Citrus
Many citrus varieties are seedless and are produced parthenocarpically without pollination. Some varieties may be capable of producing fruit either way, having seeds in the segments, if pollinated, and no seeds if not.
Citrus that requires pollination may be self compatible, thus pollen must be moved only a short distance from the anther to the stigma by a pollinator. Some citrus, such as Meyer Lemons, are popular container plants. When these bloom indoors, they often suffer from blossom drop because no pollinators have access. Hand pollinate by a human pollinator is a solution, though it is important to learn whether the variety is self fertile or self incompatible.
A few citrus varieties, including some tangelos and tangerines are self incompatible, and require cross pollination. Pollenizers must be planned when groves are planted. This last group generally requires the addition of managed honeybee hives at bloom time for adequate pollination.
Read more about this topic: Fruit Tree Pollination