<P> The peanut, also known as the groundnut or the goober and taxonomically classified as Arachis hypogaea, is a legume crop grown mainly for its edible seeds . It is widely grown in the tropics and subtropics, being important to both small and large commercial producers . It is classified as both a grain legume and, because of its high oil content, an oil crop . World annual production of shelled peanuts was 42 million tonnes in 2014 . Atypically among crop plants, peanut pods develop underground rather than aboveground . It is this characteristic that the botanist Linnaeus used to assign the specific name hypogaea, which means "under the earth ." </P> <P> As a legume, the peanut belongs to the botanical family Fabaceae; this is also known as Leguminosae, and commonly known as the bean, or pea, family . Like most other legumes, peanuts harbor symbiotic nitrogen - fixing bacteria in root nodules . This capacity to fix nitrogen means peanuts require less nitrogen - containing fertilizer and improve soil fertility, making them valuable in crop rotations . </P> <P> Peanuts are similar in taste and nutritional profile to tree nuts such as walnuts and almonds, and as a culinary nut are often served in similar ways in Western cuisines . The botanical definition of a "nut" is a fruit whose ovary wall becomes very hard at maturity . Using this criterion, the peanut is not a true nut, but rather a legume . However, for culinary purposes and in common English language usage, peanuts are usually referred to as nuts . </P> <P> Cultivated peanut (A. hypogaea) arose from a hybrid between two wild species of peanut, thought to be A. duranensis and A. ipaensis . The initial hybrid would have been sterile, but spontaneous chromosome doubling restored its fertility, forming what is termed an amphidiploid or allotetraploid . Genetic analysis suggests the hybridization event probably occurred only once and gave rise to A. monticola, a wild form of peanut that occurs in a few restricted locations in northwestern Argentina, and by artificial selection to A. hypogaea . The process of domestication through artificial selection made A. hypogaea dramatically different from its wild relatives . The domesticated plants are bushier and more compact, and have a different pod structure and larger seeds . The initial domestication may have taken place in northwestern Argentina, or in southeastern Bolivia, where the peanut landraces with the most wild - like features are grown today . From this primary center of origin, cultivation spread and formed secondary and tertiary centers of diversity in Peru, Ecuador, Brazil, Paraguay, and Uruguay . Over time, thousands of peanut landraces evolved; these are classified into six botanical varieties and two subspecies (as listed in the peanut scientific classification table). Subspecies A. h . fastigiata types are more upright in their growth habit and have shorter crop cycles . Subspecies A. h . hypogaea types spread more on the ground and have longer crop cycles . </P>

Explain why a peanut is not a nut
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