Plants move in response to both internal and external signals known as stimuli. Plant response to stimuli through movement may be categorized in two ways: tropisms and nastic movements. Unlike tropisms, with nastic movements the response bears no relation to the direction from which the stimulus is applied. That is, the response is not necessarily directed toward or away from the stimuli. Nor are nastic movements slow and irreversible growth movements like those found in tropisms. Rather, nastic movements are rapid and visible to us in real time. Furthermore, nastic movements are reversible and may happen over and over.
What Powers Nastic Movements?
How can plants make rapid movements and do so over and over when they have no muscles with which to power such movements? The answer lies with hydraulics. By pumping water into or removing water from certain strategically – placed cells (motor cells), plants can propel their parts and bodies in ways we still don’t fully understand.
Nastic movement is generally caused by elastic changes in the size of special motor cells within the plant tissue. These changes are generally produced by changes in osmotic (water) pressure due to an influx or efflux of ions which in turn cause water to move in or out of the cells.
External Environmental Stimuli Trigger Nastic Movements
Nastic movements may be generated by any number of external environmental stimuli – light, lack of light (night), chemicals or nutrients, fluctuations in temperature, gravity, and contact (touch).
Photonasty is a nastic response to light and dark. Photonasty is driving force behind the opening during the day and closing during the night of the flower of the evening primrose (Oenothera).
Nyctinasty, often referred to as sleep movements, is a nastic response to dark in which the leaves and/or flowers of some plants adopt a different posture at night than they do during the day. Many flowers, such as the morning glory, tulip (Tulipa), crocus, California poppy (Eschscholzia californica), and dandelion (Taraxacum), are open during the day or part of the day and closed at night. Other flowers are open at night and closed during the day, a situation frequently associated with pollination by night-active animals such as bats.
Chemonasty is a nastic response to chemicals or nutrients. The sundews (Drosera) have many hair-like glandular tentacles each tipped with a sticky secretion. When a nitrogenous substance, such as an insect, contacts the tentacles and is stuck in the sticky secretions, the tentacles bend inward (chemonasty) trapping the insect. Other glands are then stimulated to release enzymes which digest the prey.
Thermonasty is a nastic response to fluctuations in temperature. One example is the opening and closing of crocus flowers following an increase or decrease in temperature.
Geonasty is a nastic response to gravity. Geonastic movements coupled with geotropism function to direct root growth downward.
Thigmonasty (or haptonasty) is a nastic response to contact or shaking. A spectacular display of thigmonasty occurs in the Venus Flytrap (Dionaea muscipula). When an insect lands on a trap formed by two curved lobes of a single leaf, the trap rapidly switches from an open to a closed configuration. Without a doubt the most striking example of thigmonastic movement occurs in the sensitive plant (Mimosa pudica).
The leaf of a sensitive plant consists of a main petiole at the end of which are two to four secondary stalks, each bearing many pairs of tiny leaves or leaflets, arranged opposite one another. When the plant is subject to touch – being hit with rain drops, an insect landing, or a human finger or shock – being shaken or having a lit match held under a leaf, the leaflets fold upward and the main petiole droops. The reaction is immediate and visually striking.
Once the stimulus ceases, the plant pumps the petioles back up, opens the leaflets gradually and within an hour or so, resumes its normal posture.
There is much about nastic movements we still do not understand. They happen primarily due to changes in water pressure within cells but how does the plant control and coordinate these changes? If you firmly strike one leaf on a sensitive plant, not only will it fold up and droop but quickly all the other leaves follow suit. Some sort of signal must move through the plant. Is this signal hormonal, is it bioelectrical or is it both? At this point botanists have more questions than answers about nastic movements.
Adventures in plant growth and nastic movements for aspiring botanists of all ages may be found at:
www.TickleMePlant.com This site has kits for growing the sensitive plant (Mimosa pudica) or what they call the "Tickle Plant."
Dennis Holley - Looking back, it's clear that I have been a science nerd my entire life. A simple, almost toy-quality microscope my poor parents scraped ...
