Cross-pollination, which is when the pollen of one plant fertilizes another plant of the same species, occurs in a huge number of plants, including corn, willows, grasses and olive trees. A large number of food crops are cross pollinated, including squash, onions, broccoli and spinach.
Cross-pollination can only occur between members of the same species. For example, it is impossible to create a hybrid of a cucumber and a zucchini as they are separate species within the same family. However, it would be possible to cross a zucchini with another squash, such as a pumpkin, as they belong to the same species.
The opposite of cross-pollination is self-pollination, in which a plant produces both male and female organs that pollinate by themselves. In nature, cross-pollination can occur in a number of different ways, but the main methods are pollen being spread by wind, insects, birds and other animals. However, humans also cross pollinate plants in order to make hybrid species.
When a plant is cross pollinated, the resulting seeds contain characteristics from both parents, but the cross-pollination doesn't affect the fruit or vegetable being produced. The one exception is corn, in which the resulting ears usually contain aspects of both plants being cross pollinated.