Plant Movements
by
Harold Richman
"Plants are usually thought of as inanimate objects. You find them where you left them except that they may have grown imperceptibly. But plants actually do move according to particular stimuli.
Most plants move according to the location of the sun. The most familiar plant that follows the sun is the sunflower. When I drive down the highway in the summer, I am always in awe of a complete field of sunflowers all facing the same way. One night as I was lying in bed thinking about the sunflowers, it suddenly occurred to me that they would all be facing west when the sun set, but in the morning the sun would rise in the east. Do they do a quick take as the comedians do, or is there some other mechanism to turn the sunflowers to the east? I discussed this with Professor Pip at the University of Winnipeg. "The sunflowers keep on turning during the night and in this way they are facing east when the sun rises." she explained.
I intended to check on this with the sunflowers I had planted in my garden. In particular I wanted to know if the sunflower continues turning clockwise or does it retrace its motion and turn counter-clockwise. I asked for help from members of the Manitoba Naturalist Society via the Bulletin that, if they were growing sunflowers, to check this out and advise me by e-mail. I suggested that the best time to check would be about 2:00 a.m. If the sunflower turned clockwise, it should be facing north, and if the sunflower turned counter-clockwise, it should be facing south. I promised that the results would be tabulated and reported in the Bulletin.
The reason why sunflowers face the sun is not completely understood, but it is possible that the sunflower is more visible to the pollinating insects when it is brightly lit by the sun. The temperature of the flower will be a few degrees higher which will cause the developing embryo to mature more quickly. Most plants will lean towards the light; this exposes more of the chlorophyll to the sun.
When you plant a seed, the root always grows downward and the shoot grows upward. How can the plant determine which is up and which is down? We humans can keep our balance because of the labyrinth in our ears which can sense gravity and helps to keep us vertical. Plants do not have ears (except for corn) but they have developed their own mechanism to detect gravity. In the root cells are tiny grains of starch which settle to the bottom of the growing tip. The roots 'know' in which direction to grow. Shoots, on the other hand 'know' they grow in the opposite direction.
Dandelions and thistles have developed an ability to pull themselves down into the ground by pulling on their roots. If a seed germinates on the surface of the soil, it will develop a root system. Once established it will pull itself down into the ground for a more desirable location. If you cut off the leaves of a dandelion it will pull itself down into the ground and then develop a new set of leaves. That is why it is so difficult to get rid of dandelions.
(See questions)
A desert plant called Astragalus has the ability to move horizontally. This enables the plant to find a more suitable location. Of course it moves very slowly so you would not see it moving, but after some time you might notice that it is in a new location. The plant moves by pulling on the roots on one side.
There are many plants that respond to touch. I had heard of the sensitivity plant but had never seen one until about five years ago. We were in Puerto Rico for a winter holiday. A friend pointed out a sensitivity plant growing in a ditch. The leaves are very small, somewhat similar to those of the honeysuckle. When I touched the plant, the whole plant drooped very noticeably. It took less than a second. How did it send the message to all parts of the plant so quickly? Plants do not have a nervous system, so they use chemical signals. Even though we have a nervous system, our bodies also send out chemical signals which force us to react very quickly. Adrenaline is such a chemical.
What advantage does the sensitivity plant have by being able to wilt suddenly? It has been suggested that the sudden movement of the plant would startle a herbivore who began to eat it. Of course we have no way of knowing for sure what led to this odd evolutionary behaviour.
Portulaca is a popular plant for flower beds. I have planted them for many years but never realised that they take an active part in assuring that they are cross pollinated. When an insect lands on the flower and touches one of the stamens, the rest of them lean over and deposit some of their pollen on the insect. This movement has to be quite rapid as the insect is not likely to stay very long.
A somewhat similar movement occurs in the sundew (native to South Africa), but with a more sinister purpose. The sundew has what looks like a star burst of stalks. Each stalk has a small shiny drop of a sticky substance at its tip. To a passing insect it looks like a drop of nectar. When the insect lands on the sundew, it becomes stuck. When the insect struggles to escape, the other tentacles bend over making it impossible for the insect to escape.
If there were an Olympics for speed in the plant kingdom, the gold medal would go to the Venus fly trap. This is a carnivorous plant that has a trap at the end of each leaf which looks like an open clam shell. The inside is brightly coloured and shiny which attracts insects. On each half of the clam shell are three trigger hairs. When an insect lands and touches the trigger hair the clam shell snaps shut. This action takes only one fifth of a second. To insure that the trigger hair has not reacted to a rain drop or a piece of debris , the trap will not snap shut unless two heirs are touched within seconds, or the same hair is touched twice. The trigger hair sends an electrical signal along special tubes that act like nerves to the cells that make up the hinge. These cells are full of sap, but when the signal is received, these cells collapse causing the trap to snap shut. In fact, the trap is closed with enough pressure to crush the insect. The plant then exudes an enzyme to digest the insect. Even though the Venus fly trap has chlorophyll, it is found in locations that are short of nitrogen so the trapped insects supply this missing nutrient. I had always assumed that the native habitat of this plant was some tropical jungle, but it is actually found in the USA, in North Carolina.
Recently we saw the stage production of Little Shop of Horrors at MTC, and the movie with Robin Williams called Jamangi, both of which have man-eating plants as main 'characters'. Even though I know that they are science fiction, I think I will be a little more aware of what is behind me when I am out in my garden. You never can tell!