We take plants for granted, studying them scientifically far less than we study animals. But the complexity and subtlety of plants is greatly underestimated and merits far greater attention. The complexity of trees is engagingly described by forester Peter Wohlleben in his book The Hidden Life of Trees (HarperCollins 2019). Although the style is somewhat anthropomorphic, the description is broadly supported by science.
Plants and animals have radically different body plans. The animal body has a central command/control structure – the brain – and several organs, each carrying out a vital function – heart, kidney, liver, lungs, eyes etc. On the other hand, the whole “being” of a plant is diffused throughout its entire body – remove 90 per cent of the body and the plant still survives. Unlike animals, plants are immobile and cannot flee predators – in fact plants evolved to be eaten by predators. It would make no sense for plants to have localised brains and vital organs.
Animals have obvious social interactions through family and herd. Although plants of the same kind may congregate together, it is not obvious that they communicate with each other. Relative to animals, plants seem to be passive, uncommunicative and defenceless. But nothing could be further from the truth. Research shows that plants communicate freely with each other, defend themselves, help their offspring to thrive and alert plant neighbours to impending danger.
Communicate
Plants communicate with each other both above and below ground. Consider a forest of trees. Each tree has extensive underground roots extracting water and nutrients from the soil. The trees’ green leaves harness the power of sunlight to fuse carbon dioxide and water into glucose, the basic food on which trees subsist.
Tree rootlets live in interdependence with symbiotic mycorrhizal fungi. The fungi extract vital nutrients from soil and feed them to the tree roots. In “payment” for this service the fungi receive about 30 per cent of the glucose photosynthesised by the tree.
Tree rootlets spread out, making contact with neighbouring tree roots. Roots can discriminate between roots from other trees of its own kind and roots from different species. Tree roots share nutrient resources with their own kind but compete for resources with other tree species. Rootlets are extremely sensitive to environmental cues and, for example, can “hear” running water and grow towards the water.
Giant forest “mother trees” recognise their own saplings growing in their shade, suffering from lack of sunlight. They help these saplings survive by directing sugar to them through the root system – analogous to animal mothers “suckling” their young.
Trees communicate above ground using chemical signals. For example, caterpillars feeding on pine/elm leaves stimulate the leaves to secrete chemicals that attract parasitic wasps who come and eat the caterpillars. And giraffes feeding on acacia tree leaves stimulate the leaves to secrete ethylene gas that alerts other acacia trees to the danger, prompting them to pump tannins into their leaves making them highly unpalatable and toxic to the giraffes. The giraffes, who evolved with acacia trees, know this and feed into the wind so that upwind trees are not warned of their approach.
Awareness
Stefano Mancuso at the University of Florence studies plant behaviour, including memory (Amy Fleming featured him in a piece in The Guardian). When you drop water on Mimosa pudica it recoils its leaves. If you continue, the plant stops recoiling the leaves, realising the water is harmless. This knowledge is retained for weeks, far longer than the one- to two-day memory span of insects.
Mancuso controversially believes trees have awareness. A plant shaded by another plant quickens its growth to reach the light. However, Mancuso points out, even though all shoots in the tree-crown are shaded, they do not speed up growth because they know they’re shaded by parts of themselves. The plant is aware of itself and of the outside world. Of course, none of this amounts to conscious behaviour.
We humans see ourselves on the pinnacle of the natural world. But we are not doing so well. A species lives two to five million years on average. Homo sapiens are only 0.3 million years old, yet we have very significantly damaged our environment.
Our social/political world mirrors our top-down animal body plan, with central command/control structures and discrete vital organs. Mancuso thinks that incorporating some ideas from the plant world, such as greater diffusion of capacity and authority throughout our social and civil fabric, would greatly increase our chances of long-term survival.
William Reville is an emeritus professor of biochemistry at UCC
