Dissect a flower to explore the form and function of plant reproductive structures.
Examine your intact flower. What do you notice first about it? What are your senses drawn to? What do you think the function of the structures you notice could be? Notice how the different structures are connected to the stem of the flower. Draw what you see, if it helps you.
Carefully remove the petals from the flower (you can use scissors or gently pull the petals off). You will probably notice that the center of the flower contains long filaments (click to enlarge the photo below). Use the hand lens and/or dissecting microscope to examine the filaments. Do all the filaments look similar to each other, or do they look different? Are they the same from top to bottom? Draw and describe the different structures that you see. What do you think the functions of the different structures you notice might be? How might these structures interact?
Remove the filaments and examine the swollen structure that is left. Use a blade or scalpel to cut it in half, either lengthwise or crosswise (see photo below). (If you have two flowers, cut one lengthwise and one crosswise.) Examine the inside using your hand lens or microscope. What do you notice? What might this be?
Flowers are the reproductive structures of angiosperms, or flowering plants. While there are lots of different types of flowers with seemingly very different structures, they all have pretty much the same parts—though sometimes you have to look very closely to notice the similarities.
Flower structures have evolved to maximize the chance of pollination, or fertilization of ovules by sperm cells contained in the pollen. The color, scent, nectar, and shape of a flower all help facilitate pollination through animal pollinators like insects, birds, and bats, or through wind, rain, and other physical phenomena. The colored petals along the outside of the flower advertise its presence to animal pollinators. The petals may be surrounded by green leaflike sepals, connected to the stem, or the petals themselves may actually be modified sepals.
When you removed the petals, you probably noticed a ring of filaments with knobs on top. These are the stamens, or sperm-associated part of the flower (the outer filaments in the first and second photos below). The knobs, or anthers, on the top of these filaments contain a powdery substance called pollen (third photo below) that may come off on your fingers (and can stain your clothes!). This pollen contains the plant sperm. If you examine pollen under a high-powered microscope, you’ll see that pollens from different flowers can have very different shapes and colors.
You also probably noticed a filamentous structure that didn’t have anthers or pollen—this is called the pistil or carpel, and it’s the structure that catches the pollen (the middle structures in the first and second photos above). The tip of the pistil is called the stigma and is often sticky, nubby, or covered in fine hairs, all structures that aid in catching pollen. The base of the pistil is swollen and is called the ovary, where the sex cells called ovules are found. Ovules are unfertilized, immature seeds. The tube connecting the stigma and the ovary is called the style.
When you dissect the ovary in a cross section, you’ll see segments containing ovules similar in shape to the inside of an orange (first photo below). These shapes will vary depending on what type of flower you are dissecting. Dissecting in a longitudinal cut reveals ovules lined up like peas in a pod (second photo below). Indeed, peas are actually mature, fertilized ovules.
Fertilization occurs when a pollinator transfers pollen from the anther of one flower to the stigma of another (flowers often try to avoid self-pollination). The pollen on the stigma forms a tube that travels the length of the inside of the style and contacts an ovule. The nucleus of the pollen sperm then travels down the pollen tube into the ovule and fuses with the female nucleus—that’s the fertilization event. The fertilized ovule grows into a seed and the entire ovary ripens into a fruit.
In some types of flowers, you may find stamens but no pistil, or a pistil but no stamens. There’s great variety in the way angiosperms organize their sexual organs: many have “perfect” flowers containing both parts, while other species may have sperm-associated structures and ovule-associated structures on separate flowers on the same plant. In other species, an individual plant may have only sperm-associated parts or ovule-associated parts.
Dissect other types of flowers, such as sunflowers, iris, or snapdragons, and identify the same parts.
Fruits come from flowers. Try examining fruits (as described in the Fruit Dissection Science Snack) for the remnants of flower structures.
It’s easy for students to get overwhelmed at the amount of vocabulary that can be involved in learning about botanical structure and function. For this reason, you may not want to begin a lesson with technical vocabulary. Instead, leave botanical terms for the end of the lesson and encourage students to spend the bulk of their time examining and describing the structures that they notice using their own words, and considering their form and potential functions. You might even consider whether knowing the botanical names of flower parts is truly necessary for your students.
Studying plant reproduction is an excellent way for younger students to learn about sexual reproduction in a hands-on way. Depending on their age and prior knowledge, you can have students draw connections and identify similarities between the sexual structures of a plant and those of a mammal or another animal. For example, sperm sex cells of both plants and animals are numerous, small, and mobile, while eggs/ovules of both plants and animals are fewer in number, larger, and stay in place.
You may want to consider how you use the gendered terms “male” and “female” during the flower dissection, which are commonly applied to flower reproductive parts. While the sex cells of flowers have many similarities to human sex cells, flowers do not have genders, and the use of gendered terms to describe non-human organisms may contribute to the exclusion of trans or gender-nonconforming students by reinforcing ideas about human gender as a naturally binary phenomenon.