Article by Mike Kluk -
The first half of this article was published last month. So, if you missed it, you can click here to read it. The Light Eaters is a recently published book looking at current research that is revealing how plants interact with, respond to, and manipulate other plants and animals in their environment. It gives us a view of the plant world from an entirely new perspective.
Plants that hear a predator coming
Past research has demonstrated that many plants produce compounds to become less tasty to predators. We actually find many of these useful, the flavor of mint or oregano, the fragrance of lavender. Botanists have also demonstrated that an increase in these chemicals occurs throughout the plant when it is being browsed, not just the leaves actually being bitten. Plants generate electrical impulses, much as our nerves do, but they travel at a much slower speed. Too slow to explain the change that can quickly occur in distant parts of a plant in response to an herbal predator. Two researchers, Rex Cocroft and Heidi Appel, decided to try a theory: could sound be the transmitting medium? Sound is merely vibration, whether it is passing through air or the body of a plant. They recorded the sound of the larvae of a white cabbage butterfly chewing on a leaf. Then they played these back, using guitar pickups, through the leaves of Arabidopsis, rock cress, a brassica subject to the mastication of cabbage worms. The plant responded by producing additional compounds to become less tasty just as it would if actually being chewed by a caterpillar. Testing with the sounds of other insects that don’t eat Arabidopsis, or the disturbance of a blowing fan produced no response in the plant. It was pretty clear; it could distinguish the sound of a predator from similar sounds. Other research has shown pea roots growing towards the sound of water flowing through a closed pipe. Apparently, underground sounds may give plants important information too.
All plants have a charge
Early in the last century, scientists discovered that plants generate electrical activity. Just touching a plant can register an electrical spike on a meter hooked to the plant. Later, scientists realized that a plant could “amp up” defensive proteins throughout the plant even though only one area was attacked by insects. Communication through electrical impulses was suspected. If you attach an electrode to a leaf and touch the plant in another area, a meter attached to the electrode will register a spike in electrical activity. But just how this electrical charge is produced was a complete mystery until recently. Most higher animals have a nervous system that sends electrical current throughout their bodies, often communicating with a centralized neural organ or brain. Plants have nothing of the sort. But there are some similarities.
Animal nerves function by having “channels” through which ions, an atom or molecule with an electrical charge, pass from one to the other. Sodium and potassium ions are primarily involved. It turns out that plant cells also have channels through which ions pass. In this case, potassium, chloride, calcium, and hydrogen. The ions are different, and plant cells are not specialized but still. . .
Two researchers, Simon Gilroy and Mastastugu Toyota have done extensive research on electrical activity in plants. They use a system discovered many years ago that essentially tracks calcium output in cells. It turns out that in both animal and plant cells, electrical activity is accompanied by a burst of calcium. Researchers utilized a gene that is responsible for producing florescence in a species of jellyfish. They engineered it to be responsive to calcium and then inserted it into plants. After that, all of the offspring would fluoresce when free calcium was present. Special microscopes and cameras allowed researchers to observe an entire plant. Gilroy and Toyota were able to demonstrate that when one part of a plant was cut, waves of green florescence would course through the entire plant at one millimeter per second. That is far faster than can be explained by transmission through the plant’s vasculature. It is visual proof that the entire plant is being “notified” with an electrical message that part of it was harmed.
How this works is still a mystery. Plants do not have a centralized “brain” to process this information. But somehow distant individual cells will produce defensive proteins when notified.
Plants that can remember or at least count
A genus of plants, Nasa, grows in the harsh environment of the high-altitude Andes mountains. Known for beautiful blossoms and stinging hairs, they seem to have the unusual ability to anticipate when a pollinator will arrive, based on past experience. Since they live in a tough neighborhood with few pollinators, Nasa have developed a way to make every grain of pollen count. They hold most of their stamens sheltered within the flower with just one held up to deposit its pollen when a pollinator visits. When the next pollinator arrives, a different stamen is erect and waiting for it. Observers noted that the stamen tended to rise near the time the next pollinator arrived, thus protecting its precious pollen as long as possible. Reseachers Tilo Henning and Max Weigard, studying Nasa poissoniana, were able to manipulate the process. They physically probed the flowers to simulate a pollinator’s visit. With one group of plants, they probed the flowers on a fifteen-minute schedule. With another they probed the plants on a forty-five-minute schedule. This replicated situations with more or few pollinators in the area. Sure enough, the plants on a fifteen-minute schedule would raise their stamens for the next visit after approximately fifteen minutes and those on the forty-five minute schedule would raise their stamens on that longer schedule. When the researchers switched schedules on the plants, they adopted the new timing the next day. How these plants do this is a mystery. But they, and quite possibly others, have the ability to store past experience and use it to modify how they interact with the world.
Conclusion
The Light Eaters has been criticized for delving too deeply in the “mystical” idea of plant intelligence and consciousness. That is certainly a fair criticism given that we hardly know the meaning of those words as they relate to our own species. But the book also raises fair questions about our propensity to categorize plants as submissive nonparticipants in the active environments they occupy. Clearly, they are more than that. It is undeniable that many plants have skills and abilities unknown and unsuspected just a few decades ago. It is also undeniable that science has just begun to crack whatever codes govern plant life. It could be we are relying on old animal-oriented words and concepts far too much. Along with new research, we need to develop a new vocabulary to better describe what we are seeing.