Grades: 9-12
Essential Skills: Science
Duration: 2 class periods
OVERVIEW
Students will be able to describe sexual reproduction in plants, including the process of pollination, how insects assist in pollination, and how pollination differs from fertilization. Students will also learn about the importance of honey bees to Arizona agriculture.
Information Sheets:
Activity Sheets:
Lesson Plan
Introduction activity (20 minutes)
Have the students think of all the ways that plants can produce new plants, and make a list on the board. To stimulate their thinking, ask what they can bury in the earth to grow a new plant. They should think of seeds, cuttings, potato tubers, runner plants like strawberries or spider plants, bulbs, etc.
Explain that unlike human beings, plants can produce new plants through normal growth and cell division. This is called asexual or vegetative reproduction. Asexual reproduction is important to horticulture because the new plant is exactly the same as its parent. If a plant breeder develops or finds a useful variety of plant, he or she can propagate many additional plants of the same kind. For example, all the Golden Delicious apple trees in the world came from one seedling found in Clay County, West Virginia. Have the students indicate which of the reproductive methods on their list are asexual (all but seeds).
Explain to the students that flowers are beautiful to us, but to the plant they serve a critical function. Flowers are how plants produce seeds or reproduce by sexual reproduction (see quote in Lesson 2.4, page 2).
Activity 2 Flower parts (45 minutes)
Give each student a copy of Information Sheet 9 and Activity Sheet 29 to review. Have the students examine dissected and labelled flowers. Discuss the parts and their functions. If possible, display books with detailed drawings of a number of different kinds of flowers (such as The Clover & the Bee by Anne Ophelia Dowden).
Have the students dissect different flowers and label the parts. Ask the students count the number of stamens and petals and compare between different types of flowers. Have them observe the stamens carefully and draw several that are different in shape.
Show the students examples of composite flowers such as daisies, asters, or sunflowers. Explain that the exterior structures that look like petals are actually tiny flowers called ray flowers. The center is made up of individual flowers called disc flowers. Each disc flower has an ovary that may form a seed if fertilized. Show the students a mature sunflower seed head, if available, to demonstrate this.
Activity 3 Pollination (50 minutes)
Give each student a copy of Information Sheet 10 and Activity Sheet 10. Discuss what pollination means. Discuss how pollination differs from fertilization. Have the students list all the ways they can that pollen may be moved from the anther to the stigma (wind; bees and other insects; birds, particularly hummingbirds; bats; people, particularly by plant breeders, etc.)
Discuss the importance of pollination by insects to food crop production. Mention that insects pollinate many landscape plants, native plants and wildflowers as well as the crop plants listed. Have the students investigate the obligate relationship between the yucca moth and yucca flowers and other examples of specialized pollination (see The Clover & the Bee by Anne Ophelia Dowden). Explain that tomatoes grown in greenhouses require pollination and so the growers keep nests of bumble bees to perform this service.
Read or distribute the following materials to the students:
Alfalfa Seed Production in Arizona
Arizona has an ideal climate for growing alfalfa for seed. A long dry period following the bloom is ideal for proper maturation of seeds and for maximum harvest. However, alfalfa grown for seed production also requires good pollinator activity during bloom since plants grown from self -fertilized seed frequently lack vigor and produce less forage. When alfalfa flowers are self -pollinated a low percentage of the ovules are fertilized, few pods set, and the pods are small since only 1 or 2 seeds form.
The drawback is that alfalfa flowers have a special mechanism that must be tripped in order to be cross-pollinated and to produce seed in commercial quantities. When a bee inserts its proboscis into the flower, it separates the keel petals which bears the stamens and pistil. When released (or tripped) the sexual column strikes the bee, pollen is deposited on the bees head, and in the same action, the stigma usually receives pollen brought by the bee from another plant.
The honey bee visits alfalfa to collect pollen and nectar. Pollen-collecting bees can be recognized by the pollen pellets adhering to the outside of their hind legs. When more pollen-collecting honey bees are present in an alfalfa field, proportionally more tripping and cross-pollination of flowers occur.
Numerous surveys in Arizona alfalfa fields show that often less than 5% of foraging honey bees are pollen collectors. This is thought to be because honey bees learn to avoid being struck on the head when the flower is tripped. However, the activity of nectar-collecting honey bees should not be discounted. Although their tripping efficiency is comparatively low, their total effect is important when large populations of bees are working in a field.
Although there are usually a variety of insects present in alfalfa fields, only bees are effective pollinators because other insects do not trip the flower. Surveys in Arizona have shown that native bees leafcutters, bumble bees, and alkali bees are efficient trippers of alfalfa flowers. However they are not always present in sufficient numbers in Arizona to be depended upon as pollinators of alfalfa.
At this point, stop and ask the students to discuss the problems with growing alfalfa for seed. Have them make a list of factors which affect pollination and seed yields. For example:
1. Improper applications of insecticides kill native bees and honey bees.
2. Over-irrigation reduces the nectar sugar concentration and the attractiveness of the flowers to honey bees.
3. Other sources of nectar and pollen attract bees away from the alfalfa field.
4. Pests such as Varroa mite and trachea mites kill honey bees and weaken their colonies.
5. Cultivation reduces or destroys nesting places for native bees.
6. Honey bees do not collect as much pollen in alfalfa fields...
Then have the students suggest solutions,
for example:
1. Restrict insecticide use, choose insecticides that target pests and are soft on bees.
2. Irrigate properly.
3. Use commercially available scents (pheromones) to attract honey bees into alfalfa.
4. Supply nest sites for native bees.
5. Have beekeepers bring in hives to increase the number of honey bees in the field (to compensate for the low percent collecting pollen)...
Conclusion (20 minutes)
To conclude, discuss the economic impact of pollination of agriculture in Arizona. Reemphasize what pollination is, and how roughly one third of our diet is the direct result of pollination by insects.
Additions
Extensions:
· Africanized honey bees have recently arrived in Arizona. Have the students investigate how Africanized honey bees have impacted the beekeeping industry. Ask the students to read local beekeeping newsletters, interview beekeepers and/or attend a meeting of a local beekeeping club or association. Have the students present oral reports of what they dis cover.
Words with special meanings:
(for understanding only, not to be tested)
Anther
Bibliography:
Insects and Flowers: The Biology of a Partnership, by F.G. Barth. Published by Princeton University
Press, Princeton, NJ, 1985.
The Clover & The Bee, A Book of Pollination, by A.O.T. Dowden, published by Harper Collins, 1990.
Roses Red, Violets Blue: Why Flowers Have Colors, by S. A. Johnson and Y. Sato. Published by
Lerner, M.N., 1991.
The Story of Pollination, by B.J.D. Meeuse. Published by the Ronald Press Company, N.Y., 1961.
The Sex Life of Flowers, by B. Meeuse and S. Morris. Facts on File, N.Y., 1984.
The Pollination of Flowers, by Michael Proctor and Peter Yeo. Published by Collins, London, 1973.
