Are plants are capable of intelligent life?

PUBLISHED: 00:00 03 August 2020

The Dragon tree (c) Grzegorz Jarzynski/Getty Images/iStockphoto

The Dragon tree (c) Grzegorz Jarzynski/Getty Images/iStockphoto

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In a new series, botanical enthusiast Martyn Baguley takes a deep dive into the intelligence of plants,

The native holly tree has a fascinating defence mechanism (c) Hana Richterova/Getty Images/iStockphotoThe native holly tree has a fascinating defence mechanism (c) Hana Richterova/Getty Images/iStockphoto

I was pleased when Nathan Fearn, Derbyshire Life’s Editor, recently invited me to build on the ‘Plants that have changed our lives’ series that has appeared in the magazine since August 2017 with a new plant-related series. For me, this generous offer provided me with an opportunity to indulge myself in exploring a subject that has intrigued me for years, tentatively called ‘plant intelligence’.

Intelligent Plants? Take away the question mark and most people would say that it’s an oxymoron. But let’s go back to basics and resort to the highly respectable Concise Oxford Dictionary for some definitions.

‘Intelligence: the intellect, understanding’ – not very helpful.

‘Intellect: the faculty of reasoning, knowing and thinking’ – a bit more helpful.

The Dragon Blood tree (c) Janos/Getty Images/iStockphotoThe Dragon Blood tree (c) Janos/Getty Images/iStockphoto

‘Reasoning’ - no, I’m going to stop there.

The problem is that the wise people (notice I said ‘people’) who assembled dictionaries never entertained the idea that intelligence could possibly have anything to do with plant life.

The biologist Lyall Watson, in his 1973 book Supernature, tells the following intriguing story. On a February morning in 1966 Cleve Backster, a polygraph (lie detector) specialist with the American CIA, connected his machine up on a whim to the leaves of a dragon tree plant (Dracaena marginata) in his office to see if the polygraph would register a reading when 
he subjected the plant to stress.

His sadistic intention was to burn one of the plant’s leaves. But he didn’t get that far. Before he had even lit a match the polygraph registered a surge of electricity suggesting that the plant was experiencing stress. Had it read his mind? Since then many respected plant scientists, having tried without success to reproduce Backster’s results, have dismissed what has come to be called the ‘Backster Effect’.

Red berries on a holly tree (c) Olivier Guiberteau/Getty Images/iStockphotoRed berries on a holly tree (c) Olivier Guiberteau/Getty Images/iStockphoto

But other equally respected scientists have come to believe that plants are able to sense and respond to 
many environmental influences - light, temperature, toxins, soil structure, gravity, chemical signals from other plants - and may have some sort of information-processing 
system to analyse external data and coordinate their behavioural responses.

I first learned about the fascinating interactions between acacia trees and giraffes from a television nature programme - it probably featured David Attenborough. Species of acacia are the principal sources of diet in Africa for herbivores, particularly giraffes.

The trees’ first line of defence is to grow thorns but, despite these, giraffes still manage to browse the foliage. So, what do the trees do? They release a toxin into the leaves that makes them inedible and even lethal if ingested, encouraging the giraffe to seek more palatable vegetation. But that isn’t all; acacias have another trick up their branches. They also release ethylene gas that floats downwind and is picked up by other acacia trees as much as fifty metres away and within fifteen minutes their leaves also become unpalatable.

The trees don’t have brains – they can’t reason like we do – so how do they do it? Incidentally, the giraffes are crafty (excuse the anthropomorphism). To ensure they have enough to eat they try to fool the trees by browsing downwind and only for a short time before moving on.

Coming nearer to home, our native holly trees have a fascinating defence mechanism. Look carefully for some holly trees; it doesn’t matter where they are but Derbyshire Wildlife Trust’s Holly Wood Reserve might be a good place to start if you live near Ashbourne.

Look carefully at their leaves 
and you will usually see that they 
don’t all have sharp points - the leaves higher up the tree will often have smoother margins. The long-held belief that leaves on lower branches 
are prickly to deter deer and other animals from grazing them gained credence in 2013 when the results were published of some research done in Spain on forty holly trees.

Scientists found that chemical changes (called methylation) can take place in the leaves that don’t affect the genes (the genotype) but, by modifying the DNA, do influence how prickly they 
are (phenotype/appearance). In the sample studied there was a significant relationship between deer and goat browsing intensity and the number of prickly leaves up to a height of 2.5 metres above ground level, the average maximum reach of an adult red deer.

In urban situations browsing animals are replaced by garden shears or hedge cutters. Somehow, holly trees recognise when their leaves are being attacked so when they fall off the next generation of leaves have more prickles. How do they do it?

There is a plant-related parallel world around us about which we have much to learn. There are few books on the subject but it is attracting the attention of scientists, so there is a steadily growing collection of academic papers.

So, I invite you to join me on a journey of enquiry over 
the coming months into the mystical world of plant intelligence, continuing next month by addressing the question ‘Can plants see?’

 

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