Pollination is the process of transferring pollen from one anther to the receptive surface of the pistil in plants.(Purves, G-21) Pollination is most common in angiosperms, which are flowering plants. Although pollination occurs mostly in angiosperms pollen and pollen tubes are also used in gymnosperms. There are four types of pollination: cross pollination, self pollination, water pollination, and wind pollination. Cross pollination is the most common type of pollination in angiosperms, but the other techniques are also used. Cross pollination also includes animal pollination. This is where and animal or insect is involved in the pollination of a plant. Some examples of animals that are used in cross pollination are bees, ants, beetles, butterflies, moths, and hummingbirds.
Pollen is the tiny grains that are produced in the male organs of flowering and cone bearing plants. Pollen is produced in the stamen, which is the male part of the plant. Pollen is transferred from the male parts to the female parts of the plant, which is called pollination. Pollen is found in seed plants and are the microscopic grains containing the male gametophyte and gamete. Pollen is needed in order for pollination to occur.(Purves, 752)
Once the pollen grain is germinated it then develops a pollen tube. The pollen tube either traverses the spongy tissue or, if the style is hollow, it grows down on the inner surface of the female organ until it reaches the ovule. In order for the pollen tube to grow it requires calcium ions and cell adhesion proteins. These are taken up by the growing tip. The pollen tube is guided by chemical signal within the ovule. If the ovule is destroyed the plant can no longer be fertilized. (Purves, 752-753)
There are many characteristics that characterize the angiosperm. These include: double fertilization, flowers, produce a triploid nutritive tissue called the endosperm, ovules and seeds are enclosed in a carpel, produce fruit, xylem contains vessel elements and fibers, and their angiosperms are mostly animal pollinated. The yucca genus are only pollinated by one specific moth. The moth that pollinates the yucca is known as the Prodoxidae or the yucca moth. This specific moth transfers pollen from the stamen of one plant to the stigma of another plant. The moth then lays an egg in its flower. The moth's larvae then feed on the plant's seed. The moth larvae does not eat the whole seed, but only eats that yucca seed. This plant genus is exclusively pollinated by the yucca moth and the yucca moth, larva specifically feeds on the yucca seed.
Gymonosperms use the production of male gametophytes in the form of pollen grains. The pollen grains free the plant from depending on water to fertilize the plant. In this case, wind assists the conifer in the first stage of pollination. The wind carries the strobilus to the female gametophyte inside the seed cone. The pollen tube is then used in the transporting of the sperm to the sporophytic tissues.(Purves,592)
Cross pollination occurs between flowers of the same or closely related species. Cross pollination happens in different ways using pollinators, which are insects or mammals that are used to transfer pollen to other plants. Cross pollination is also called syngamy.(World Book)
The advantages of cross pollination is that the beneficial genes are transmitted faster to their offspring. Their offspring will the spread that beneficial gene to their offspring. Cross pollination introduces the plant to different genes which causes the plant to be able to deal with weather changes easier. Also, with cross pollination the plant is more likely to not be extinct because if one plant dies because of the weather or predators the new genes make it possible that the plant can still survive. 
Self pollinated seeds inherit its genetic information from one parent. In this case the new plant would be genetically identical to the parent plant. The advantage to this type of plant is that whether or not there is a pollinator, the plant can still reproduce. The plant can reproduce the entire garden from one plant. An example of this plant is a weed, because the weed uses rapid propagation to produce a large amount of weeds from one plant. Along with the advantages of self pollination also come disadvantages. The genetic uniformity of the population makes the plant easier to become extinct. The reason being that if a single disease destroys one plant all the genetically identical plants will then be susceptible to that disease. Also the beneficial genes do not spread as rapidly because if one plant has a beneficial gene it can only transport to a plant of its own offspring. 
Examples of some self pollinated plants are pansies and violets. Some plants are unable to self pollinate. Cranebills and Spiderworts are two plants that their stamen ripen before the pistil. In this case the pollen is shed from the stamen before the pistil ripens. Some plants have imperfect flowers. The Willow tree plant bears flower with a stamen or a pistil but not both. (World book)
An example of a pollinator is the honey bee. Honey Bees collect pollen and nectar from flowering plants to make honey. The Honey collects pollen in small cavities on their back legs and carry it to the hive. Pollen clings to their bodies and is carried to other plants. This is the most common of the insect pollinators. Also insect pollinators are ants, moths, beetles, and flies. The pollen grains of the entomopholy plants tend to be larger then those of the wind pollinated flowers. This is so that the pollen grains are more visible to their pollinators. Also their pollen has a nutritional value to the insect. The insect will use the pollen for food and by doing that will then pollinate the flower.
Pollination by vertebrates is called zoophily. This includes birds and bats. Both of these types of pollination are known as biotic pollination. Another pollinator is the Hummingbird. The Hummingbird inserts its beak into the flower to drink its nectar. The pollen sticks to the beak and is then carried to the pistils of other flowers. The pollination of birds and bats is the most common in the zoophily pollinators. The brightly colored flowers and the scent of the nectar is what attracts the different birds and bats to the different types of flowers.
Plants that use wind pollination have a sticky of feather like stigma. They also produce a large number of pollen grains so that the pollen is more likely to be dispersed. Wind can carry some pollen grains up to 100 miles away and further. Wind pollination is used in cone bearing plants when wind carries pollen from male pollen cones to female seed cones.(Purves,752)
Water pollination is the most uncommon of all the pollination techniques. When this form is used it takes place in rivers or streams. there are two types of water pollination surface pollination and under water pollination. Surface pollination is observed to be a transitional phase between wind pollination. Some examples of water pollination include pondweed, and waterweed.
- Pollination Encarta
- Hydrophily Wikipedia
- Pollination Wikipedia
- Purves, William. Life the Science of Biology. 2005. Couriers Company Inc.
- "Pollination." The World Book Encyclopedia. Chicago: World Book Inc. 2006
- Yucca Wikipedia
- Zoophily Wikipedia
- Entomophily Wikipedia