Fifty Shades of Green

This piece was published recenrly in Prom – The Aberystwyth University Alumni magazine

It gives you a flavour of

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It is frequently but apocryphally claimed that Eskimos have 50 words to describe snow. Closer to reality, but almost never quoted is the observation that there are 45 words for shades of green in the Icelandic language. In fact in most languages there are many more words to differentiate shades of green than there are for any other colour. This is because we live on a planet dominated by the colour green, where the forces of natural selection have equipped our species with eyes that are particularly sensitive to light in the green sector of the spectrum. When offered photographs of landscapes, people prefer the more intensely green images. There are good biological reasons behind this bias and this biology is reflected in our electronic gadgets that capture or present coloured images. To be able to represent the world in a way that humans recognise televisions, cameras etc. need to include more green receptors or colour cells, than those for other shades.

You might not think so, but humans have evolved as botanists with acute abilities to differentiate plants species. The ability to differentiate between subtly different shades of green and thus different species of plant is a talent, which greatly enhances your ability to survive in the wild. If you eat the wrong plant then you are poisoned. Conversely brighter shades of green tend to indicate elevated nutritional values. It is therefore no surprise that our crop plants tend to be particularly bright green, and agricultural fields are a brighter green than the wider countryside. It is not a coincidence that many of the wild relatives of our crops are found along coasts and in floodplains. These habitats are naturally highly fertile, as they are regularly supplied with nutrients courtesy of defecating seabirds or inundation by rich sediments. Wild cabbages, carrots, wild-beets, asparagus, peas, kale and various members of the spinach family all have maritime distributions, while many cereals are at home in fertile river valleys.

The fact that crop plants tend to be bright green and highly nutritious have not been the only factors in determining which species of plants our ancestors chose to domesticate. There are approximately 400,000 species of plant on earth. However, we regularly only eat about 200 of them, which equates to less than 1% of what is possible. So what makes our crop plants so special? The answer to the question why do we eat the plants we do appears to be related to their sex-lives rather than the more obvious – we avoid the poisonous ones. In fact many important crops contain toxic chemicals, classic examples being; potatoes, tomatoes, cassava, even wheat if you are gluten intolerant. The Jamaican national dish of akees, regularly results in lethal poisons of its citizens.

The most compelling evidence that a plant’s sex-life limits its ability to be domesticated is provided by the orchids. There are 20,000 species of orchid. They are the most species rich of all plant families and yet we cultivate just one of them for food. The fact that we don’t exploit orchids for food is perhaps a little surprising given that many of them have starch filled root tubers. Historically wild orchid tubers were boiled to make a starchy drink called salep, which was a common street food in London and is still drunk Turkey. Salep was thought to have aphrodisiac powers, this belief was linked to the fact that orchid tubers typically grow in pairs, which in turn is linked to the derivation of the word – orchid from the Greek for testicles.

The reason we don’t cultivate orchids as food crops is linked to their bizarre sex lives. Orchid flowers are highly complex and typically specialised for pollination by a single species of insect. Both orchid and insect species are interdependent on each other for their survival. If you try and cultivate an orchid away from its specially adapted pollinator, it will fail to set seeds and ultimately fail as a crop. The one species of orchid that is grown commercial therefore relies on hand pollination. This species is the vanilla orchid and it remains viable as a crop because natural vanilla pods command a price high enough to cover the cost incurred by hand pollination.

If crops are going to be successfully grown over wide geographic ranges, then they need to have generalist pollination mechanisms that rely on either the wind or on the services of common insects such as bees. The ten most important crops on earth are: maize, wheat, rice, potatoes, cassava, soybeans, sweet potatoes, sorghum, yams and plantains. Of that list most are wind-pollinated cereals, or root crops that don’t require insect pollinations. Plantains (bananas eaten as a vegetable) are fruits that don’t contain seeds. From this list only soybean produces a seed crop that is not depended on wind. However, soybeans tend to self-pollinate without the need for insects.
If the world’s most important food crops are not depended on insect pollination, why then does the UN claim, “seventy out of the top 100 human food crops, which supply about 90 percent of the world’s nutrition, are pollinated by bees”?

Although the majority of our staple crops are wind pollinated, it is striking that many other crops are insect pollinated, including virtually all temperate fruit trees. This is particularly strange since the majority of these trees originally evolved in deciduous woodlands that are dominated by large wind pollinated species such as oak, ash, beech etc. In their natural habitats, the ancestors of apples, pears, cherries, almonds, peaches etc are never as abundant or as tall as the wind-pollinated species. There is a simple biological explanation responsible for this fact. Wind pollination is an effective mechanism to ensure fertilisation if you are abundant. But smaller, less common species cannot reply on such a random delivery method and have to utilise the more precise pollen delivery service operated by insects.
This observation begs the question -why are so many fruit crops derived from smaller, less abundant, insect pollinated species rather than the larger, more abundant, wind pollinated trees such as oaks, ash and beech? For example why have we domesticated the almond rather than an acorn? Even without domestication both of these species produce large nutritious nuts and as it happens both are naturally poisonous. Wild almonds are full of cyanide and acorns are full bitter tannins. The answer is simple, if our ancestors had been lucky enough to find a tannin free oak tree and tried to establish an edible acorn farm, then this new palatable variant would have quickly become swamped by pollen from the local oak forests and the next generation of oaks would revert to the bitter wild type. In contrast, if you stumble across a cyanide free almond tree, it’s a relatively easy task to prevent it crossing with its poisonous wild relatives, as in the wild these are much more uncommon than are oak trees. If you establish an almond orchard the chances of any of your trees crossing with a wild toxic almond is fairly remote.
It appears therefore that many of our fruit crops tend to have sex-live that are less exotic than those of orchids but are less promiscuous than oak trees that throw caution to the wind. Unfortunately there is a very pressing reason behind this botanical voyeurism. Populations of many of the world’s pollinating insects appear to be declining and this could have important implication for our food security. As we have seen our most important food crops are not depended on insect pollination. So the good news is that we are unlikely to starve if bees become extinct. However, many of our fruit crops do require insect pollination. In the wild, these species tend to occur, as scattered trees within the forest, so are the focus of much insect activity. In contrast today, many of these crops are grown on an industrial scale in monocultures often outside the range of their native pollinators. Thus, even without the problems resulting from chemical pesticides, disease and climate change, insects are always going to struggle to pollinate all flowers we need to maintain a well-balanced and interesting diet.


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