Other plants have unisexual (imperfect) flowers. These unisexual flowers are either; staminate (male) flowers and pistillate (female) flowers. These unisexual flowers can be on the same individual plant (monoecious) or on two different individual plants (dioecious). Corn (Zea mays) is an example of a plant that has unisexual flowers but is monoecious (one household).

Marijuana (Cannibis sp.) is an example of a plant that has unisexual flowers and is dioecious . Marijuana plants with female flowers are far more valuable (and illegal) than those with male flowers. With dioecious plants, a male plant and a female plant must both be present in order to produce fruit and seeds. Female marijuana plants grown in the absence of males produce no seeds and their flowers are called sinsemilla (without seeds).

Thus the range of sexuality in plants is very broad: from self-crossing hermaphrodites to out-crossing hermaphrodites, to unisexual flowers, to unisexual plants.

As we will see, the eggs of flowers need to be fertilized. Pollination is the process of moving the pollen grain from the anther of a stamen to the stigma of a pistil.

Bio 113 pollination animation

There are a few flowers that can self-pollinate all on their own, but this limits them to inbreeding. Most species rely upon some kind of pollination vector to accomplish pollination. The vector can be any agent that moves pollen from the anther to the stigma.

Most flowers have evolved to use a particular type of animal as a pollen delivery vector. The vectors have sensory organs to locate flowers, they have locomotion to get them to the flowers, in spite of large spaces between the individual flowers, and they have enough intelligence to remember that they can depend upon a reward if they visit one particular species repeatedly.

The Birds and the Bees (short movie, new page)

Bees are the most important group of flower pollinators The bees live on the nectar and feed it to their larvae. With special mouth parts, hairs, and other body adaptations, they are especially suited to collect and carry these materials. Bees are guided by sight and smell. Bees see yellow and blue colors. They also see ultraviolet light as a distinct color and many flowers have ultraviolet markings. Bees are not able to see the color red, so they rarely pollinate red flowers. Some flowers have “honey guides” that guide bees to the nectar. They often have a "landing platform".

Why Are So Many Bird Flowers Red? Article

Birds - Birds have a good sense of color and are attracted to colorful red flowers, many of which form long tubes for the beak of the bird. They do not have a keen sense of smell, so bird-pollinated flowers usually have little odor. Hummingbird-pollinated flowers usually have a long, tubular corolla suited to their long beak.

Wind - The flowers are small in comparison to animal-pollinated flowers and the pollen grains are light, small, and are not sticky. The stamens are very exposed so that the pollen can easily blow in the wind. The stigmas are feathery and also exposed to the wind so that they can catch the pollen. Wind pollinated flowers are often grouped in inflorescences (a group of flowers on a single branch or stem) to facilitate pollen release and capture.

Wind pollinated plants like corn have evolved to grow in large populations so that it is likely a stigma will be downwind from an anther. Of course it is important that we cultivate corn in that way too. If you are only putting a few corn plants in your garden, remember that a single row will only pollinate well if the prevailing wind is down the length of the row. The stigma of the female corn flowers is called a silk. Each stigma must receive a pollen grain for its kernel to develop properly.

Once pollinated, the ovules within the ovary of the pistil are fertilized and develop into seeds. The seeds send a signal (hormone-like) to the surrounding Ovary which causes it to swell and develop into a fruit.

You can see that the ovary of this legume (bean or pea) already resembles the fruit it will become. In most instances, without pollination/fertilization there would be no formation of fruit and definately no seeds. This makes plant sex an important topic for humans, since seeds and fruit are such a large part of our diet.