Things plants know

By Dorothy Dobbie

Over the past few years, we have been discovering that plants “know” a lot more than we used to think

They have memories and learn from experience. They can sense objects around them and can detect presences beyond their own. They have complicated inner clocks that tell them when it its time to wake up or go to sleep or set blossom, seed or drop leaves. Plants can learn the habits of animals and take advantage of those habits.

They adapt to invaders and form co-operative relationships.

They know when there are enemies in the area and produce chemicals to ward them off or even to lure them to their demise. They have a secret underground relationship with families of fungus and bacteria that not only carry messages to other plants but practice trade, exchanging digestible minerals for carbohydrates produced by the plant.

Physically, although they aren’t mobile, plants have learned how to have long distance sex, rejuvenate lost body parts and, what’s more, some can live for hundreds, even thousands, of years.

Animals learn to beware the numbing effects of eating dieffenbachia.

Don’t laugh. All these things are being seriously stud- ied. Leading the debate on plant intelligence is Australian Monica Gagliano, an evolutionary ecologist, who started out as a marine scientist. “I realized only in hindsight that the same question I had about the animal transferred onto the plant. The main realization for me wasn't the fact that plants themselves must be something more than we give them credit for, but what if everything around us is much more than we give it credit for, whether it's animal, plant, bacteria, whatever?”

Researchers have found the sensitive plant (Mimosa pudica) can learn.

Plant memories

Mimosa pudica, otherwise known as the sensitive plant, has taught us through some scientific experiments that it can learn from experience, remembering what happened when it went through a jarring experience created by Gagliano, who dropped the plant gently over and over again. The first few times, the sensitive plant reacted as one would expect, closing its leaves. But after many repetitions, it started ignoring the action even though it reacted to other stimuli. Monica left the plant for a few days, came back, and it still didn’t react to the drop. Even after a month had passed, the learned behaviour prevailed.

House plant growers will know this if they think about it. Plants that are stressed often learn to adapt to the stress by modifying their responses, storing more water in leaves and roots if their keeper is a light waterer, for example, or becoming less sensitive to cold when in a persistent draft. So, when you hear someone exclaim. “Oh, I only water my plants once every two weeks and they are all right,” you might want to credit the cleverness of the plant over the ingenuity of the gardener.

Plant memories can be stimulated by one or several things: cold, light, hydration, air vibrations caused by insects or other stimuli, chemical triggers. . . it all adds up to a little more than we once gave plants credit for. Plant defenses

Plants have a powerful arsenal that can be used to defend themselves or go even further and attack. It all depends on the type of plant and the nature of the threat. The list of plant-based chemical defenses is long and deadly and includes cyanide, glycolates and razorshaped crystals of calcium oxalate as in what dieffenbachia, a common houseplant, can release to predators that dare to taste their broad leaves. This nasty defence mechanism can paralyse a browsing animal’s tongue, giving the plant the common name of dumbcane.

Chemical defenses can be delivered through aerosols, roots, touch and even by employing the assistance of beneficial bacteria and fungus such mycorrhizae that live in symbiosis with the plant.

Acacia trees and ants work together.

Mutualism

Symbiosis or mutualism is another way plants have of protecting themselves and taking advantage of resources. One of the most common examples is the relationship between the plant and the pollinator. The plant gets the ability to propagate while the pollinator gets food during the exercise.

The African acacia gives a home to the acacia ant, which in turn prevents rampant vines from invading their tree-home by chopping off the vines’ tendrils as soon as they being to wind around the tree. The ants will also go after invading insects as large as grasshoppers. The tree feeds as well as shelters the ant in hollowed out thorns where they also raise their ant offspring.

This is just one of the many ways plants have of using outside resources to survive and thrive. We all know how they employ birds and squirrels and certainly humans to disperse their seeds.

Plants also respond to sound, something that perceptive gardeners have been saying for years. We now know that plants will respond with a chemical weapon when they “hear” a caterpillar chewing

How were outside tobacco plants able to tell indoor tobacco plants about aphids?

Plant feelings

Lately we have been learning that some plants “scream” when under distress from events such as drought. They produce ultra sonic squeals, but we aren’t quite sure why, yet, unless it is the answer to the old question of how a tobacco plant in a closed environment was able to signal an aphid attack to another tobacco plant outside. In that case, it seems that the warning was to protect the species and we have known for some time that plants will warn others in their vicinity of danger from predator attack. Some do this by sending out chemicals. Some change their shape or colour when they are under stress.

This new research from Israel was conducted to see if detecting sound could help in developing more precision response to the needs of agricultural crops. Using tobacco plants as well as tomatoes, scientists then introduced different artificial stresses to different plants and attached listening devices to each. They responded with sounds which were different in response to each set of challenges.

We also know that plants produce electrical signals throughout their structures, from the petals of flowers to the roots, leaves and stems. Some scientists believe that electrical signals in plants could carry and process information.

In conclusion, Monica Gagliano says, “ …plants have taught me a lot and through the process of working with them I have learned a lot about who they are and who I am as well in relation to them, I needed to go through that first phase of interacting with the animal world and realizing that there is much more there than we are allowing them to show. There is more here than we're actually acknowledging.”