Botany 101. A peanut is not a nut but a legume, closer biologically to a pea or a bean than a walnut or pecan. Each ovary of the plant usually releases one seed per pod, and all normal shells contain more than one ovary.
But not all peanut shells contain two seeds. We are most familiar with Virginia peanuts, which usually contain two but occasionally sprout mutants that feature one, three or four. Valencia and Spanish peanuts boast three to five seeds per shell.
Traditionally, breeders have chosen to develop two-seeded pods for a practical reason. Two-seeders are much easier to shell. There is little taste difference among the varieties of peanuts, although the three-seed peanuts are more difficult to shell, requiring tremendous pressure to open without damaging the legume.
http://www.peanut.com/aboutpeanuts.asp
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