The typical flower contains the necessary parts for enticing pollinators and producing seeds.
The center of a flower usually contains the female, pollen-receiving pistil. The stigma at its tip is often sticky, feathery, folded, or otherwise designed to trap pollen. When they're ready to accept pollen, stigmas ready themselves for the transfer. They may be pushed upward by the long style that supports them, lean toward the male parts, or become stickier, for instance. As your students observe live flowers, encourage them to look for such subtle changes and try to infer how these may serve the plant. (Amaryllises and other types of lilies reveal a dramatic display!) At the base of the pistil, the generally hidden ovary protects ovules (eggs), which can be fertilized to become seeds.
The male parts, or stamens, typically surround the pistil. They may be quite long, to maximize exposure to wind and pollinators; hidden inside the flowers, to force pollinators to touch the stigmas on their way in or out; or able to lengthen and shorten over time, as needed. The anther, which is held up by the filament, produces and releases huge quantities of dust-like pollen in a variety of ways: by splitting open and curling back to release pollen or by twisting, for example. Once a pollen grain lands on a stigma of the same type of flower, it grows a pollen tube and sends it down the style in search of an egg. A sperm cell travels down the tube to fertilize the egg, leading to seed production. Once fertilized, the ovary wall takes in moisture and swells, becoming the fruit, which surrounds and protects the developing seeds.
The petals, which are typically the most noticeable parts of flowers, are designed to attract and provide platforms for insects, bats, birds, and other roving pollinators. The base of many petals contains nectaries, which produce the sugar-laden nectar. Since this food treasure is typically tucked deeply in the flowers, pollinators are coaxed into touching the flower's reproductive organs, and thus transferring pollen, in their search for sustenance. (Consider having students explore the petals of flowers such as nasturtiums or columbines, which have nectaries hidden deep in the long petal spurs.)
Although the major parts described above are standard in most flowers, the actual parts, numbers, and arrangements vary greatly. Plants such as cucumbers and squashes have separate male and female flowers, for instance. Others, such as holly bushes, have separate male and female plants, each featuring one type of flower. Different plant groups and families tend to have distinct numbers of parts. Lilies and other monocots (plants with one cotyledon) usually have reproductive parts and petals in multiples of three. Most dicots (plants with two cotyledons) have parts in multiples of four or five. (Invite students to cut through an apple horizontally to see the star-shaped seed container, which is a remnant of the five-chambered pistil. Then explore an apple blossom, if available, and see if any correlation exists.)