The natural world is rich with well-adapted plants that have evolved intriguing means of meeting their needs. Carnivorous plants have developed mechanisms for trapping and digesting nitrogen-rich prey in nutrient-deprived environments. Although they remain in one place, sessile plants (leaves, flowers, or fruit that lack a stalk and sit directly on the stem) have developed movement unlike that of animals. They respond to gravity, light, and, in some cases, touch in ways that promote their survival. Plants vary most in the ways they reproduce, but most produce seed that they disperse in a grand variety of ways. Seeds can be seen as tiny packages that a plant sends to many sites. If the sites are suitable for growth, they allow seeds to germinate and grow into adult plants. Some plants use animals as subcontractors to deliver their seeds, such as when birds eat berries and excrete seeds. Others, such as grasses, broadcast seed on the winds. The coconut can float its seed vast distances in the oceans, colonizing islands thousands of miles from its starting place.

BACKGROUND To understand and admire how plants disperse their seeds effectively, students can build a "plant" that launches its seed as far as possible and then study the dispersal strategies of various plants. This activity motivates students to think about the many ways plants broadcast their seeds. Students use their knowledge of physics, botany, writing, art, and other disciplines. They also use their understanding of mechanics. The activity is appropriate for use in the K-12 biological science curriculum with adjustments appropriate for the grade level. It takes roughly 5 days to complete the activity (students' work habits may help to determine the time factor). This activity is open-ended; there is no one correct answer to be discovered. Students work in pairs so they communicate with one another and share various skills to create effective mechanisms.

Do the activity in the early to midautumn, when many plants are going to seed. Some common plants widespread in the United States that demonstrate a variety of dispersal mechanisms include milkweed (Asclepias species), cattail (Typha species), apple (Malus species), cactus (Opuntia species and others), maple (Acer species), oak (Quercus species), burdock (Arctium lappa), and jewelweed (Impatiens species). Jewelweed, which is common in low lying moist areas, features seed pods. When squeezed gently between thumb and forefinger, the pods snap suddenly and mechanically eject their seed some distance. Apples and cacti, along with many types of berries, hide their seeds inside an edible fruit. The seeds pass through the digestive tract of an animal, most often a bird, and are expelled some distance from the parent plant. Burdock has prickly burrs that, like Velcro, attach to fur and fabric and disperse seeds as they move around. Maples use the single wings that resemble helicopters as they drift to the ground, whereas oaks depend on squirrels to bury their acorns. Maple and oak seeds can be saved for repeated classroom use.

MATERIALS

Orange paint--enough to paint all the sunflower seeds

TEACHER INSTRUCTIONS Describe the project and tell students that the goal of the activity is to test their designs in the field. This will motivate them to apply their full attention when various plant dispersal methods are discussed. Different groups can work on different problems. Some students may want to test how far they can send a single seed. Others may want to see how many seeds they can send a minimal distance. The key point of this lab is for students to understand the adaptations plants use to disperse seed. Novel approaches help students with this understanding. For example, some plants compensate for poor dispersal ability by producing greater amounts of seed, while others develop highly effective methods of dispersal yet create small amounts of seed.

STUDENT INSTRUCTIONS

1. Have students select seeds from those listed in the Background section. Give them time to examine and work with these plants and to explore their strategies for delivering seed. Ask students to report their findings in journals. Students should share their journal entries with one another and with the class so that they gain understanding of their peers' and their own work.

2. Once students have a working knowledge of seed dispersal, they can determine class policy for the activity. They should consider the conditions that might affect their outcomes. These variables include the following:

Policy standards help make the activity as fair as possible. Students will suggest many possible rules such as restricting the heights of their plants to 2 meters (6.6 feet) for safety reasons. Sunflower seed is a good seed to broadcast; the kernels can be painted bright orange so that they are easier to find after they are launched.

3. Students should develop an assessment rubric. (The teacher can facilitate the rubric design, making sure that the students emphasize communication through journals, use of reasoning skills, explanation of and support for their ideas, participation, creativity, and knowledge of the ways real plants disperse seed. The actual performance of the devices is less important than the self-confidence students gain from the activity.)

4. Once these preliminary steps are completed, the students can design and build their devices. This can take one or two class periods. Any other work is done at home or during periods when the teacher is available. (The teacher can circulate among the groups, asking questions, pointing to interesting insights, and learning from the students.) Students should record and explain their ideas, tests, and observations in their journals, as real scientists do. They can illustrate their findings with diagrams and sketches.

5. To help develop communication skills, the students should describe their projects to the class before going outside to test them. (The teacher can ask students for their estimates of the distance their device will send the seed, what mechanism they have chosen, and how a plant might use this mechanism. Some students will base their designs upon nature; others will design contrived machines that would not be found even in the strangest environment. Allow for a question-and-answer period.)

6. The class then goes outside to test the various devices. Students can decide how many trials each group is allowed. Using a tape measure, designated students measure traveled distance from the spot where the seed was launched to the actual destination of the seed. The entire distance covered should be measured, including distance gained (or lost) from bouncing or floating. The winners (the students whose devices best fit the class's criteria for success) earn a bag of sunflower seeds that they can plant in their own yards. Students can also be rewarded on the basis of the "strangest" or the "most-plantlike" device.

7. Finally, the students should write a reflective exercise. They should assess their own designs and those of fellow students. Exercises can include interesting features of the devices and how they mimic plant strategies for seed dispersal. (Note: It is important to maintain connections with plant seed dispersal throughout this project. Students tend to forget the reasons for performing this project as they get caught up in what they view as a competition. To help keep the students focused, the teacher should ask them questions about how their structures are similar to and different from plants. Their journal writing will also help them remain focused.)

EXTENSIONS Elementary school students can create class books cataloging a wide variety of plants and their strategies for reproduction. A sixth-grade class could make books and then share them with first graders. Older students can collect plants and seeds and show them to younger children. The different levels could also work together to design their mechanisms.

In more advanced high school courses, students can expand on this activity. For example, if students are interested in physics, they might study physical aspects of the jewelweed mechanism such as, How is the seed pod designed? How fast does the seed move when released? How far does the seed travel? How does the seed pod move? More advanced students can calculate the velocity and acceleration of their projected seed to determine how much work is done by the plant.

CONCLUSION Students enjoy open-ended projects such as this seed dispersal activity. They feel motivated because they get to design their own mechanisms, and they take a high degree of ownership in their work. In addition, the activity promotes many of the National Science Education Standards (National Research Board 1996). The activity is inquiry-based because it gives students a problem to solve and requires them to design and conduct an investigation. It is student-centered because it requires the learners to generate their own solutions to the problem of dispersing seed rather than re-creating the teacher's solutions.

Assessment criteria for the projects are developed with student input and are agreed upon democratically. The focus is on scientific reasoning expressed through writing and oral communication and on the application of knowledge gathered through observation. The activity can be adapted easily to meet the national standards particular to the age group doing the activity.

STUDENT RESOURCES Bix, C. 1982. How seeds travel. Minneapolis: Lerner. Caduto, M., and J. Bruchac. 1994. Keepers of life: Discovering plants through Native American stories and earth activities for children. Golden, Colo.: Fulcrum. Hunken, J. 1994. Ecology for all ages: Discovering nature through activities for children and adults. Old Saybrook, Conn.: Globe Pequot. Katz, A. 1986. Naturewatch: Exploring nature with your children. Reading, Mass.: Addison-Wesley. Lauber, P. 1981. Seeds, pop, stick, glide. New York: Crown.

REFERENCE National Research Board. 1996. National Science Education Standards. Washington, D. C.: National Academy Press.

AUTHOR: DOUGLAS J. BUEGE

DOUGLAS J. BUEGE is a teacher-in-training at the University of Wisconsin, School of Education, Madison, Wisconsin. A former philosophy professor, he is being certified to teach secondary philosophy and biology. He is interested in designing curricula that explores the philosophy and history of science in the biology classroom. Example of a milkweed seed pod. Device ready to launch its seed.

SOURCE: Science Activities 35 no4 10-12 Wint '99 The magazine publisher is the copyright holder of this article and it is reproduced with permission.