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| Life Science | Diversity And Adaptations Of Organisms H1.00 | Process Of Science | Observing 1.1 a |
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Digital Library For Earth System Education Sponsored by the National Science Foundation |
GRADE: Seventh
CONTENT STANDARDS: Life Science
CONTENT TOPIC: Diversity And Adaptations Of Organisms
CONCEPT: Organisms may respond to specific environmental stimuli.
CONTENT OBJECTIVE: 7H1.00 To learn that an organism may respond to environmental stimuli and that behavior is one such response.
INSTRUCTIONAL OBJECTIVE: The learner will:
TN COMPONENT OF SCIENCE: Process of Science
TN GOAL:
BENCHMARK By incorporating prior knowledge with the process of observation, a better understanding of one's environment may develop.
TIME REQUIRED:
Most living things have adaptations that help them survive. Adaptations are responses by organisms to an internal or environmental stimulus that help them survive. A polar bear is adapted to a cold climate. It has a heavy fur coat and lots of body fat. A polar bear would not be able to survive in a desert. Plants also have adaptations. Cacti have adaptations for living in deserts. They have stems that store water.
Responses by plants to light:
Some plants need a lot of light and others need very little. Let's conduct some experiments to observe how light affects plants.
(Light and the growth of leaves) Observe plants growing in the field. Note that leaves are oriented in relation to the angle at which light strikes. Indoor, healthy, full-grown geranium and coleus plants show the same tilting of leaf petioles as a positive response to light.
(Light and the growth of stems) Soak seeds of radish, oats, wheat, or bean, or kernels of corn overnight and plant them just under the surface of moist, clean sand in paper cups or flowerpots. Students may place several of these cups of seeds under a box to exclude light. Arrange a second box covering similar cups of seeds, but with a slit on the side at about the level of the cups, and set the boxes where moderate light can enter the slit. Within a few days, depending on the kind of seeds used, a marked growth or bending of the stems of the shots should be apparent in the box in which light enters from one side. In the other box, seedling grow upright in random fashion.
Roots show a negative phototropism, that is, they grow away from light.
(Light and the growth of roots) Soak radish or mustard seeds overnight. Spread cheesecloth across several tumblers of water and fasten with a rubber band, allowing some slack so that the cheesecloth remains wet. Then sprinkle seeds on the surface of the cheesecloth. Enclose some of the tumblers in a darkened box. Cut a small slit in a similar box and cover other preparations with this box. In this way light enters the box from one side (as described above for shoots of seedlings). You may find it more convenient to make small black paper boxes which fit snugly over individual preparations (aluminum foil may be used instead of black paper). Just cut a small slit along one side of some of the paper boxes. After the response of roots (and stems) is apparent, rotate the position of the slit 180°. Notice the change in the growth of stems and roots over the next few days.
Responses by animals to light:
The fact that animals respond to light may be shown in several ways.
(Flatworms) [Planaria] Planarian worms are found in streams on the underside of rocks or leaves of water plants, or they may be obtained through a biological resource supplier. In the laboratory they thrive best when kept in shallow, darkened jars of pond water containing some small stones under which they can hide. Remove the cover of the containers holding planarians and watch their movement away from a source of light. They exhibit a negative response to light.
(Earthworms) Students may keep earthworms in wet sphagnum moss in a dark box or covered dish, or they may use a layer of humus to line the container. Suddenly lift the cover and direct a beam of light from a flashlight on the anterior region of the worms. Observe the way they avoid the light by contracting or burrowing into the moss.
(Other organisms) Perhaps pond snails, land snails, or earthworms can be obtained easily. Can students use these organisms to design their own investigations on different light sources at different angles, on the effects of alternating light and dark periods, on the effects of polarized light, as well as light of different wavelengths? How do these responses compare with growing oat seedlings?
Response of plants to water:
There are several ways to show responses to the stimulus of water.
(Pocket garden) Use two squares of glass to fashion a pocket garden. Place two thicknesses of blotting paper between the glass squares so that a clear channel remains in the center. See illustration below. Along the center arrange small seeds, such as radish, mustard, or lettuce, which have been previously soaked, Then plug the ends of the row of seeds with cotton so that they will not fall out. Fasten the glass plates together with rubber bands. Now stand one end of the preparation in water until one blotter becomes soaked. Attach a strip of filter paper to the edge of this wet blotter to form a wick. Rest the pocket garden on a finger bowl of water and immerse the wick as shown. Be sure the other blotter remains dry. Within a few days observe the roots of the germinating seeds growing in the direction of the wet blotter rather than toward the dry blotter. Since the preparation is in a horizontal position, the stimulus of gravity will not interfere with the responses.
(Hanging garden) Shape a ball of sphagnum moss (or use a sponge), soak it in water, and then hold it together with cord. Insert soaked seeds of oats, corn grains, bean , radish, or wheat seeds at different locations: top, sides, and bottom. Hang this ball from a hook somewhere in class. Within a few days (depending on the kind of seed) the seeds will sprout. Students will see that roots grow into the sphagnum rather than downward in response to gravity. That is, the roots show a positive hydrotropism. The stimulus of water is greater than the stimulus of gravity. Should students suggest that the roots grow inward as a negative response to light, have them devise a plan for a control experiment (the same preparation growing in the dark) so they may refute their own suggestions.
Response to gravity by plants:
Line large test tubes (about 10 in. long) with blotting paper and insert a plug of absorbent cotton into the bottom of each tube. Wet the blotting paper thoroughly. Between the glass and the blotter, arrange soaked seeds of oats, radishes, or wheat. Fill the tubes again with water; pour off the excess. If you attach string or wire to both the top and bottom of each of the test tubes you have a loop through which they can be suspended in different positions from hooks around the classroom. When the test tubes hang at a slant, the growth of stems shows a negative geotropic response. Change the position of the tubes and watch for the change in direction of the growth of stems. In addition, set up replicas covered with aluminum foil or hang some in the dark. Students should see a negative response to gravity in the stems and a positive response in the roots.
Response to gravity by animals:
Live land snails are available from a fish market. A pound of snails will be sufficient for many investigations in behavior. Place some snails in a tall battery jar (with a cover, or they may soon be walking up the walls). You should observe that they quickly move upward. What happens when the jar is placed on its side? (Also, now is an opportunity to test the effect of other stimuli: a bristle near the antennae (contact); a bit of lettuce (food source); a lighted match (heat); ammonium hydroxide near, but not touching (chemical).
Reflexes in man:
Have students observe their own reactions in the following situations.
Blinking. Have one student stand and hold a sheet of clear plastic in front of his face. Then ask a second student to throw cotton balls or crumpled paper at the plastic. The student cannot avoid blinking even though he is protected by the plastic.
Contraction of the iris. Have students cover one eye for a minute, or darken the room for a minute. When they remove the hand have them look at once into a mirror. Note the dilation of the pupil of the eye that was covered. As light strikes the eye observe the change in size of the pupil (as the iris contracts in bright light). Students will need to try this several times. Have students observe their eyes in a mirror as the light is suddenly turned on. The contraction of the iris is marked. Also note the size of the pupil when the eye accommodates for close and distant vision.
Flow of saliva. Slice a lemon in front of the class, or describe its sour taste. Recall a favorite food by means of a full description (or pictures). Ask students to describe the results of a reflex that occurs within them.
Patellar reflex (knee jerk). Have a student sit on a chair or a table with legs crossed or freely suspended. When the subject is completely relaxed, strike a blow just below the patella bone with a rubber hammer or with the side of the hand.
2. Show a film showing instinctive behavior, such as Dance of the Bees available from Wilner Films and Slides or show Behavior in Animals and Plants (Coronet). Also preview films in the American Institute of Biological Sciences (AIBS) series on behavior (McGraw-Hill).
3. Establish how an experiment might be designed so that only one factor is tested at a time. This is a good device for teaching the elements of a controlled experiment.
behavior- a complex set of responses
geotropism- the response to gravity
hydrotropism- the response to water
negative phototropism- the act of growing away from light
petioles- a slender stem that supports the blade of a foliage leaf
phototropism- the response to light
response- a reaction to a stimulus
thigmotropism- the response to touch
tropism- growth responses of plants and the orientation movements of some sessile animals.
This is the time this file has been accessed since 04/02/98.
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