Plants respond to different stimuli, including light, gravity, water, nutrients, and touch. Flowers grow toward the Sun. Roots push downward into the soil. Peas and beans have tendrils that wrap around a trellis and help them climb. Usually, plants move slowly, by growing. A plant bends or twines over a period of hours or days because one side grows faster than the other—the outer, faster-growing side curls around the inner, slower-growing side.
Some plants move much faster, however. Their movement is not due to growth, but to an immediate chemical response, much the way an animal's muscle moves in response to touch. The most famous sensitive plant is the Venus's-flytrap. This plant is a carnivore (meat eater), trapping tiny flies in its clawlike trap and digesting them. There are many other species of sensitive plants that move in response to touch, in order to trap insects or as a defensive response. Most sensitive plants are semitropical or tropical.
The Venus's-flytrap gets nutrients from flies rather than from the soil.
- 1 or more sensitive plants, such as cat-claw mimosa (Mimosa pigra), sensitive plant (M. pudica), Venus's-flytrap (Dionaea muscipula), wild sensitive pea (Cassia nictitans), or bladderwort (Utricularia purpurascens or U. roseopurpurea) (available from a nursery or catalog)
- notebook and pencil
- tissue paper
- white paper
- graph paper
- cuticle scissors
- assorted liquids (water, oil, rubbing alcohol, nail polish remover, etc.)
Some carnivorous plants, such as the sundew (Drosera aliciae) and pitcher plant (Cephalotus follicularis), trap insects with sticky goo and slippery sloping surfaces. You can feed them insects or small bits of hamburger with a pair of tweezers.
- Identify the part of the plant that is sensitive to touch, triggering a response. You can use a toothpick to gently touch different parts of the plant to see what part responds. In the case of the Venus's-flytrap, small hairs inside the trap will cause it to close when stimulated. In the wild sensitive pea, look for a pulvinus, an enlargement at the base of the leaflet and at the base of the leaf stem. Determine how long it takes your plant to return to its normal, untriggered state.
- You can conduct various experiments to test the sensitivity of your plant. Keep a detailed log of all your activities and experiments.
- Determine the degree of physical stimulation needed to trigger a response: Use a small straw to drop different size balls of tissue paper onto the triggering mechanism. What is the minimum mass (measure of matter) required to achieve a triggering response? Do larger forces cause the leaves or trap to move through greater angles? Use paper, pencil, and a protractor to make a goniometer like the one shown, to measure the angle of closing traps or curling leaves. Graph your results, showing the angle as a function of the mass.
- Determine whether the level of water or light affects the response: Repeat the experiment in step 2a, but use a constant mass and change the amounts of light and water present. (Be sure to test these variables one at a time, not simultaneously, so that you know which created the effect. Let an hour pass after making your light or water changes, to allow the plant to adjust to the new environment.) Does the plant respond differently at night? When wet or dry? Again, you can measure the extent of movement with a goniometer.
- Determine the effect of certain organs on the response: Cut the trigger hairs or pulvinus from your plant with cuticle scissors and see how it responds without these organs.
- Determine whether the plant responds to chemical stimuli: You can use an eyedropper to administer common household liquids, such as water, alcohol, oil, and so on. You can examine the effect of vapors by using a Q-tips swab dipped in a liquid and held close to, but not touching, your plant. Rubbing alcohol (isopropyl alcohol) and nail polish remover (acetone in water) have vapors that evaporate (change from a liquid to a vapor) readily into the air. Avoid inhaling the vapors of these liquids.
The pulvinus is a special motor organ. The center of the pulvinus contains a strand of vascular (vein-containing) tissue. Fluid pressure inside the plant helps it to sense its surroundings and move.
Carnivorous Plants (Lerner Natural Science series) by Cynthia Overbeck and Kiyoshi Shimizu (Minneapolis: Lerner, 1989).
Meat-Eating Plants (Weird and Wacky Science series) by Nathan Aaseng (Berkeley Heights, N.J.: Enslow, 1996).
Sundew Stranglers: Plants That Eat Insects by Jerome Wexler (New York: Dutton, 1995).
Carnivorous plant database: www.hpl.hp.com/bot/cp_home
Warning is hereby given that not all Project Ideas are appropriate for all individuals or in all circumstances. Implementation of any Science Project Idea should be undertaken only in appropriate settings and with appropriate parental or other supervision. Reading and following the safety precautions of all materials used in a project is the sole responsibility of each individual. For further information, consult your state’s handbook of Science Safety.