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.