Intelligence in Nature - Jeremy Narby [34]
Trewavasâs desk stood against a bay window overlooking Edinburgh. He sat facing me, with his back to his desk. He looked straight at me as he spoke. His eyes had a piercing quality, but his tone was generous. He said he had spent years pondering the behavior of plants in the light of Stenhouseâs definition. Though most plants do not move at a speed perceptible to the naked eye, they respond as individuals to signals from their environment and develop in adaptively variable ways. Even plants growing in pots inside houses turn their leaves to the light to optimize light collection and send their roots down in the soil and their shoots up into the air. And wild plants manage to compete with other plants for resources. Research now shows that growing shoots can sense neighboring plants. They can detect shifts in infrared light indicative of nearby greenery, predict the consequences of that presence, and take evasive action. Plants can alter the shape and direction of their stems to maintain an optimal position relative to sunlight. They can adjust their growth and development to maximize their fitness in a variable environment. According to Trewavas, this means they are intelligent, if one refers to Stenhouseâs definition.
To illustrate his point, Trewavas described the behavior of the stilt palm. This tropical tree has a stem raised on prop roots and moves toward sunlight by growing new prop roots on the sunny side and letting those in the shade die off. By doing this over several months, the stilt palm actually changes places. It âwalksâ around in this manner, fending off competitive neighbors and foraging for light, at a speed imperceptible to humans. Trewavas considers this a clear example of âintentional behavior.â
Ground ivy is another plant with measurable foraging skills. This perennial weed creeps along the ground as a vine, and when it reaches a patch of optimal size and nutrient content, it puts down roots and generates leaves to catch the light. Scientists recently tested ground ivy in a controlled environment in which nutrients were distributed unevenly. The plant demonstrated that it senses resources by starting to grow roots much earlier in its development in the locations containing nutrients and by skipping over the poorer ground between rich patches. Trewavas finds it âdifficult to avoid the conclusion of intention and intelligent choiceâ in the case of ground ivy.
Such examples cannot be dismissed as preprogrammed rote responses, he said. Rather, they demonstrate plasticity. He explained that an individual plant has an enormous capacity for changing its morphology, its branching structures, to accommodate the environment in which it finds itself. The transformation occurs very slowly from a human point of view, over a period of months, rather than milliseconds. âBut the way in which it is conducted and the success with which it has occurred must indicate that a lot of computation goes into the decisions which are actually made, otherwise plants would not dominate this planet in the way that they actually do.â
Trewavas had obviously argued in favor of plant intelligence many times. I was willing to consider that Western cultures, and science in particular, had misjudged the vegetal world. But I wondered about the extent of plantsâ capacities. I asked Trewavas if he thought plants think when they make decisions. He replied that he did not. In his opinion, they compute what is actually