For humans, Flowers have become symbols of fragile purely ornamental beauty. In real, non-symbolic life, however, flowers are the hard-working and highly successful reproductive tools developed by one group of plants, called angiosperms. Flowers vary widely in size, from those of the aquatic Wolffia, the size of a sesame seed, to the jungle flower Rafflesia, which has the diameter of a washtub. They range in colour and beauty from tiny, grey-green flowers without petals to large, brightly coloured garden flowers and exquisite tropical orchids. Some flowers look like a child’s drawing of a flower, five petals with a round centre. Others look like trumpets, pitchers, or even insects. All these flowers have some structures in common. Male parts called stamens consist of a stalk, or filament, and a pollen head, or anther. Female parts, called carpels or pistils, consist of an ovary where the seed develops, and a stalk, or style, tipped by a sticky stigma, which receives pollen. Stamens and carpels may occur on the same flower, on different flowers of the same plant, or on different plants. Most plants species avoid pollinating themselves, thus increasing variety in their offspring. Instead, flowers rely on a vast variety of pollinating methods, from the wind to insects to bats.

Flowers in most plant species contain both male stamens and female carpels. But both species have evolved with separate sexes. In such plants, all the flowers on an individual plant are male with stamens or female with one or more carpels. These plants cannot self-pollinate, they produce offspring that are exactly the same as the parent plant. If environmental conditions should suddenly change, however, a population of identical plants could die out. A varied population on the other hand, might contain some individuals that could survive. So a species grains an advantage if it does not self-pollinate.

Plants in which each flowers has both stamens and carpels are called hermaphroditic. Those with separate male and female flowers are called monoecious. And those in which each individual is either male or female are called dioecious.