Millennium Seed Bank blog

I am a bit behind with my blog this month, having only just returned from a trip to Manaus (Brazil) where I taught one of my courses on the structural diversity of fruits and seeds. However, while there I encountered another pretty amazing fruit which is worth devoting a blog post to. 

The Amazon is a great place to discover new exotic fruits... and the Tarzan way-of-life (image Wolfgang Stuppy)

When travelling to such far-flung places, one of my passions is to try all the available exotic fruits, most of which I have at least heard about before. However, when I got to Manaus for the first time, I was almost shocked to find I had never heard of cupuaçu (pronounced ‘coo poo asoo’) before. Everyone told me this is the most famous and original fruit of the Amazon basin. In fact, it is considered by both locals and non-local connoisseurs to be the ‘taste of the Amazon’ and in March 2008 it was even declared the national fruit of Brazil. Sure enough, I was burning to try this mysteriously delicious fruit and find out what it actually ‘is’, from a botanical point of view.

The fruit of a cupuaçu (Theobroma grandiflorum) growing in the Amazon (image Wolfgang Stuppy) 

What’s chocolate got to do with it?

I was surprised to learn that cupuaçu (Theobroma grandiflorum) is a very close relative of cocoa (Theobroma cacao), the main ingredient in chocolate. Both plant species are indigenous trees of the Amazon rainforest and native tribes have used their fruits as a food source for centuries, if not millennia. The cupuaçu’s shared ancestry with cacao is clearly reflected in their very similar flowers and fruits, although at a height of just up to eight metres, cocoa is dwarfed by its cousin the cupuaçu, which can grow up to twenty metres tall. Incidentally, the flowers of cupuaçu are also much larger and more heavily built than those of cocoa.

Left: flower of cupuacu (Theobroma grandiflorum); right: flowers of cocoa (Theobroma cacao) - (image Wolfgang Stuppy) 

These very interesting-looking flowers give rise to large pods with a thick and tough brown (cupuaçu) or yellow to orange (cocoa) rind. Although similar in length (15-35 cm), the fruits of cupuaçu are more plump (up to 16 cm in diameter) and weigh up to 2 kg whereas cocoa pods are more slender and weigh only around 500 g. Inside their hard shell the fruits contain a white or yellowish juicy lump which consists of numerous large seeds covered in soft, fleshy seed coats (similar to the ice cream bean I have blogged about previously). 

Fruits of cocoa (Theobroma cacao) on the tree (left) and cut in half (right), revealing the fleshy seeds, one of which has already started germinating (image Wolfgang Stuppy) 

What most chocolate eaters won’t know

Chocolate owes its heavenly taste to cocoa butter extracted from the fermented seeds (‘cocoa beans’) of Theobroma cacao. Unbeknown to most people outside South America, the seeds of Theobroma grandiflorum yield a fat very similar to cocoa butter that is also used to make a type of chocolate called ‘cupulate’. Another fact that most chocolate eaters are unaware of is that the soft seed-coat-derived pulp surrounding fresh ‘cocoa beans' is also edible and it tastes delicious, just like the pulp of cupuaçu. However, neither of the two tastes even remotely of chocolate. Disappointing as you may find this - wait until you read about the flavours these fruits yield as they unfold on our palates.

Dried cocoa ‘beans’ on display in a market stall in Mexico City

Strange flavours

The pulp of cocoa pods has a fresh, sweet and sour taste that has variously been described as reminiscent of apple, lychee, mangosteen, banana, rambutan or soursop. This sounds pretty exciting, but to me cupuaçu has a far more exotic and unusual smell and taste. In fact, it doesn’t taste like anything else so it is really hard to explain its aroma, but I will try. The cupuaçu’s rich, voluminous flavour is sweet, sour and slightly tart at the same time, with a very pleasant but heavy, fruity component reminiscent of a mix of pear, banana and pineapple. On top of all this, almost like a blanket, lies a rather strong hint of something bizarre, almost artificial, that to me tastes like a mix of aniseed and wintergreen or perhaps the resinous aroma of mango skin. In short, it simply tastes like cupuaçu! Some people claim that cupuaçu also has a hint of chocolate but this seems more of a fantasy inspired by its close relationship to cacao. 

Amazonian superfood

The fresh pulp of cupuaçu is either eaten raw or turned into refreshing drinks, ice cream, pastries, candies, jams etc. Because of its high levels of antioxidants (with anti-ageing effects!) cupuaçu has been touted by some as the next Amazonian ‘superfood’ after the fruits of the açai palm (Euterpe oleracea, Arecaceae) and guaraná (i.e. the seeds of Paulinia cupana, Sapindaceae), both also from the Amazon region. The latter two have already caused some recent ‘health-food excitement’ in North America and Europe. 

Left: seeds of guarana (Paullinia cupana) for sale at a market in Manaus. Right: fruits of Paullinia pinnata which look very similar to those of guarana (image Wolfgang Stuppy)

 Some natural history ...

As exciting as the unusual flavours of tropical fruits are, far more exciting is to find out why these fruits have evolved these flavours in the first place. Soft and fleshy tissues in fruits are usually there to attract fruit-eating animals which, in exchange for food, help with seed dispersal. Co-adaptation to a certain group of dispersers is reflected in a distinct set (‘syndrome’) of fruit characteristics.

For example, the yellow- to orange-coloured fruits of Theobroma cacao are protected by a thick, tough rind that is typical of primate-dispersed fruits. In order to open the fruits to get to the sweet pulp, an animal must have a certain strength and dexterity, such as that of monkeys. Although monkeys avoid swallowing the soft, bitter-tasting seeds, they still disperse them. They remove the cohesive lump of up to fifty fleshy seeds from the cocoa pods, and then take their bounty to a safe place in the canopy where they nibble off the luscious pulp.

After birds and bats, primates like these Emperor tamarin monkeys (Saguinus imperator subgrisecens) from the Amazon basin are among the most important seed dispersers in rainforests (image Wolfgang Stuppy)

The story isn’t quite that simple with the harder and much heavier fruits of Theobroma grandiflorum. There is no doubt the fruits are adapted to be dispersed by a certain type of animal, albeit one with a very big mouth and perhaps a taste for the extraordinary. However, when trying to match the cupuaçu’s suite of functional traits against present-day dispersers in the Amazon rainforest, there is nothing that really fits.

The available animals are either too small to handle the heavy, hard-shelled fruit, or stuff like this is simply not part of their diet. The fascinating truth is that the cupuaçu’s texture, size, colour, taste and odour indicate that it is a typical ‘megafauna fruit’, adapted to be dispersed by the huge beasts that inhabited the Americas until the end of the last ice age, between 10,000-15,000 years ago. Among them were giant ground sloths, mastodons and gomphotheres (four-tusked elephant-like creatures). Like many other fruits in the Amazon rainforest, the cupuaçu really is a typical gomphothere fruit!

Top: horse cassia (Cassia grandis; Leguminosae; the white bar); left: tagua palm (Phytelephas macrocarpa; Arecaceae); middle: stinking toe (Hymenaea courbaril, Leguminosae); right: genipap (Genipa americana, Rubiaceae) (Image Wolfgang Stuppy)

Some more Amazonian megafauna fruits that used to be on the menu of the big beasts of the last ice age. Note the dull colours - large mammals are colour-blind!

For a more detailed explanation of the megafauna dispersal syndrome see my earlier blog about the Texas Mountain Laurel. To readers who have a special interest in fruits that are adapted for dispersal by extinct animals (‘anachronistic fruits’) I recommend the excellent paper by Guimarães et al (2008) which also discusses the cupuaçu. 

Oh, one last thing....

Because of its low melting point, the fat (‘butter’) extracted from the seeds of both cupuaçu and cocoa is also used as a base for suppositories.

- Wolfgang - 

Related links

• Beyond the Gardens: The Millennium Seed Bank Partnership - A short film showcasing behind the scenes work of Kew's scientists
• Kew's Projects in Brazil
• Loving Wolfgang's blogs? Discover more from his awesome books ...

This is not a regular ‘Seed of the Month’ blog post. Rather than writing about the seeds of a particular plant, I decided that this time, I want to share my ideas of the ‘bigger picture’ with you, which lays bare the ultimate reason why we, here at Kew’s Millennium Seed Bank, are so passionate about saving seeds from all over the world. The following ‘essay’ is about life on Earth, the future of mankind and the true relevance of seeds. And so here it goes...

Science, washing powder and beyond

When scientists are lost for words they create technical terms which help them to communicate with each other more easily and precisely. Most of these technical terms don’t ever percolate into the public domain. However, nowadays everybody talks about biodiversity - some still think it’s a new kind of washing powder. But behind this abstract word lies a meaning that goes far beyond that of washing powder, in fact, far beyond anything anyone can imagine. 

A (very!) small selection of fruits and seeds from all over the world
[Images from ’SEEDS – Time Capsules of Life’ by Rob Kesseler & Wolfgang Stuppy and ‘FRUIT – Edible, Inedible, Incredible’ by Wolfgang Stuppy & Rob Kesseler; Copyright Papadakis Publisher, Newbury, UK]

All creatures great and small

‘Biodiversity’ refers to the cornucopia of all life forms on Earth, most of which are still unknown to us. Estimates range from 3 million to 100 million species. A smart scientific study from 2011claims to have calculated that there are altogether almost 9 million species of plants, animals, fungi, bacteria and so-called protists (= microscopic critters that are neither plants, animals, fungi or bacteria). Of all these, only about 1.2 million are known to science and formally described. As for the other 8 million or so species, we kind of know that about 6.5 million of them are animals (mostly very small ones) but we don’t yet know who they are.

A tiny sample of planet Earth’s ‘life forms’ - all photos Copyright 2012 Wolfgang Stuppy
 

Kingdom of plants

After animals, the second largest kingdom of life on Earth are plants. At about 300,000 to 450,000 species, there are a lot less of them than animals but that doesn’t mean that they are less important. Quite the contrary! Many of us love plants, due to their sheer omnipresence, most of us can’t but take them for granted. But when we stop and think for a moment, I mean really think, plants, even the scruffiest of weeds, are amazing. Actually, they are not only amazing, they are, in fact, our life support system.

Unlike animals, plants have the remarkable ability to use sunlight to make sugar from just water and carbon dioxide during a complicated process called ‘photosynthesis’. In doing so, they not only produce their own food but also feed – either directly or indirectly – literally all life on Earth. Furthermore, as a ‘waste product’ of photosynthesis, they produce the oxygen in our atmosphere. To put it simply, without plants we would neither be able to breathe nor eat.

Flooded forest (‘Igapo’) on the Urubu River, Amazonas – Photo: copyright 2012 Wolfgang Stuppy

 
What’s the point of biodiversity?

Good question. Why would we need so many species if we don’t even know most of them? Let me try to illustrate this point using a not too far-fetched allegory.

Imagine your continued existence depends on a complicated life support system which, should it fail, would mean that you are definitely going to die. Scary stuff! Now imagine that your life support system is a very large, robust and luxurious one, one that has built in a great deal of redundancy and backup systems. This means that if one or several parts fail or break, there are plenty of other parts which can make up for the failure without you even noticing. Phew! Not too much to worry about then. In fact, you can almost forget that you are on a life support system. After all, it’s so huge and so fail-safe that you might take it for granted as much as the sun that rises every morning. And because it’s so huge and apparently so fail-safe, you probably think it’s no harm to make it a bit smaller by getting rid of some parts. After all, by making it smaller you can create more room for other important things, such as growing food to feed your family or to earn money (preferably lots of it!). Now imagine this huge and ever so reliable life support system is not a metal box with loads of man-made parts inside but a living system that consists of all the plant species on Earth as well as all the other organisms which, although they also depend on plants, still play important regulatory roles.
 

A colourful grasshopper in the Amazon rainforest - Photo: copyright 2012 Wolfgang Stuppy

Welcome to reality!

If you manage to take this imaginary step you have actually crossed from my very simple allegory into real life. By turning ever more ‘wild’ places like tropical rainforests into ‘cultivated’ land for agriculture, mines, factories, housing and so on, to produce ever more stuff for ever more people, we are causing the extinction of countless species. This extermination of species – unintended or not - is equivalent to the dismantling of our life support system which we can now give a name. It is actually called ‘biodiversity’.

Although ‘biodiversity’ is very robust and can cope with severe losses, there comes a point when it begins to ‘malfunction’ and finally fail if too many parts are removed. Here's a good analogy: remove part after part from your car until it stops working. You will find that some parts will make hardly any difference if they are missing, others will be crucial for the running of the engine and the turning of the wheels. It is just that nature is far more complicated than even the most sophisticated car and there are no mechanics out there who know exactly how to fix it.

A slab with plant fossils on display at the Canterbury Museum in Christchurch, New Zealand. These specimens belong to a mix of flowering plants, podocarps (a kind of conifer) and cycads, and are about 100 million years old (Photo: Wolfgang Stuppy)

Species have always come and gone in the Earth’s history, as we can tell from all the million-year-old fossils of strange-looking animals and plants displayed in museums. Even though it is true that extinction is a natural process, the current rate of extinction is about 1,000 times greater than the natural rate and this is solely due to human activities. Compared to the alarming loss of species, climate change, although currently dominating the media, is only the bitter ‘icing on the cake’.

Climate Change is just the bitter icing on the cake (Photo: Wolfgang Stuppy; Cake design: Michelle Wibowo)

Forever is not for ever – or is it?

Extinction of a species is forever. Each species that disappears takes with it one elaborate piece of our vast and complicated life support system, which itself is the result of millions of years of evolution. Tragically, on top of this, the extinction of a species also affects many other species with which it has shared the same environment for thousands if not millions of years. The fossil record tells us that life on Earth has already experienced five global mass extinctions.

After each disaster the recovery of global biodiversity took between four and twenty million years. Four million years at least! Nobody can imagine such a vast amount of time! To illustrate the dimensions, modern humans like us, have existed for no longer than about two hundred thousand years. It is pretty clear that we can’t wait for biodiversity to restore itself. For us, the vast geological time spans involved in the evolution of life mean that with every species that we lose, we lose a part of our life support system – forever!

Seeds are miraculous! (Images from ’SEEDS – Time Capsules of Life’ by Rob Kesseler & Wolfgang Stuppy, Papadakis Publisher, Newbury)

Why saving seeds is more than just a good idea

At this point it is worth remembering that plants are the only solar-powered primary producers, and, as such, support literally all other life on Earth. Because of their crucial importance, one trait of plants almost seems like a miraculous gift to mankind: their ability to produce seeds.

Unlike animals, most plants (apart from algae, mosses, ferns and other spore plants) can survive extensive periods of unfavourable conditions in the form of seeds. As long as they are kept dry, seeds can hold a tiny plant safe and alive (albeit in a quiet, coma-like state) inside their hard shell for years, decades, centuries or even millennia.

A typical seed with the tiny green baby plant sheltered in its centre. This particular seed belongs to the South African gifboom ('poison tree'; Hyaenanche globosa, Picrodendraceae) (Photo: Elly Vaes)

The ability of seeds to survive for long periods of time in the dry state is their most astonishing and momentous quality. Above all, it holds the key to the survival of a species! And this is exactly what we, Kew’s Millennium Seed Bank Partnership, are capitalising on. Because of their small size and longevity, seeds provide an extremely efficient means of saving species from extinction. Rather than accepting the fact that we will lose thousands of unique species of plants in the years to come, we make every effort to collect their seeds now in order to ensure their survival in the future. Saving seeds is the best chance we have to enable us to re-build essential parts of our life support system, should it eventually begin to fail.

Collecting seeds of prickly pears (Opuntia sp.) for the Millennium Seed Bank Partnership in Mexico (Photo: Wolfgang Stuppy)

To date, the Millennium Seed Bank Partnership has managed to collect the seeds of 10% of all plant species (c. 32,000 species). Until 2020, we aim to collect the seeds of 25% (75,000 species) of the world’s plant species. If you agree that seed conservation is a good idea you can help us achieve our goal by adopting a seed for £25 or saving an plant species outright.
 

A ‘cool view’ of the cold storage at the Millennium Seed Bank (Photo: Wolfgang Stuppy)

- Wolfgang -
 

Related links

• August's 'Seed of the Month' blog post - scrumptious ice cream bean
• Millennium Seed Bank Partnership - find out more about our work
• Visit Kew's Millennium Seed Bank at Wakehurst
• Adopt a seed, save a species - how you can help
• Kew species profiles - search through hundreds of profiles of plants and fungi
• One in five plants are at risk of extinction - find out more about plants at risk

Ever since I can remember I have been fascinated by plants, especially all things edible that come from them. And the more exotic the plant, the better! After blogging about the cashew nuts, a very well-known exotic treat, I decided to now choose a seedy delicacy that is less well-known, at least in our temperate climate.  

Sweet seeds

The legume family (Leguminosae or Fabaceae) is an amazingly diverse (nearly 20,000 species worldwide!) and incredibly useful group of plants. They feed us with beans, peas, lentils, soya and loads of other pulses and they treat us with delicious peanuts, tamarinds and liquorice. When travelling to far-away tropical countries, we are likely to encounter lesser spotted members of this miraculous family and some of them bear strange-looking but excitingly delicious fruits. One of these scrumptious exotics is the South American ice cream bean, Inga edulis, locally called ‘guaba’ (pronounced ‘wuba’). Behind this mouth-watering name lies a beautiful tree with a remarkable fruit.

 Fruit of the ice cream bean tree (Inga edulis, Leguminosae) photographed in its native Brazil 

A two metre long candy bar!

Reaching a height of up to 30m (96ft), an ice cream bean tree produces cylindrical and often spirally twisted bean-like pods that grow up to more than two metres(!) in length. This is no hoax! One of my colleagues at Kew, Dr. Terry Pennington, is a specialist on the genus Inga. Whilst discussing this blog with him, he told me that he was supervising Inga trials in Amazonian Peru in the early 90’s, they had a competition on the local radio to find the longest ice cream bean. The winner brought one which measured 2.07 m long.

Inside, lined up in one row, the pods contain numerous large purple-black seeds embedded in an edible, translucent-white pulp. The sweet flavour of the spongy pulp resembles that of vanilla ice cream, hence the name. Basically a yard-long candy bar that grows on a tree, most people will be sufficiently impressed by the size of this fruit and its delicious flavour. As a practicing ‘seed nerd’, however, I find something else quite extraordinary, too. 

Inside an ice cream bean lie the juicy delicious seeds

Seeds of Inga edulis with and without their white fleshy seed coat. What is left after the white juicy part has been eaten is the bare naked embryo

Unlike most fleshy fruits, the sweet pulp of the ice cream bean is not produced by the fruit wall but by the seed itself. Usually, seeds are covered by a hard seed coat to protect the precious embryo inside against both the elements and hoards of seed-eating critters. In the ice cream bean, however, the entire seed coat becomes soft and fleshy. Once you have nibbled a hole in it at one end, you can basically strip it off like a sock and suck the sugary treat! What remains is the purple-black embryo. Although it looks very dark on the outside, underneath its black skin, the embryo is actually green.

Cross section of the embryo of an ice cream bean (Inga feuillei, see below) showing that underneath its dark purple skin the embryo is actually green.

The genus Inga contains about 300 species of shrubs and trees, all native to the warm and tropical parts of the Americas (including the Antilles) and most of them occurring in the Amazon rainforest. Although Inga edulis (edulis meaning ‘edible’) is the most common species that is referred to as ‘ice cream bean’, there are a number of other Inga species that spoil us with equally delicious fruits, for example Inga feuillei (called ‘pacay’ in Peru), I. rhynchocalyx, and I. spectabilis. Their fruits differ in shape and size but inside they are very similar.

Because of their delicious taste, ice cream beans are very popular in many parts of Central and South America, where they are almost always eaten raw. During the tropical wet season when fruits are abundant, monkeys and birds feast on the sweet pulp and scatter the soft embryos.

But aside from their edible fruits, Inga trees are also useful in other respects. Like many members of the legume family, Inga trees have the ability to fix nitrogen with the help of Rhizobium bacteria which they house in specialized root nodules. Because they introduce nitrogen from the air into the soil, these root nodules act like fertilizer and help to improve soil fertility. Moreover, Inga trees have long been grown by indigenous Amazonians for food, shade and timber and they have also proven very useful as shade trees in cacao, coffee, tea, black pepper and vanilla plantations.

Another delicious ice cream bean: Inga rhynchocalyx, a native of the Amazon rainforest

But it’s not only the fruits that make Inga trees so very special. Their flowers, which produce the delicious ice cream beans, are pretty amazing too. They only open for one night and wither very quickly in the early morning. Like many other nocturnal flowers, they are large, white in colour and arranged in dense clusters (inflorescences). The individual flowers consist mainly of a brush-like tuft of stamens with long filaments. Flowers of this type are typically pollinated by bats. Their furry faces get powdered with pollen as they search for nectar at the base of the flower. Recent research into Inga has shown that at least in some species (e.g. Inga sessilis), hummingbirds are the first visitors just after the flowers open, followed by bats as well as hawkmoths throughout the night.

The nocturnal, brush-like flowers of Inga rhynchocalyx only last for one night and are mainly pollinated by bats 

One last thing...

In my blog post on the cashew nut I was raving about eating embryos and how delicious they are. Those of you who read this blog might wonder as to why the birds and monkeys that feast on ice cream beans don’t also eat the embryos wrapped inside the juicy seed coat. Good question indeed since hungry animals usually devour everything they find palatable and digestible. The answer to this is not only a fascinating example of the co-evolution between plants and animals but also a lively illustration of how plants pursue different strategies to achieve the same goal: in seeds it’s all about the safe dispersal of their embryos.
 

Why would animals only eat the fleshy seed coat and not the large soft embryo (here cut in half) inside?

When it comes to protecting their seeds, animal-dispersed fruits that offer edible rewards to attract animal dispersers have two options to increase the chances of their seeds to escape destructive mastication: mechanical or chemical protection. Plainly this means that either seeds are encased in a really hard shell (see my blog on the Texas mountain laurel) or they are laced with bitter-tasting substances (usually tannins) and/or poisonous chemicals that teach uninitiated animals enjoying a particular fruit for the first time a lesson they won’t soon forget. If you need experimental evidence for this, try chewing the seeds of grapes, apples, oranges, avocados or papayas. In the unlikely event that you actually enjoy the experience, please do note that the seeds of apples and other members of the rose family (e.g. plums, apricots and bitter almonds) contain highly toxic cyanide (actually a cyanogenic glycoside called amygdalin, to be precise). But fear not! You are unlikely to have sufficient apple seeds at your disposal to actually poison yourself. For example, one kg of Fuji apple seeds contain the equivalent of c. 700-800 mg hydrocyanic acid (see this article from Food Chemistry for more information). When ingested, the lethal dose is about 50 mg of cyanide (depending on your body weight) so you would have to eat about 60-70 g of fresh apple seeds. That doesn’t sound like a lot but how many seeds is that? Since I couldn’t find any reliable figures for the weight of an average apple seed, I chopped up my Braeburn that I bring for lunch every day. I found three seeds in it that weighed 211 mg altogether, so that’s 70 mg per seed. To get to the potentially ‘lethal dose’ of 70 grams of seeds I would need 1,000 apple seeds. My cherished lunchtime Braeburn yielded only three fully developed seeds so I would need over 300 more Braeburns before I could contemplate to seriously poison myself. Hmh...

Returning to the actual matter of this blog, in the case of the ice cream bean this means that in the absence of even the slightest mechanical protection, the embryo is almost without doubt (I haven’t tried it!) very bad-tasting, usually bitter, if not actually poisonous.

- Wolfgang -

 All photographs by Wolfgang Stuppy

Related links

• Growing legumes of your own? Start saving your seeds for The Great Seed Swap at Wakehurst Place!
• Kew's projects in Peru
• Are you making the right consumer choice when it comes to soya products?

I absolutely love nuts! They are healthy, taste great and being a Seed Morphologist, I obviously have a very special relationship with them. The latter is fuelled by the strange awareness that every time I eat a nut, I eat an embryo. Yuk? Well, read on!

To me, the most delicious nut of all is the cashew. No other nut can match its fine flavour and soft creamy texture. If you are also nuts about nuts, you will undoubtedly know cashews, even if you (incomprehensibly) prefer macadamias, Brazil nuts or pecan nuts. I wonder, though, how many people know the plant that cashew nuts come from and how unusual and amazing the actual fruit they are borne in looks like. Hence this blog...

The vibrantly colourful fruits of the cashew tree, Anacardium occidentale

Cashew apples and cashew nuts

Cashew nuts grow on trees with large, beautiful bright green leaves. Nowadays cultivated and naturalised almost all over the tropics, the cashew tree (Anacardium occidentale, Anacardiaceae) is originally native to the coastal plains of north-eastern Brazil, where it forms part of the so-called restinga* vegetation. Long before European colonisation in the sixteenth century, Brazilian Indians have relished the delicious fruits. Called ‘acajú’ by the members of the Tupi tribe, the name was converted by the Portuguese into ‘cajú’ and eventually became ‘cashew’ in English.

A cultivated cashew tree (Anacardium occidentale) in Brazil

Young cashew fruits with their stalks starting to swell, top right: flowers 

From very humble, tiny, first white, then pink flowers arise rather large, brightly coloured (there are orange-red and yellow varieties) and somewhat weird-looking fruits. When ripe they resemble a very soft pear with a hard, kidney-shaped nut tucked in at one end. The pear-like part, also called ‘cashew apple’ (whoever came up with that name has never seen an apple next to a pear!) is extremely juicy and sensitive to pressure which is why you can’t find them in our supermarkets. In Brazil I have seen them offered in roadside stalls where they were carefully displayed in egg trays.

Ripe cashew fruits on the tree  

'Cashew apples' are so soft, they need to be treated like raw eggs (here in a roadside stall in Lindóia, Brazil) 

Hard sell methods aside, what’s most unusual about the cashew fruit is that the fleshy bit is not formed by the swollen ovary of the flower as in ‘normal’ fleshy fruits. Rather, the cashew apple is formed by the hugely swollen stalk of the flower which is why you wouldn’t find any seeds in it. If you want to find the seed you must open the fertile part that is formed by the ovary of the flower, namely the hard, kidney-shaped ‘appendix’, better known as the ‘cashew nut’.

A longitudinal section of a 'cashew apple' proves its origin from the flower stalk; there are no seeds, just flesh 

Ever had to stuff your mouth with a cotton ball soaked in milk?

Sounds delicious but here’s a word of warning to those who love to travel in the tropics. If you ever encounter a fruit-laden cashew tree, restrain your enthusiasm, at least for the ‘nutty’ bit. Whilst the amazingly succulent cashew pear, err... apple, is harmless and best enjoyed by sucking out the sweet juice and discarding the stringy-fibrous residue, the shell of the cashew nut is poisonous owing to an acrid phenolic oil, called urushiol. Urushiol causes dermatitis which is why the cashew nut was once also called ‘blister nut’. The same nasty oily chemical is found in other members of the Anacardiaceae. Among these close relatives of the cashew tree are notorious plants such as the poison ivy (Toxicodendron radicans), poison sumac (Toxicodendron vernix) and poison oak (Toxicodendron pubescens & T. diversilobum) but also mango (Mangifera indica; urushiol is found in the sap and fruit peel!).

If you try to crack a fresh cashew nut with your teeth, you will soon be blessed with a painful blistering rash in and around your gob (if it’s any comfort, the pain can be soothed by stuffing your mouth with a cotton ball soaked in milk). That’s also the reason why you will never find unshelled cashew nuts on supermarket shelves whereas other nuts like walnuts, hazelnuts, almonds and pistachios are quite often sold ‘whole’. 

How to prepare 'home-grown' cashew nuts: roasting the nuts in a fire destroys the poison and the charred shells can easily be removed

Why cashew nuts are expensive

Because of the problems caused by the toxic shell of the nut, Latin Americans, West Indians and West Africans have long used only the succulent ‘cashew apple’ making it into wine and refreshing beverages, similar to lemonade, such as the Brazilian ‘cajuado’. However, on a worldwide scale the seed of the cashew tree is still the main commercial product, despite the laborious cleaning process that makes cashew nuts one of the most expensive of all nuts (at least in the UK macadamias cost a lot more than cashews). The safest way to enjoy ‘home-grown’ cashew nuts is to roast them in a fire and then remove the charred shell as shown in the pictures above.

In the wild, the brightly coloured, 5-10 cm long cashew apple acts as a tasty reward for the animals it needs for seed dispersal. Fruit bats, monkeys and bats pick the fruits to feed on the yellow- to scarlet-coloured apple but discard the poisonous nut, leaving the seed inside unharmed. 

Cashew nuts off the supermarket shelf as we all know and love them

About eating embryos

Every nut contains a seed and as such, logically, also an embryo (i.e. a little baby plant that ‘hatches’ from the seed upon germination). Therefore, every time you eat a nut, you eat an embryo. But this is not like eating a microscopically small embryo when you eat a chicken egg. The edible part of the nuts we eat as nibbles consists of nothing else but the embryo. Here’s an experiment for you that proves my point: next time you eat a cashew try pulling it apart and you will see that it splits into two halves, the cotyledons of the embryo. There is even a tiny shoot axis with miniscule leaves in between them! 

A whole cashew nut, botanically an embryo, and one split in half, revealing its true nature: a baby plant with two leaves (cotyledons) and a shoot axis

Oh, one more thing....

We botanists use the term ‘nut’ in a very different and much more rigorous sense than ‘ordinary people’ do in their everyday language. For the food industry, chefs and 'regular' consumers who enjoy a tasty nibble, any large edible kernel that requires forceful liberation from a hard shell before consumption is unscrupulously addressed as a ‘nut’. In a botanical sense, a ‘nut’ is only a nut if it consists of nothing but the mature ovary of an indehiscent (= non-opening) fruit with a hard, dry shell, usually harbouring a single seed. This is true for hazelnuts (Corylus avellana, Betulaceae), walnuts (Juglans regia, Juglandaceae), pecan nuts (Carya illinoiensis, Juglandaceae), acorns (Quercus spp., Fagaceae), and unshelled peanuts (Arachis hypogaea, Leguminosae), though the latter often (and annoyingly to botanists!) contain more than one seed. Other culinary ‘nuts’ are, in fact, the stones of drupes (= stone fruits), such as unshelled almonds (Prunus dulcis var. dulcis, Rosaceae), pistachios (Pistacia vera, Anacardiaceae), and, in all honesty, also cashew nuts (Anacardium occidentale, Anacardiaceae). I didn’t want to make things too complicated but, working for Kew, I finally have to break the truth about the cashew nut. Although hardly recognizable as such, the shell of a cashew nut does indeed display the three defining layers of a drupe: an outer skin, a very thin, quick-drying but nevertheless soft middle layer, followed by the dominating hard, woody stone.

As a final blow to the culinary nut-concept, it has to be unveiled that unshelled Brazil nuts (Bertholletia excelsa, Lecythidaceae), macadamias (Macadamia integrifolia and M. tetraphylla, Proteaceae), ginkgo nuts (Ginkgo biloba, Ginkgoaceae) and pine nuts (Pinus pinea, Pinaceae) are purely seeds in a botanical sense because their shell consists of the seed coat.

- Wolfgang -

All photos by Wolfgang Stuppy

*Restinga: a distinct type of tropical and subtropical forest found on acidic, nutrient-poor soils at the the Atlantic coast of Brazil.
 

Related links

• "Difficult Seeds Project" - Anacardium occidentale
• Black Walnut on Kew's species pages
• More about the work of the Millennium Seed Bank Partnership
• Read Wolfgang's other blog post

In my last blog I debunked some sadists of the plant world and now it is time, as promised, to expose the fraudsters of the plant kingdom. Hang on a moment, you may think! How could plants commit fraud when they can’t think? Well, it seems that evolution has done the ‘thinking’ for them.

Take the pretty but dangerous crab’s eye or rosary pea (Abrus precatorius), a member of the legume family (Leguminosae/Fabaceae) that is found throughout the tropics. When its fruits open, they reveal some very attractive looking shiny bright red-and-black seeds. The gaudy display catches the attention of birds. After all birds have very similar colour vision to humans (even better, in fact - they can see part of the UV spectrum!). In ‘dispersal sign language’, the colour ‘red’ is generally understood to advertise something juicy and edible, such as a sweet berry or stone fruit (think cherry!), for example.

Dispersing seeds of Abrus precatorius (Image: Gwilym Lewis)  

The lollipop effect

By the way, this colour scheme already works with very small children. As many parents can testify, children clearly understand the message sent out by a red lollipop from an early age, and – worryingly dangerous - also from any red fruit they encounter in the garden. So to birds, the shiny round seeds of the crab’s eye represent the equivalent of sweet lollipops. But in reality these seeds are very hard and dry and have nothing edible to offer. Birds hungry or curious enough to pick up one of the seeds may carry it for a short distance but discard it as soon as they discover the scam. Those birds that are not so clever might even swallow the seeds and as long as they haven’t got a strong gizzard, like the specialised seed eaters, the seeds pass through the bird’s gut without harming it.

 

 Bright red seeds of Abrus precatorius (Image: Wolfgang Stuppy) 

Pretty but dangerous

Whilst the seeds of the crab’s eye are useless to birds, they are treasures to makers of botanical jewellery. In many tropical countries, seeds are strung together into beautiful necklaces, earrings and bracelets.

Botanical jewellery made from the seeds of Abrus precatorius (Image: Wolfgang Stuppy)

However, handling some seeds is a very dangerous business and those of the crab’s eye are the most dangerous of all. Despite their great beauty, the seeds of Abrus precatorius contain abrin, one of the strongest plant poisons known. With an estimated lethal dose for humans of between 0.1 and 1 microgram per kilogram of body weight (experiments to determine the precise dosage are difficult to conduct!), less than 0.003 grams could kill a child. Fortunately, the seeds have a very hard seed coat and as long as they are not damaged, they are harmless, even if swallowed (don’t try this at home!). However, the manufacture of botanical jewellery can become a hazardous occupation if holes have to be drilled to thread seeds together. The dust from crab’s eye seeds can cause blindness on contact with the eyes, and inhalation or contact with open wounds has even worse consequences. 

What a way to go!

Symptoms of poisoning appear only hours or perhaps days after contamination; they include nausea, vomiting, severe abdominal pain, diarrhoea and a burning sensation in the throat. Later, drowsiness and ulcer-like lesions in the mouth and oesophagus, convulsions and shock finally lead to coma and death. Not a nice way to go! For adventurous types of the kind that indulge in fugu (Japanese for puffer fish), there is a simple way to detoxify the perilous seeds. At temperatures above 65°C, abrin breaks down so that the seeds are even allegedly edible after boiling (and don't try this at home either!). 

The crab’s eye is not the only cheater

Apart from the crab’s eye, there are other examples of fraudulent seeds, most of which are also members of the legume family. Because of their hardness and shiny colours, all of them star prominently in botanical jewellery. Among the favourites are the pure red seeds of coral trees (members of the genus Erythrina) and others such as the red bead tree (Adenanthera pavonina) from south-east Asia and Australia, and the Texas mescal bean (Dermatophyllum secundiflorum), a native of the south-western United States and Mexico (see my previous blog on the Texas mountain laurel). 

Image left: Flowers of Erythrina caffra; image right: Seeds of Erythrina herbacea (Images: Wolfgang Stuppy)

Even more eagerly sought are bicoloured red-and-black seeds like those of Abrus precatorius and others. Apart from the crab’s eye, such seeds can occur in certain species of Ormosia and Rhynchosia from Central and South America as well as in coral trees (Erythrina spp.), such as the Malagasy Erythrina madagascariensis. 

Botanical jewellery made from Ormosia spp. (Images: Wolfgang Stuppy) 

Seeds of Ormosia coccinea (photo by Wolfgang Stuppy) 

One last thing... some people like to toy with their necklaces by putting them into their mouth. If you happen to be an aficionado of botanical jewellery, this might not be a good idea!     

- Wolfgang-

All above images by Wolfgang Stuppy are copyright Board of Trustees, Royal Botanic Gardens, Kew

Related Links

• Discover more amazing plants with David Attenborough's new 3D film; Kingdom of Plants
• Find out more about Kew's Millennium Seed Bank Project