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Dissect-A-Flower Huntington Library, Art Collections, and Botanical Gardens Dissect-a-Flower Overview Students dissect an Alstroemeria or similar flower to familiarize themselves with the basic parts of a flower. They then use that information as they dissect and classify the parts of other species. Introduction Flowers mean something. We use flowers to say ―congratulations,‖ ―I’m sorry,‖ ―I love you,‖ or ―feel better soon.‖ We send and give flowers for many purposes. But for a plant, the flower has one purpose: sex. It’s true! Flowers are the sexual reproduction organs of plants. Some plants don’t have flowers, and they have to sexually reproduce by sending off spores or by using water to transport sperm cells. But a flower is the most evolutionarily advanced method for a plant to sexually reproduce. And every part of the flower is involved. There are four layers of a flower, called whorls, and each one has a duty in facilitating sexual reproduction. Starting from the outside in, the first layer we encounter in a flower is the layer of sepals. Collectively, this layer is also referred to as the calyx. The job of this layer is to protect the rest of the flower as it develops, making sure it reaches maturity. If you look at a flower bud, chances are most of what you will see are the sepals protecting everything else. Moving towards the center of the flower, we next encounter petals, or the second whorl (which is known collectively as the corolla). The job of the petals, often with help from fragrance produced by glands within the flower, is to attract pollinators. Pollinators carry pollen from one flower to another, which is a key step in uniting egg and sperm, as we will see. Petals are often showy and colorful, with beautiful patterns and markings. We love this about flowers, but they aren’t pretty in order to impress us; their appearance reflects the interests of the particular animal or bird that does the pollinating job. Red, tubular flowers often attract hummingbirds. White petals that open at night may attract moths. The petals of the Stapelia, coupled with a foul smell produced by its scent glands, attract flies that think they are encountering rotting flesh! Usually the pollinator gets a reward for visiting the flower in the form of nectar, but that’s not always the case. Sometimes a plant can trick a pollinator into thinking it will receive a reward, and by the time it finds out the truth, it has already collected or distributed pollen. Our next whorl is known as the stamen. There are often many stamens per flower. A stamen is made up of an anther on top—the part that holds the pollen grains—and a filament, the part that holds up the anther. On each anther are thousands and thousands of pollen grains. Each pollen grain is a protective wall surrounding sperm cells. When pollen is transported to another flower, the sperm cells are united with the genetic material of the egg, and sexual reproduction occurs. So, then, where is the egg? The egg lies well within the last whorl, the layer known as the pistil. The pistil lies at the center of the flower. The top part of the pistil is called the stigma. The stigma is often sticky; its job is to collect pollen. Once deposited on the stigma, the pollen grain sends out a pollen tube which contains the sperm cells. The pollen tube travels down the long shaft of the pistol called the style. The traveling tubes next enter a swollen cavity called the ovary at the base of the pistol. The ovary houses many ovules, tiny chambers of cells, one of which is the egg cell. The sperm is then delivered to one of the egg cells within the ovule, and fertilization takes place. The fertilized egg is called the zygote. The entire ovule, whose egg cell has been fertilized, will now mature to become the seed. Sexual reproduction takes place in every ovule within a flower that is fertilized by the sperm of a pollen grain. One pollen grain can fertilize only one ovule. Have you ever thought about a flower in such a way?! It’s important to note that not all flowers have all four whorls. Some flowers are either ―male‖ or ―female.‖ A ―male‖ flower would have stamens but no pistil, while ―female‖ flower would have the just opposite – a pistol but no stamens. Other flowers, like those pollinated by the wind, probably lacks petals. This makes sense because, as we know, the petals job is to be attractive to insects and birds that pollinate the flower. It takes a lot of energy for a plant to produce beautiful colors, patterns, and scents, and that would be a waste if the flowers primary pollinator is the wind, which has no interest in such things! The lab that follows starts with an Alstroemeria flower dissection. The Alstroemeria is similar to standardized drawings or models of flowers in number, appearance, and arrangement of its different parts. This flower is used to help identify and examine the parts involved in sexual reproduction. Students are then encouraged to look at flowers that vary from this basic setup, either by fusion of parts, loss of whorls, or aggregation together in an inflorescence. Motivation Describe a flower. Often descriptions include words like ―pretty‖ or ―colorful‖. Why are flowers colorful and attractive? Can you think of any flowers that don’t have pretty, colorful petals? What is the purpose of a flower? Objectives Upon completion of this lab, students should be able to 1. Name and describe the function of the four whorls of a flower. 2. Predict the possible pollinator of a flower based on its colors, whorls, or shapes. 3. Locate the structures that contain the reproductive cells (sperm and egg) in a flowering plant. Materials Alstroemeria or another lily-like, simple flower (one per student) Razor blades, scalpels, or dissecting scissors Dissecting scopes or hand lenses Forceps Dissecting probes (optional) Other flowers for study, ex: weeds, daisies, salvias, wild mustard, passionflower, orchids, etc. Mixed bouquets from a grocer sometimes have flowers whose parts have ―disappeared‖ through years of breeding--or have been removed--and can be difficult. Grass flowers, if available Associated California State Biology Standards 2a. Students know the differences between life cycles and reproduction methods of sexual and asexual organisms. 5f. Students know the structures and processes by which flowering plants generate pollen, ovules, seeds and fruit. Procedure 1. Pass one Alstroemeria flower out to each student. Also give each student (as you deem it appropriate) proper dissection tools: forceps, probes, and razor blades, scalpels, or dissecting scissors. 2. Introduce the activity with the motivation above. Discuss how a dissection of the flower will help to discern the functions of each of the parts. 3. Introduce the term whorl. A whorl is a layer of a flower with a specialized function. Flowers generally have 4 whorls. Tell the class they will now each dissect a flower and their challenge is to identify, describe, and determine the function of the four whorls. 4. Before beginning, outline objectives for the dissection: o Determine how many layers the flower has. o List the identifying characteristics for each whorl of the flower. o Form a hypothesis about the function of each of the parts of the flower. 5. Make sure students understand guidelines for safe use of the dissection tools. 6. Give students or student teams time to fully dissect a flower on their own. As they dissect, they should fill out Table 1 on the Student Sheet, discussing and deciding on responses together with their team members. You may wish to offer some guidance as they dissect-- suggesting, for instance, that they examine the inside of the ovary. 7. After students have had time for their exploration, lead a class discussion about their findings. Start with very general, open-ended questions (―What did you find?‖ etc), moving then to more specific questions about the number of layers, function, etc. 8. Synthesize the information by reviewing the flower they dissected, starting from the outer whorl and moving inward, describing form and function. 9. Once the class has an understanding of this information, you may wish to provide them with additional flowers, challenging them to properly identify the component parts and how they a similar and/or different from the Alstroemeria. 10. Finally, if grass flowers (or other flowers from plants in the grass family: corn, wheat, barley, bamboo, etc) are available, pass one out to each student or student group. Ask them to examine the flower and compare it to the other ones they have dissected. After generating a list of similarities and differences, challenge the class to offer a hypothesis as to why this flower lacks showy petals. You can tell the class, if needed, that grass is wind pollinated. Then ask again why they might lack petals (no need to use energy producing colors and scents if not pollinated by animals). 11. Students may now revise any ideas about the 4 whorls by filling out Table 2 on their Student Sheet. Evaluation The following questions are listed under the Analysis section of the student handout and maybe used as part of a report, class discussion, or assessment. 1. Define pollination. 2. What do animals gain from pollinating flowers? What do the flowers gain? 3. What kinds of things attract animals to flowers? 4. Where are sperm cells located in a flower? Where are they egg cells located? 5. What whorls are most likely to be missing in a wind-pollinated flower? Why? Extension Activities 1.
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