Abstract
The term ‘nut’ is widely used for edible fruits or seeds that have a hard pericarp, but botanical authorities fail to agree on any aspect of its definition. We use a combined definition here to evaluate 22 species edible by humans and commonly called ‘nuts’, or with nut-like characteristics. The only fruit that meets the criteria is the hazelnut (Corylus spp.). The criterion least often included is that the fruit must be derived from a compound ovary, but even removing this restriction there are no other edible nuts. The vast majority of species of true nuts are not eaten by humans. The syndrome of a large, edible seed with hard outer covering derived from either the testa or pericarp is found in a wide range of families. It is an adaptation to animal dispersal by gathering, hoarding and forgetting, and is a notable example of parallel evolution.
Introduction
Scientists should be concerned with precision in their writing. If vernacular words are used as technical ones, as they often are (e.g. ‘leaf’, ‘flower’, ‘growth’, ‘bog’), their scientific usage should be defined and adhered to. The example of ‘nut’ is used here to illustrate the point.
We talk loosely of nuts, even of ‘pine nuts’ (Pinus L. spp.), though a nut must have a woody pericarp (fruit wall) and the pine, being a gymnosperm, has no pericarp at all. Some other ‘nuts’ stand out from the minimum rule that the pericarp must be woody, with one seed inside. The woody covering of the Brazil ‘nut’ (Bertholletia excelsa Humb. & Bonpl.) is the testa (seed coat, derived from the integument[s]). The fruit of B. excelsa contains several of these seeds, and, despite having a woody wall that must be opened by rodents such as agoutis to release the seeds, it is not a nut either. The almond (Prunus dulcis (Mill.) D.A. Webb) seems to have a woody fruit wall, but that is only the endocarp, the fleshy mesocarp having disappeared. The nature of the almond pericarp is clear in comparison with the closely related apricot (Prunus armeniaca L.), where humans eat the fleshy parts of the pericarp (exocarp and mesocarp) and discard the rest (woody endocarp and seed).
What is a nut?
This is actually a difficult question. In eight botanical dictionaries and glossaries we found no single criterion on which they all agree (, Appendix 1). The requirement for the fruit to be derived by the fusion of more than one carpel is absent in several definitions, but it does distinguish the nut from the achene, as in dandelion (Taraxacum Cass.), buttercup (Ranunculus L.), sedges (Carex L.) and strawberry pips (Fragaria L.). Here, we take the sum of those definitions and apply them to edible ‘nuts’ ().
Table 1 Definitions of ‘nut’ from eight botanical dictionaries/glossaries.
Table 2 Matching ‘nuts’ to the definition of ‘nut’.
Which are nuts?
Several ‘nuts’ fall at the same hurdle as the almond, that is, a more-or-less-fleshy mesocarp originally covered by the woody endocarp that is cracked open to obtain the edible seed, so the fruit wall is not all dry, and the fruit is best classified as a drupe (Mabberley 2008). Such ‘nuts’ are the pecan (Carya illinoinensis (Wangenh.) K. Koch), walnut (Juglans L. spp.), pistachio (Pistacia vera L.) and water chestnut (Trapa natans L.) (). The cashew (Anacardium occidentale L.) has a swollen, fleshy ‘apple’, but that is not part of the fruit itself, it comprises swollen pedicel and receptacle. The whole could be called a glans (Spjut 1994), but the fruit itself is a drupe with a hard endocarp but a spongy, non-lignified mesocarp. Some botanists have called it a nut, some a drupaceous nut and others a nutty drupe, but the consensus and logical conclusion is that it is a drupe. The most familiar drupes are those like the apricot where the endocarp is woody and the mesocarp is fleshy and edible, but a rather drier mesocarp meets the definition of drupe too. The mesocarp of the coconut (Cocos nucifera L.) is fibrous, for which reason it is normally classified as a drupe (e.g. Mabberley 2008). Spjut (1994) uses ‘nuculanium’ for the coconut, though he has 95 fruit types, which seems to defeat the purpose of a classification of fruit types. Kaden and Kirpicznikov (Citation1965) took fruit-type splitting to extremes, with 84 types for only 49 of the angiosperm families, often a specific type for each genus (e.g. ‘alsinocarpium’ for Alsine, ‘berberidocarpium’ for Berberis, ‘cannabocarpium’ for Cannabis), and, they reassure us, ‘considerably less than a thousand’ fruit types in total for the angiosperms.
It is clear that the pericarp of the peanut (Arachis hypogea L.) is non-woody; they are sometimes sold whole with two seeds inside the indehiscent fibrous pericarp, immediately breaking two of the rules. The peanut is in a Fabaceae follicleFootnote1, like the pea, derived from a simple ovary. The related Moreton Bay chestnut (Castanospermum australe A. Cunn. & C. Fraser ex Hook.) is similar. It has a more woody pericarp, but it still has more than one seed (three to five) and is from a simple ovary. The Bambara groundnut (Voandzeia subterranea (L.) Thouars ex DC. ≡ Vigna subterranea (L.) Verdc.) does have a single seed, but fails to be a nut because the pericarp is not woody and it comprises only one carpel. All three are members of the Fabaceae. The fruit of the macadamia (Macadamia integrifolia Maiden & Betche and M. tetraphylla L.A.S. Johnson; Strohschen 1986) has been called a drupe or follicle, depending how fleshy the pericarp is judged to be, but the pericarp is removed before sale and the ‘macadamia nut’ bought in shops is just the seed, with the hard shell of the ‘nut’ being the testa. In the kukui nut/candlenut (Aleurites moluccana (L.) Willd.) no part of the pericarp is woody or even dry, the hard shell being the testa, so it is a berry.
Most misleading of all are the Chinese water chestnut (no taxonomic relation to the water chestnut) (Eleocharis dulcis (Burm. f.) Trin. ex Hensch.), which is not a fruit at all but an underwater corm, and the tiger nut (Cyperus esculentus L.), a stem tuber.
That leaves only seven possible nuts. The flesh we eat in the litchi/lychee (Litchi chinensis Sonn.), longan (Dimocarpus longan) and rambutan (Nephelium lappaceum L.) is an aril, not part of the seed and arguably not part of the pericarp. (The aril is sometimes called a ‘sarcotesta’ on the grounds that it is derived from the integument, but other workers report that it is derived from the funicle, perhaps with the obturator. Some workers restrict ‘sarcotesta’ to the Cycadophyta. Many do not distinguish between ‘aril’ and ‘sarcotesta’, including outgrowths of the integuments, funicle, hilum and/or obturator as ‘aril’.) The litchi/lychee, longan and rambutan are technically close to being nuts, but the pericarp is hardly woody. Mangosteens (Garcinia mangostana L.) are very similar, but the pericarp is even further from woody: they are berries. However, the main reason these four fruits are not called nuts in the vernacular is probably because humans eat only the fleshy aril, not the seed. Sweet chestnut (Castanea sativa Mill.) is closer to being a nut. An inflorescence of usually three flowers is enclosed in an involucre of fused, woody bracts which then form a cupule around three fruit. The fruit itself corresponds to the definitions of ‘nut’ except that it is doubtful whether the pericarp could be called woody. The nut plus cupule forms a glans. Oaks (Quercus spp.) are related, but with only one fruit in each cupule, and similarly the acorn is a nut only if one regards the pericarp as being woody. They were eaten as nuts by humans in pre-historic Europe and North America (Karg & Markle Citation2002). The chestnut and oak are close to being nuts, but it is our judgement that the pericarp of neither could reasonably be described as wood.
Conclusion and evolution
The only edible fruit/seed that is clearly a nut is the hazelnut (Corylus L. spp.). It meets all the criteria. Even if we exclude the least common criterion, that is, a compound ovary (), no other candidate meets them (). Three Corylus species are involved. The common hazel nut is C. avellana. ‘Cob nut’ is an old (AD 1440 Simpson & Weiner Citation1989) synonym for C. avellana fruit, but it is also used for a cultivar of that species. The fruit of C. maxima and C. americana is often called the ‘filbert nut’, but that name and ‘hazel nut’ can be used for any of the three Corylus species.
The berry and drupe have evolved to attract a predator, who will leave the seed or the endocarp + seed to germinate. However, there are 17 species treated here that have large, hard, edible seeds: almond, Bambara groundnut, Brazil nut, cashew, coconut, hazelnut, kukui nut/candlenut, macadamia, Moreton Bay chestnut, oak, peanut/groundnut, pecan, pine nut, pistachio, sweet chestnut, walnut and water chestnut. They belong to 12 families: Anacardiaceae, Arecaceae, Betulaceae, Euphorbiaceae, Fabaceae, Fagaceae, Juglandaceae, Lecythidaceae, Lythraceae, Pinaceae, Proteaceae and Rosaceae, which suggests repeated parallel evolution. They include five types of fruit—berry, capsule, drupe, follicle and nut—plus naked seeds in the pine. In most cases, the meat of the seed comprises the cotyledons of the embryo, the three exceptions being the pine nut where it is the haploid female gametophyte, the coconut where it is the cellular endosperm and the Brazil nut where it is undifferentiated, mainly hypocotyl. It seems that the nature of the endosperm does not allow it to form a hard tissue, and even the meat of the coconut is not hard as in the other fruits, with the centre being liquid cytoplasm of endosperm, with some DNA and RNA material but no whole nuclei.
A large seed has evolutionary advantages and disadvantages in terms of dispersal to suitable sites and in seedling establishment. However, once the seed is large and nutritious, granivores, mammal or bird, are encouraged to gather them (Vander Wall 2001). In 15 of the 17 species above there is a hard outer covering, even if it is in some cases the testa, and even though in most cases the fruit is not a nut. The clearest examples are probably the almond, Brazil nut, coconut, hazelnut, macadamia, Moreton Bay chestnut, pecan, pistachio and walnut. Such a woody wall is difficult to open, so many are not eaten immediately but hoarded, usually in sites removed from the fruit-bearing tree (Muñoz & Bonal Citation2011). In masting species, a higher proportion of the fruit tends to be hoarded in mast years. The fruits may be recovered by the hoarder, by conspecifics, or by other species such as wild pig (Sus scrofa). However, the hoarder may have died, and anyway the almost odourless fruit are not easy to re-find (Vander Wall 2010). Some therefore survive to germinate, the percentage differing markedly between sites and years (Pulido & Díaz Citation2005; Muñoz & Bonal Citation2011), but perhaps in the order of 5%–10% (Vander Wall 1994). In the process of hoarding and forgetting, the seeds are dispersed tens or hundreds of metres from the mother tree, depending on the fruit and especially on the hoarder/disperser (Jansen et al. Citation2004). The occurrence of this syndrome in a wide range of families, and in many species not listed here because they are not eaten by humans, is a notable example of parallel evolution.
We have applied the definition of ‘nut’ strictly. In the case of the almond and apricot it is clear that the fleshy mesocarp renders the fruit a drupe. However, other cases are more marginal, more nut-like, and terms have been used such as ‘dry drupe’, ‘drupe-like’, ‘drupaceous’, ‘nut-like drupe’, ‘drupe-like nut’, ‘nutty drupe’ and ‘drupaceous nut’. We have made our best judgement under ‘Fruit type’ in , but marginal cases result in different workers using a different terms. For example, the fruit of A. moluccana (candlenut) has been described as a ‘berry’, ‘drupe’, ‘capsule’ and ‘nut’.
There are many plant species with genuine nuts, but of the edible ‘nuts’, the hazel is the only true nut. ‘I'm Charley's aunt from Brazil, where the nuts come from’, says character Lord Fancourt Babberly in Charley's Aunt, impersonating the real aunt (Thomas 1892). But the Brazil nuts were impersonating too.
Notes
1‘legume’, but this is a type of follicle.
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Appendix: definitions of ‘nut’
- Allaby 2006 : a dry, indehiscent, woody fruit .
- Allan 1961 : an indehiscent one-seeded fruit with hard woody wall .
- Bailey 1999 : a dry indehiscent fruit that is usually shed as a one-seeded unit. It forms from more than one carpel but only one seed develops, the rest aborting. The pericarp is usually lignified and is often partially or completely surrounded by a cupule. True nuts include the acorn, hazelnut, and beechnut. The term is often loosely applied .
- Jackson 1928 : a hard and indehiscent, one-seeded fruit .
- Mabberley 2008 : technically a hard, one-seeded brittle fruit, (in common parlance …)
- Stace 2010 : a dry, indehiscent one-seeded fruit with a hard woody wall, often large
- Usher 1966 : a hard, dry, usually one-seeded indehiscent fruit, derived from a syncarpous ovary .
- Willis 1925 a dry indehiscent fruit, the product of more than one carpel.