generation of plants. Despite their popularity around the world, the plant itself gets very little attention outside of agricultural circles. This is a shame as the ecology of the peanut is truly fascinating.

The plant that produces the peanuts that the world either loves or fears is known scientifically as Arachis hypogaea. It was originally native to South America and it is believed to have first been domesticated in Paraguay over 7,000 years ago. Domestic peanuts are amphidiploid or allotetraploid meaning they have two sets of chromosomes from two different parent species. 

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Research points to a natural hybridization event between Arachis duranensis and Arachis ipaensis, which produced the tetraploid Arachis monticola. It is A. monticola that gave rise to the domestic peanut we know today. As a plant, it looks rather much like most pinnate legumes. Its yellow flowers are unmistakably pea-like. It is only after they have been pollinated that they become truly bizarre. 

Members of this genus exhibit what is called "geocarpy." This relatively rare form of plant reproduction involves the plant literally planting its own seeds. After fertilization, the flower stalk elongates and bends towards the ground. Once it touches soil, the stalk pushes the developing seed pod down into the dirt. Underground, the seeds mature and germinate. The embryos will only become active in the dark, subterranean environment. This is especially useful in habitats where soil disturbance is a frequent occurrence. Geocarpic plants like the peanut increase the likelihood that their offspring will survive long enough to germinate and grow by skipping over that pesky seed dispersal step. Keep that in mind the next time you tuck into a bag of peanuts.

Photo Credit: [1]

Further Reading:

http://www.biomedcentral.com/content/pdf/1471-2164-14-517.pdf

http://www.amjbot.org/content/94/12/1963.abstract