There are two main ways that pollen is moved: it can be carried by the wind or through the aid of animals.
A small number of plants use other means, and some self-pollinate, especially when they don't get the help they need to cross-pollinate.
Wind pollinated plants usually have small, drab flowers or no flowers at all since they don't need to attract pollinators. Instead, their strategy is to put out huge quantities of pollen.
These are the kinds of pollen that cause allergy symptoms in many people. The pollen of insect-pollinated plants is typically heavier and less likely to be blown by the wind, so the pollen grains of those plant species are less likely to be an issue to allergy sufferers; another reason to appreciate bees!
Elegance of Pollination
In the video on this page, Maryann mentions that insect pollination is elegant and efficient, assuming there are enough insects to do it. Therein lies the major concern of many fruit and vegetable farmers and other agricultural experts. Honey bee populations have dropped significantly, over 25% since 1990, while food production has increased during that time.
This stretches the capacity of migratory beekeepers as they move fewer colonies all over the country in an effort to pollinate all the food crops that require insects to do the work of transferring pollen from one blossom to another.
Honey Bee Health and the Future of Pollination
If the honey bee population drops too low, it will not only likely jeopardize human food supplies around the world, but also reduce the wide variety of fruits and vegetables in the human diet. Some farmers and entrepreneurs are trying to cultivate larger populations of other species of insect pollinators, like bumble bees and blue orchard bees in an effort to capitalize on the niche left by the declining honey bee, but it is unclear which practices will prove to be sustainable.
Honey bee health has impacts beyond agriculture because bees pollinate all kinds of flowering plants that are integral parts of the complex food web and ecosystem of our planet. Halting the decline of the honey bee population is a better approach than finding substitutes that can do their jobs. This may require changes in human behavior and honey bee management practices so scientists continue to study and explore the life and world of the honey bee in an effort to bring helpful new (or old) ideas to light.
Since pollination is dependent on many factors, there are times when it doesn't happen or doesn't happen enough.
Some species of plants, like melons and cucumbers, have male and female flowers and they may not always open at the same times. Plants may grow in places where pollinators can't get to them, like on a high balcony of a tall building or in a non-native location. Maybe only a few pollinators stop by at the appropriate time due to weather.
Whatever the causes, if a plant is not pollinated, the fruit of that plant will not grow. If it is partially pollinated, it will grow, but it will look small, possibly misshapen, and could be a different color than a well-pollinated example.
Aside from hand-pollinating every flower, the best way to ensure thorough pollination is simply to have lots of pollinating insects around!