In many plants, pollination--the transfer of pollen from anther to stigma--is accomplished by insects (including bees, wasps, and butterflies) or birds, especially hummingbirds. These creatures are attracted to a flower by its color and fragrance, and by its nectar, on which they feed. Other plants, such as grasses, oaks, and birches, have inconspicuous flowers whose pollen is spread simply by wind. Wind is also responsible for pollinating gymnosperms, which evolved before the advent of insects.
The production of seed from the union of pollen and egg is called sexual reproduction. In nature, sexual reproduction most often occurs between plants of the same species: if pollen from a different species lands on a stigma, pollination often cannot take place. But many kinds of plants have developed ways to prevent self-pollination--the pollination of a flower by pollen from that same flower--and ensure cross-pollination, which involves two different parents. For example, some varieties of fruit trees produce sterile pollen; another variety must be present and in bloom at the same time to ensure pollination. Most holly (Ilex) plants are either male or female, and both sexes must be present for the female to bear fruit.
Cross-pollination is also possible between species; the resultant offspring are called hybrids, and they may or may not resemble either parent.
Because sexual reproduction allows for mixing of genes from different parents, it is important as a source of genetic variation. Such variation can lead to the development of plants better able to survive and adapt to environmental changes.
In contrast to sexual reproduction, asexual (vegetative) reproduction yields offspring that are genetically identical to the parent. Examples of asexual reproduction include propagation by cuttings or layering or by division.