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Walnuts (genus Juglans) are plants in the family Juglandaceae. They are deciduous trees, 10 - 40 metres tall (about 30-130 ft.), with pinnate leaves 200 - 900 millimetres long (about 7-35 inches), with 5 - 25 leaflets; the shoots have chambered pith, a character shared with the wingnuts (Pterocarya) but not the hickories (Carya) in the same family.
The 21 species in the genus range across the north temperate Old World from southeast Europe east to Japan, and more widely in the New World from southeast Canada west to California and south to Argentina. The Latin name Juglans derives from Jovis glans, “Jupiter’s acorn”: figuratively, a nut fit for a god.
The word walnut derives from Old English wealhhnutu, literally “foreign nut”, wealh meaning “foreign” (wealh is akin to the terms Welsh and Vlach; see *Walha and History of the term Vlach). The walnut was so called because it was introduced from Gaul and Italy. The previous Latin name for the walnut was nux Gallica, “Gallic nut”.
The best-known member of the genus is the Persian Walnut (Juglans regia), native from the Balkans in southeast Europe, southwest & central Asia to the Himalaya and southwest China.
The scientific name Juglans is from the Latin jovis glans, “Jupiter’s acorn”, and regia, “royal”. Its common name, Persian walnut, indicates its origins in Persia (Iran) in southwest Asia; ‘walnut’ derives from the Germanic wal- for “foreign”, recognising that it is not a nut native to northern Europe. In Kyrgyzstan alone there are 230,700 ha of walnut-fruit forest, where J. regia is the dominant overstorey (Hemery and Popov 1998). This is the species which is widely cultivated for its delicious nuts. J. regia is also called English walnut because English merchant marines once controlled its world commerce.
The Black Walnut (Juglans nigra) is a common species in its native eastern North America, and is also widely cultivated elsewhere. The nuts are edible, but have a smaller kernel and an extremely tough shell, and they are not widely grown for nut production.
The Butternut (Juglans cinerea) is also native to eastern North America, where it is currently endangered by an introduced disease, butternut canker, caused by the fungus Sirococcus clavigignenti. Its leaves are 40-60 cm long, and the nuts oval.