Studies challenge tree perception

The traditional view of trees and other plants as being cruder organisms than animals is being overturned by a plethora of recent research, last week's Treework Seminar in London's Kew Gardens heard.

Trees: researchers point to functions including breathing, digestion, problem-solving, vision and even memory - image: HW
Trees: researchers point to functions including breathing, digestion, problem-solving, vision and even memory - image: HW

"For a long time, plants have been thought of as inferior," said University of Montpelier emeritus professor of botany Francis Halle. "Though they face the same constraints, the solutions they have developed to these are not only different, but in many ways opposite. You should bear in mind their otherness."

International Laboratory of Plant Neurobiology director Professor Stefano Mancuso pointed out: "They have no discrete organs yet can perform the functions of these, including breathing, digestion, problem-solving, vision and even memory. In fact they can detect 15 different properties, which is more than any animal."

Among these, he said: "They can detect the sound of a chewing insect, while plant roots give out a kind of click, which we think may give them an idea of their spatial distribution. Roots have a very specific architecture underground. Do they have a kind of echolocation?"

He added that as long ago as 1880, Charles Darwin wrote in a work on phototropism that a plant's radicle "acts like the brain of one of the lower animals". Though it is challenging to study in real time, the root tip "has a lot of electrical activity, much like the brain", said Mancuso. "We also have strong evidence that some plants have a primitive kind of vision - they can detect form as well as light." Leaves of the South American vine Boquila trifoliolata, for example, change shape to resemble those of its host plant "in a matter of weeks", he pointed out.

Allied to this is spatial awareness. A wild lima bean will grow towards a nearby pole, while parasitic plants of the genus Cucscuta (dodder) "will always choose to grow up a tomato plant over wheat, but will take wheat over nothing", he said, while passiflora has even been shown to anticipate the location of a slow-moving target.

Even slime moulds have been shown to establish networks with a high degree of efficiency, mimicking or even surpassing man-made designs, he added. "It prompts the question, where is this information stored? But we don't even know how we store information."

Aberystwyth University emeritus professor of biology Howard Thomas said: "For most of plant science, what we thought plants were like came from crops and other domesticated plants, but they are massively desensitised - for example, in their tolerance of being in close proximity to others."

In contrast to animals, "Plants seem able to reset their biological clock," he said. "Can you even speak of the age of a plant? There is no deterioration in function in the gerontological sense. Growth rates in larger trees are sustained through old age." Some plant cells remain totipotent, or able to turn into any cell specialism, while animal cells "are not immortal, even under optimal in-vitro conditions".

University of Western Australia associate professor Monica Gagliano said: "Since Aristotle, we have thought of plants as insensitive, distinguished from animals by a lack of physical awareness." Gradually experiments showed first that plants are sensitive to light, while later the hormonal signals governing plants' responses to light were understood. "Plants have more genes for signalling than we do," she said, adding of the well-known response of Mimosa pudica to being touched: "It's not just an instantaneous response but a learned one - and you can't learn without memory."

She concluded: "We don't need more science so much as a change in the way we think about the animal/plant divide." Taken to its conclusion, this might lead to humans having moral and even legal obligations towards plants, she said, pointing out that in 2008 the Swiss government's ethics committee said research proposals should consider the "dignity" of plants. She added: "Other cultures see the relationship between plants and animals very differently from the way we do."

Conference organiser, Treework Environmental Practice principal consultant Neville Fay, said: "These are questions we don't normally challenge ourselves with but they can change our understanding of and relationship with trees and plants."

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