CHAPTER 81 Plants
Without Garrison Keillor on "Prairie Home Companion", Saturday evenings just aren't the same any more. I truly miss listening to his tales on life in the mythical Lake Wobegon, the adventures of Guy Noir, private eye, the two cowboys, Dusty and Lefty, and other sketches with Keillor's songs tailored to the occasion or location of his broadcasts. Once in a while he would have trees talking to each other, typically with a grizzled older tree's observations on the passing scene. Obviously, trees talking to each other is ridiculous. Or is it? I'm not so sure after reading two articles by the same author, Marta Zaraska. One article, titled "Veggies with Vision", appeared in the January 2017 issue of Scientific American; the other article, "Smarty Plants", appeared in the May 2017 issue of Discover magazine. I didn't realize the same authorship until starting this column and think it's the first time I've cited two articles by the same author in different journals in these columns.
In the first article, Zaraska cites a hypothesis back in 1907 by Charles Darwin's son Francis, who proposed that leaves contain organs that act like a lens and are also light sensitive. Apparently, work in the early 1900s confirmed the existence of such structures, known as ocelli. The implication that plants might be able to "see" lay dormant for the rest of the 20th century but now the possibility is being investigated again. In an article published in Trends in Plant Science last year, Frantisek Baluska and Stefano Mancuso report that a type of single=celled cyanobacteria acts like ocelli, using its entire cell body as a lens to focus light at the cell membrane. This is similar to focusing light in the retina of the eye in an animal. Another interesting point is that the plant Arabidopsis, a plant I often see cited in the scientific literature, makes proteins that are involved in the making and functioning of eyespots in single-celled organisms such as green algae. (These eyespots are different from the eyespots found on butterflies or other animals in which the eyespots apparently serve as markings that warn predators to stay away.) In the algae the eyespots detect light and the algae will change course when encountering a bright light.
Can the climbing wood vine Boquila trifoliolata "see"? According to the article, it can change the shapes and color of its leaves to match the color of its host plant. How can it do this if it doesn't somehow know the host's color? Speaking of color, in Zaraska's second article I learned that, when growing, common pea plants prefer blue light. She cites the work of Monica Gagliano and her colleagues at the University of Western Australia on the choices made when growing pea plants encountered forks in the road. As Yogi Berra said, "When you come to a fork in the road, take it." Well, the Australian workers wanted to see if they could apply Pavlov's approach to training dogs to the pea plant. They constructed a maze made of plumbing pipes and planted pea seeds below the maze. They wanted to see if they could train the peas to learn that a burst of air from a fan was or was not associated with a source of blue light.
The pea plants were split into three groups. In one group a fan burst was associated with a following burst of blue light in that direction at a fork in the maze. In a second group, a fan burst would be followed by a burst of blue light in the opposite direction. In the third group there was no association between the fan and the light. Most animals under these conditions apparently behaved randomly but not the pea plants. The peas soon learned to follow the direction of the anticipated blue light pulses. This implies that the pea plant has some kind of memory.
In a further test of plant memory, Gagliano and colleagues from Australia and Italy carried out experiments with a leafy plant called Mimosa pudica. When you stroke a leaf of this mimosa plant, the leaves will fold immediately in response to a perceived threat. The researchers carried out a clever experiment in which they dropped mimosa plants from a height of 6 inches. This was high enough that the plants would fold their leaves. When the workers continued to drop the plants, at first the plants continued folding their leaves but gradually the plants seemed to realize that they were not being harmed by the drop from such a low height. After 60 drops they ignored the threat completely and didn't fold the leaves at all. To make sure the plants weren't just tired from all the dropping the experimenters shook the plants vigorously and, sure enough, the plants responded by very quickly folding their leaves. They do remember!
Plants not only see and remember but they also talk to each other. I believe I've written before about certain trees producing chemical compounds that emit an odor when a predator starts chewing on its leaves. If a neighboring tree detects the odor it will also start manufacturing the same bug repellant compounds. These days, with all the talk about secure communications via back channels, it should be noted that plants also have the ability to carry out underground communications. There's something called a mycorrhizzal network, an underground network of roots connected by woven networks of bodies of fungi. It's akin to a telephone system that permits the plants to exchange information.
To prove that plants can use the underground system to communicate, scientists in the UK ran experiments in which two groups of fava bean plants were connected by an underground network, while a third group of bean plants was isolated from the network. The first group was covered with aphids munching away on the plants. Sure enough, the second group of beans started producing chemicals that repel aphids while the third, isolated group of beans did nothing. How did the researchers know the beans weren't communicating through the air rather than underground? They covered the plants with plastic bags to prevent any odors from being used to communicate via the air.
Peas come into the picture again in experiments designed to test whether plants care about the welfare of other plants. Workers in Israel checked out this possibility in experiments with pea plants expressing a drought alarm. In these experiments a pea plant stressed by not being watered responds by emitting chemicals that its neighboring pea plants detect. Those neighboring plants then close their stomata, which are small holes in the holes in the leaves of the plant. Closing these holes reduces the loss of water from the plant, helping it survive drought conditions. So, does this plant, now better prepared for a drought, keep quiet or does it spread the word? It spreads the word by itself making the chemicals that alert its neighbors that a drought may be on the way. It does this even though it is not stressed. Actually, the article points out that the plant spreading the word is also looking out for its own best interest in that being surrounded by healthy plants discourages pests that tend to attack dried out or unhealthy plants.
To me, one of the most fascinating plants is the Venus flytrap. It not only has evolved to eat animal life but in the process has learned to count. Researchers have shown that the flytrap counts the number of times a hapless insect bumps into sensory hairs in the plant to determine when to shut its leaves to trap the critter and also to determine how much digestive juice is needed to manufacture to enjoy its meal. I was thinking that sometime in the future a plant may evolve to grow large enough to actually trap and eat us humans. Then I remembered that I had been beaten to this speculation by the Broadway play and later the film "Little Shop of Horrors", which tells the tale of a plant that had a taste for human flesh and blood. Hey, how often have people written or speculated in the past about things that have actually come true?
Finally, I did some Googling about plants and came upon a BBC Web site and a fascinating entry by Josh Gabbatiss titled "Plants can see, hear and smell - and respond". We've already talked about seeing and smelling but I hadn't come upon anything about hearing. The BBC article cites work by Heidi Appel and Rex Cocroft at the University of Missouri in Columbia. They found that playing recorded sounds of caterpillars munching on leaves caused plants to flood their leaves with chemicals that deter caterpillars from attacking the plant. If plants are close enough together, the sound vibrations from a caterpillar munching on one plant may be detected by its neighbor, which can then make the appropriate chemicals to try to ward off the predator.
In summary, plants are much more interesting than generally thought. As noted on the BBC site, plants are just as alive as any animal, fighting for territory, fending off predators and even trapping prey. Without a brain they still manage many of the activities we humans engage in. A big challenge is to figure out how they accomplish what they do. They obviously know when they are under attack. Do they experience something akin to pain? A question that occurred to me is how similar are a plant's DNA compared to our DNA? We often see reports that talk about how similar our human DNA is to the DNA of our cousins, the chimpanzee.
Finally, I'm posting this column after having celebrated Memorial Day. As has become my custom, I once again observed the occasion by taking my copy of the book "Heroes Among Us" down from the shelf. The book, as I've mentioned in previous years, is a 388-page tribute to the veterans from Mechanicsburg High School who fought in World War II. In our small town of Mechanicsburg, Pennsylvania 30 men died in that war. One of them, William Guyer, lived across the street from us while another, Ernest Martin, lived just a few doors down the street. In my class of 1944, we lost one member, Edgar Wickard. I just want to pay tribute to them and to all those who have served our country in the military. I've probably mentioned before that we kids used to play baseball and football on a field we cleared across the street from the Ent household. And my hat is off to John Ent, a combat veteran of the Korean and Vietnam wars with 31 years of service in the U.S. Army!
Next column on or about July 1, hopefully. .
Allen F. Bortrum