Flowers are the sexual part of a group of plants called angiosperms. Angiosperms produce blossoms to carry out reproduction, with fruits and seeds as the end result. Indeed, looking at many fruits you can see evidence of their flowery origin. The bottoms of apples, for instance, have papery bits that are dried floral parts. Flowers are very good at fulfilling their reason for being: angiosperms are the most prevalent plant form on land.
Flowers can have up to four kinds of parts, these arranged in whorls as you move from the outside of the flower toward the middle. If a flower has all the parts, you'll find carpals in the middle, usually fused into a single pistil. Next out are the stamens, then the petals. The whorl farthest from the middle is made of sepals, which are there to protect the flower in bud as it prepares to bloom.
Male and Female Parts
Being sexual reproducers, flowers can have male and/or female parts. Stamens are "male," each composed of a filament capped by an anther. Pollen is produced in the anther, and within that pollen is sperm. The pistil, meanwhile, is "female." At its base is an ovary containing an unfertilized egg. A style rises up from the base, ending in a stigma, there to catch pollen so the egg can be fertilized.
Depending on its parts, flowers can be categorized. First, flowers are typed by whether they have all their parts. Flowers that do are called complete flowers; those that don't have the four parts are called incomplete flowers. Flowers lacking female parts are called staminate or male; those lacking male parts are pistillate or female. If a flower possesses both male and female parts, it is called perfect; if not, it is imperfect. All complete flowers must also be perfect.
Being rooted, flowering plants need help getting pollen to the stigma and use wind or creatures to accomplish the task. Small, modest flowers use wind to blow pollen to other flowers. Large, showy flowers use creatures to deliver pollen. The flowers' scent, beauty and nectar draw creatures like bugs and birds. While visiting one flower, creatures pick up pollen they then deliver to other flowers. Once on the stigma, the sperm within the pollen can travel down the style to fertilize the egg.
Wind-pollinated flowers don't need to go to great evolutionary lengths to change their design. Those employing creatures, though, need to be as enticing and accommodating as possible to attract pollinators, and scents, nectar, colors and shapes have all developed over time. Some showy flowers have even evolved to attract certain kinds of pollinators. For instance, flowers that smell like rotting meat are trying to attract pollinators like flies that typically visit dead plants. Flowers featuring tube-shaped plants are probably trying to accommodate long-tongued butterflies or long-beaked hummingbirds.