A nut may be defined as a one-seeded fruit with a hard pericarp (ripened ovary wall). One
or several nuts may sit in a cup-shaped structure called an involucre. In oaks (Quercus) the
involucre is composed of small scales and the entire structure (involucre plus nut) is called
an acorn. In chestnuts (Castanea) and beech (Fagus) the involucre is spiny, while in filberts
and hazelnuts (Corylus), the involucre is leafy or tubular, depending on the exact species.
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Assortment of true and false nuts: A. Hazelnut (Corylus americana), B. Pecan (Carya illinoensis), C. Peanut (Arachis hypogea), D. Macadamia Nut (Macadamia integrifolia), E. Almond (Prunus amygdalus), F. Brazil Nut (Bertholletia excelsa), G. Chestnut (Castanea dentata), H. Kukui Nut (Aleurites molucanna), I. Water Caltrop (Trapa bicornis), J. Walnut (Juglans regia). [True nuts: A. & G.; Drupes: B., D., E., H., I. & J.; Seeds: F.; Legumes: C. Note: The pecan (B) and walnut (J) are also considered to be a true nuts by some botanists.]
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Introduction
Not all nuts fit the above definition. In fact, Webster's New World Dictionary of the American Language (1970) also defines a nut as a foolish, crazy or eccentric person, or one of the two testicles in a male. Many so-called botanical nuts are more appropriately termed "drupes" or "dry drupes." These "false nuts" are really the seed-bearing, hard, inner layer (endocarp) of a fruit called a drupe. In dry drupes the outer layer or husk sometimes splits open or withers. This outer husk is part of the ovary wall (pericarp), and the hard inner wall surrounding the seed represents the inner part of the pericarp. Dry drupes are technically not true nuts because in true nuts the hard outer wall constitutes the entire pericarp. The coconut (Cocos nucifera) is a classic example of a dry drupe, with a thin, green, outer layer called the exocarp, a thick, fibrous middle layer called the mesocarp, and a very hard inner layer surrounding the large seed called an endocarp. These same three layers are easily visible in fleshy drupes such as the peach (Prunus persica), plum (P. domestica), and apricot (P. armeniaca). Unshelled almonds (P. amygdalus) are seeds still contained within an endocarp layer.
Almond & Peach: Rose Family (Rosaceae)
The pit of a peach (Prunus persica) showing the seed that is contained inside the hard, woody endocarp layer. The endocarp is the inner layer of the fruit wall or pericarp. It is surrounded by a fleshy mesocarp and a thin outer skin or exocarp. Fruits with a distinct endocarp layer surrounding the seed are called drupes. The endocarp protects and aids in the dispersal of the vulnerable seed, especially when it is swallowed by a hungry herbivore.
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The fresh, greenish fruit of an almond (Prunus amygdalus) contains the familiar one-seeded endocarp (unshelled almond) that is commonly sold in supermarkets during the holiday season. Each hard-shelled endocarp contains a single seed.
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Other examples of drupes include date palm nuts (Phoenix dactylifera) and pistachio nuts (Pistacia vera). There is still considerable disagreement and controversy over the classification of some of these so-called nuts, particularly English walnuts (Juglans regia), black walnuts (J. nigra), and macadamia nuts (Macadamia integrifolia & M. tetraphylla). Botanists have devised all sorts of ingenious names for these "borderline nuts," such as dry drupe, drupe-like, drupaceous, drupaceous nut, and nutty drupe. Some readers may think the author of this essay has been driven to a mental condition with the same spelling as the plural of nut.
Mango, Pistachio & Gum Mastic: Sumac Family (Anacardiaceae)
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The mango (Mangifera indica) is a drupe with an outer leathery skin (exocarp), a fleshy mesocarp and a hard, stony endocarp (pit) surrounding the large seed. Wave-worn, sun-bleached endocarps often wash ashore on tropical beaches, probably thrown overboard from ships and boats at sea.
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Pistachio (Pistacia vera), a dioecious tree in the sumac family (Anacardiaceae). It is native to the eastern Mediterranean region and central Asia where it has been cultivated for over 3,000 years. Like the almond, the fruit is drupaceous with a fleshy, greenish outer layer (exocarp and mesocarp) surrounding the hard, seed-bearing shell or endocarp. The seed has a papery seed coat and two greenish cotyledons. Commercial pictachio "nuts" are split, seed-bearing endocarps with the surrounding, fleshy fruit wall removed. The roasted, salted, greenish seeds are eaten raw and are the delectable ingredient in ice creams, cakes and nougat candies. Another species called Chinese pistache (P. chinensis) is commonly cultivated in southern California for its beautiful reddish autumn foliage.
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Chinese pistache (Pistacia chinensis), a dioecious tree in the sumac family (Anacardiaceae). Native to China, Taiwan and the Philippine Islands, it is grown in southern California for its colorful compound (even pinnate) leaves during the autumn months and its bright red "berries." The red "berries" are actually small, one-seeded drupes which are not edible. A related species from the eastern Mediterranean region (P. lentiscus) is the source of "gum" mastic, an oleoresin used in perfumes, chewing gums, pharmaceuticals, dental adhesives, and in high grade varnishes for protecting pictures. Mastic is one of the oldest known high grade resins utilized by people, and it is extensively cultivated on the Greek island of Chios.
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Chinese pistache (Pistacia chinensis) photographed in November at the San Diego Wild Animal Park. This tree produces some of the most spectacular red autumn foliage of any species in southern California. In fact, the fall coloration is reminiscent of deciduous trees of the eastern United States.
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Gum mastic or "Chios Tears," raw resin globules from the mastic tree (Pistacia lentiscus). Mastic is one of the oldest known high grade resins utilized by people, and it is extensively cultivated on the Greek island of Chios. Mastic resin (technically an oleoresin) is used in perfumes, chewing gums, pharmaceuticals, in high grade varnishes for protecting pictures, and in adhesives for dental caps.
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An ant entombed in a resin globule from the mastic tree (Pistacia lentiscus). The insect-bearing resins of some trees become buried in the soil and gradually polymerize into amber.
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Miscellaneous Nuts In Different Plant Families
The fruit of kukui nut or candlenut (Aleurites molucanna) is usually classified as a drupe or drupaceous nut, but the actual "nut" is really a woody, thick-walled seed, typically one or two inside each fruit. Drupes usually have one seed, but the kukui nut appears to be an exception. Goat nuts or jojoba are considered to be seeds within a dehiscent capsule. Brazil nuts (Bertholletia excelsa) are also seeds produced in a thick-walled capsule. The cashew nut (Anacardium occidentale) is actually a thick-shelled, seed-bearing drupe produced at the summit of a fleshy stalk (pedicel) called a cashew apple. Pine nuts and pignolia nuts are seeds produced in woody cones from several species of pines, including the pinyon pines (Pinus monophylla & P. edulis), and the Italian stone pine (P. pinea). The peanut (Arachis hypogea) is actually a seed with a papery seed coat, typically two seeds enclosed in a dehiscent pod called a legume. After fertilization, the flower stalk of the peanut curves downward, and the developing fruit (legume) is forced into the ground by the proliferation and elongation of cells under the ovary. The peanut pod subsequently develops underground. Ivory-nuts are seeds from the ivory-nut palm (Phytelephas aequatorialis). Ivory-nut palms have an extensive distribution along banks of tropical American rivers, from Panama and Colombia to Peru. They are most abundant in the Amazon Basin of Bolivia, Ecuador and Peru. Female palms bear clusters of large, brown fruits, the size of grapefruits or melons. Each fruit is studded with numerous woody, pointed horns and contains four or more large seeds. The seeds are the source of vegetable ivory, a hard, natural material used as a substitute for elephant tusks.
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A Hawaiian kukui nut necklace, made from the polished, thick-shelled "nuts" (seeds) of the candlenut tree (Aleurites molucanna). The green fruit is cut open to show the brown, thick-walled seed within the endocarp layer. The leathery, green outer layer (mesocarp and exocarp) is similar to the husk of a walnut. [According to most authorities, the kukui fruit is a drupe or drupaceous nut rather than a true nut.] Several rough, woody "nuts" (seeds) are shown before polishing.
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The candlenut tree (Aleurites molucanna). The fruit structure shown in the above
image is conclusive evidence that kukui nuts are actually seeds produced in a drupe.
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Cashew tree (Anacardium occidentale). Left: A cashew "nut" attached to its swollen stalk (pedicel). The shell of the "nut" contains the poison oak allergen urushiol, and may cause dermatitis in hypersensitive people. Right: The fleshy "apple" is the swollen stalk (pedicel) to which the one-seeded "nut" is attached. Technically, the "nut" is a dry fruit called a drupe. It consists of a thin outer layer (exocarp and reduced mesocarp) surrounding a thicker, seed-bearing endocarp. Some botanists prefer not to commit themselves and call the cashew fruit a drupaceous nut.
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Another interesting Malaysian "nut" that is actually a seed is called "buah keluak" or "kluwak nut" (also spelled kloowak). Kluwak nuts come from the kepayang tree (Pangium edule) of Indonesia & Malaysia, a member of the flacourtia family (Flacourtiaceae). The oily, hard-shelled seeds superficially resemble Brazil nuts. Meaty seeds are edible after the poisonous hydrocyanic acid is removed by soaking and boiling them in water. Fermented kluwak nuts become chocolate-brown, greasy and very slippery. Cooked seeds are used in a number of popular Malaysian and Indonesian dishes.
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Peeled kluwak nuts from the kepayang tree (Pangium edule). The fermented seeds become chocolate-brown, greasy and slippery. Cooked seeds are used in a number of popular Malaysian and Indonesian dishes.
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Oak & Chestnut: Beech Family (Fagaceae)
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The true nut of an acorn sits in a cup-shaped involucre composed of numerous overlapping scales. These acorns are from the cork oak (Quercus suber), the bark of which is the source of natural cork.
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The true nuts of the chestnut (Castanea dentata) are produced in a spiny, cup-shaped involucre.
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Two angular nuts of American beech (Fagus grandifolia) within a spiny-bracted involucre. The three-sided nuts resemble the miniature one-seeded fruits (achenes) of buckwheat (Fagopyrum esculentum). This resemblance led to the German name "buchweizen" (beech-wheat) which became corrupted to the present name of buckwheat.
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One-seeded achenes of buckwheat (Fagopyrum esculentum), an important crop plant native to central Asia. The three-sided achenes resemble miniature nuts from the beech tree (Fagus). This resemblance led to the German name "buchweizen" (beech-wheat) which became corrupted to the present name of buckwheat. The hulled achenes or groats are used in several brands of hot and cold breakfast cereals. The seeds are ground into flour which is used for pancakes, noodles and breads. In Russia, a nutritious porridge called "kasha" is made from buckwheat flour.
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Filbert: Birch Family (Betulaceae)
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The American filbert of the eastern United States (Corylus americana) produces a true nut enclosed in an involucre of leafy bracts. In the closely-related species of the Pacific northwestern United States C. cornuta, the nut is produced in an elongate, tubular involucre.
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The native filbert of the Pacific northwestern United States (Corylus cornuta) produces a true nut enclosed in an elongate, tubular involucre. In this photo the tubular involucre has been sectioned lengthwise to expose the hard-shelled nut inside.
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Coconut & Date: Palm Family (Arecaceae)
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The fruit of the coconut (Cocos nucifera) is technically a dry drupe composed of a thin outer layer (exocarp), a thick, fibrous middle layer (mesocarp), and a hard inner layer (endocarp) surrounding a large seed. The endocarp (upper left) contains germination pores at one end, one of which the sprouting coconut palm grows through. The "meat" (upper middle) of the seed is endosperm tissue and a small, cylindrical embryo is embedded in this nutritive tissue. "Coconut water" (upper right) is liquid endosperm that has not developed into solid tissue composed of cells. Copra comes from the meat of dried coconuts, while coir fibers are derived from the fibrous mesocarp.
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Sprouting fruit of a coconut Cocos nucifera. The hard inner layer (endocarp) contains the actual seed composed of a minute embryo and food storage tissue (endosperm). The base of the embryo (cotyledon) swells into an absorbing organ that fills the entire cavity of the seed as it digests the endosperm. The endocarp has three germination pores, one functional pore and two plugged pores. [In "blind coconuts" all three pores are plugged.] The three pores represent three carpels, typical of the palm family (Arecaceae). Just inside the functional germination pore is a minute embryo embedded in the endosperm tissue. During germination, a spongy mass develps from the base of the embryo and fills the seed cavity. This mass of tissue is called the "coconut apple" and is essentially the functional cotyledon of the seed. [The white color has been altered in order to clearly differentiate it from the endosperm.] It dissolves and absorbs the nutrient-rich endosperm tissue to supply the developing shoot with sugars and minerals. Eventualy, the developing palm becomes self sufficient, as its leaves produce sugars through photosynthesis and its roots absorb minerals from the soil. The coconut "apple" is rich in sugars and is a sweet delicacy in tropical countries. The endosperm is the coconut "meat" which is dried and sold as "copra." The coconut "water" is multinucleate liquid endosperm inside green coconuts that has not developed into solid tissue composed of cells. Before the liquid endosperm forms a solid "meat" it is jellylike and may be eaten with a spoon. This stage of the endosperm development is called "spoon meat." The "coconut milk" used in many Asian recipes is made by soaking grated coconut meat in water and squeezing out the oil-rich liquid. "Coir" fibers are derived from the fibrous mesocarp. The saturated fat called "coconut oil" is derived from the meaty endosperm.
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Close-up view through the inside of a coconut seed showing a small, cylindrical embryo (A) embedded in the fleshy meat or endosperm (B). The base of the embryo (pointing into the coconut) swells into an absorbing organ (cotyledon) that fills the entire cavity of the seed as it digests the endosperm. The wall of the endocarp (C) is a hard, woody layer that makes up the inner part of the fruit wall. The thick, fibrous husk (mesocarp) that surrounds the endocarp has been removed. The coconut "pearl" apparently develops where the embryo is located.
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Left: Unripe, seedless, unpollinated fruits of the date palm (Phoenix dactylifera) from a female tree in Coastal San Diego county. Right: Pollinated, dried, Medjool dates from Coachella Valley, California. Dates are technically referred to as drupes because the outer fleshy tissue is part of the pericarp. The inner, hard endocarp layer (pit) encloses the seed. The date palm is a dioecious species, like willows, cottonwoods, marijuana and people. Since the fruits occur on separate female trees, they must be pollinated by male trees in order to mature into sweet, delicious dates.
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Walnut & Pecan: Walnut Family (Juglandaceae)
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Note: Many botanists say that the husk of pecans and walnuts
contains tissue from the outer pericarp, and insist on referring
to these dry fruits as "drupaceous nuts" rather than true nuts.
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According to "The Morphology of the Flowers of the Juglandaceae" by W.E. Manning (1940), American Journal of Botany 27 (10): 839-852, the fruits of Juglans and Carya are drupe-like but not a drupe or dry drupe. The fruit is sometimes called a "tryma" but can be described as a nut. Webster's Third New International Dictionary describes a tryma as a nutlike drupe (as the fruit of the walnut or hickory) in which the epicarp and mesocarp separate as a somewhat fleshy or leathery rind from the hard 2-valved endocarp. The tryma is also defined as a drupe with a dehiscent husk, which fits the genus Carya perfectly.
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English walnut (Juglans regia). In the textbook Botany: An Introduction to Plant Biology by T.E. Weier, C.R. Stocking, M.G. Barbour and T.L. Rost (1982), the walnut is classified as a nut. According to these authors, the green husk or shuck of the walnut is composed of involucral bracts, perianth (calyx) tissue, and the outer layer of the pericarp. The hard shell surrounding the seed is the inner layer of the pericarp. In true nuts, the hard, indehiscent layer surrounding the seed is the ripened ovary wall or pericarp. In oaks and chestnuts of the beech family (Fagaceae), the nut sits in a cuplike or spiny involucre composed of involucral tissue (or fused calyx tissue) that is not part of the ovary wall (pericarp). The walnut does not exactly fit the definition of a true nut because the hard shell is from the inner pericarp and not the entire ovary wall. Unlike the closely-related pecan (Carya), the husk does not split into four sections and actually resembles the outer fleshy pericarp of a drupe. According to most botanical references, the outer green layer or husk of a walnut is part of the pericarp and the hard shell surrounding the seed is the endocarp layer as in coconuts. Therefore, walnuts and closely-related pecans probably fit the dry drupe category rather than a true nut. However, if the walnut husk contains tissue from involucral bracts, perianth (calyx), and the outer pericarp, it doesn't exactly fit the definition of a drupe. Some authors eloquently avoid this dilemma by calling these fruits drupe-like or "drupaceous nuts." To really impress (confuse) someone, refer to the walnut as a tryma.
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Black walnut (Juglans nigra), a deciduous hardwood tree native to the eastern and central United States. A. Mature dry fruit containing outer green husk. B. Hard (woody) inner layer surrounding the seed. This is what some botanists call the actual "nut." C. Sectional view showing cotyledons of inner seed and outer green husk (red arrow).
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A California black walnut (Juglans californica) and the unusual small-fruited walnut (J. microcarpa) native to desert streambeds and canyons of the southwestern United States. Walnuts are called nuts or dry drupes, depending on how you define the fleshy husk tissue surrounding the seed-bearing "nut." If you consider the husk to be part of the ovary wall (pericarp), then walnuts would fit the dry drupe category. Some botanists refer to walnuts as "drupaceous nuts."
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Pecan (Carya illinoensis): The green, fleshy outer husk or shuck splits into 4 valves, exposing a single large, one-seeded "nut" surrounded by a thick, woody pericarp. If the outer husk is composed of calyx tissue and is not part of the pericarp wall, then it fits the description of a nut; however, some botanists say that the husk contains tissue from the outer pericarp. In the walnut and butternut (Juglans), also members of the Juglandaceae, the husk does not split into sections and actually resembles the outer fleshy pericarp of a drupe. In true nuts, the hard, indehiscent layer surrounding the seed is the ripened ovary wall or pericarp and the outer husk is composed of involucral or calyx tissue that is not part of the ovary wall or pericarp. The "tryma" is a fruit type defined as drupe with a dehiscent husk, which fits the pecan perfectly. The "hican" is a Carya hybrid resulting from a cross between the pecan (C. illinoensis) and the shagbark hickory (C. ovata).
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Macadamia Nut: Protea Family (Proteaceae)
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Macadamia nuts (Macadamia integrifolia) are dry drupes containing a very hard seed-bearing endocarp. According to most botanical references, the outer husk represents part of the ovary wall or pericarp. However, according to the The Macadamia Nut in California (1978), published by the California Macadamia Society, the fruit is a follicle consisting of a single seed (sometimes 2) in a husk which usually splits open at maturity. Technically, a follicle is composed of a single carpel that splits open along one seam. If the outer husk represents one carpel, then the macadamia nut is indeed a follicle. I have fluctuated on the classification of the macadamia fruit for years, but at this time I am leaning toward a follicle rather than a drupe. The following two images may shed some light on this contoversy.
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An immature macadamia nut (Macadamia integrifolia) released from its outer dehiscent husk (pericarp). The seed shows a distinct attachment scar (hilum) where it was joined to the placental region on the inner ovary wall (pericarp). In a mature seed, the attachment scar is covered over by the thick, woody seed coat. If the outer pericarp represents a single carpel, then this fruit is indeed a follicle rather than a drupe.
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A mature macadamia nut (Macadamia intregrifolia) showing the thick, woody seed coat and white seed. The white, micropyle region of the seed extends through the woody wall and is visible as a white dot on the exterior. In a true drupe, there would be no extension of the seed or micropyle through the woody endocarp wall.
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Two views of a mature macadamia seed (Macadamia integrifolia). The left view shows the hilum where the seed was attached to the inner ovary (pericarp) wall. The right view shows a white dot that represents the micropyle region where the pollen tube entered the ovule wall (integument layer) prior to double fertilization.
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A Hawaiian macadamia nut necklace made from the woody seeds of Macadamia integrifolia. Polished macadamia nuts are typically blond or light brown, compared with glossy, dark brown or black kukui nut necklaces.
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Peanut: Legume Family (Fabaceae)
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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 flower stalk (pedicel) of the peanut curves downward, and the developing fruit (legume) is forced into the ground by the proliferation and elongation of cells under the ovary. The peanut pod subsequently develops underground. As in other members of the enormous legume family (Fabaceae), the roots bear nodules containing nitrogen-fixing bacteria.
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The peanut (Arachis hypogea) is a dehiscent legume that is harvested from below the soil. The legume was originally formed above ground following pollination. After fertilization, the flower stalk of the peanut curves downward, and the developing pod is forced into the ground by the proliferation and elongation of cells under the ovary. 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.
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Buckeye: Horse Chestnut Family (Hippocastanaceae)
Some "nuts" are actually large, hard-shelled seeds produced inside a dehiscent capsule that splits open into sections called valves or carpels. The buckeye (Aesculus) is a good example of this kind of nutlike seed. There are several species of buckeye, including the California buckeye (A. californica), Ohio buckeye (A. glabra), and the European horse chestnut (A. hippocastanum), all members of the horse chestnut family (Hippocastanaceae). Another hard-shelled seed called Texas buckeye (Ungnadia speciosa), belongs to the interesting soapberry family (Sapindaceae).
The outer capsule wall of the European horse chestnut (Aesculus hippocastanum) is covered with stout spines. They are collected in autumn and the large seeds are removed from the spiny casing and allowed to dry. In England, the dried, hard-shelled seeds are used in a curious children's game called "conkers." A hole is drilled through the seed (called a conker) and a string in threaded through the hole. The game is played between two opponents, each with one conker. One player dangles their conker by the string, holding it steady, while the opponent swings their conker on its string and attempts to strike the dangling conker. Players take turns until one conker is so badly damaged that it dislodged from its string. The winner is the player with the intact conker. Serious conker players try to harden the shell (seed coat) of their conker by drying techniques and various protective finishes. In Australia, a similar game is called "bullies." One of the choice seeds used for this game comes from the fruit of the quandong tree (Eucarya acuminata), a member of the sandalwood family (Santalaceae). Like the Hawaiian sandalwood (Santalum), this tree is a root parasite on a nearby host plant.
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Left: California buckeye (Aesculus californica), a member of the horse chestnut family (Hippocastanaceae). Right: Texas buckeye (Ungnadia speciosa), a member of the soapberry family (Sapindaceae). The "nuts" of these species are actually hard-shelled seeds produced in dehiscent capsules that split open at maturity.
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