In regards to tree fruits, what is meant by the term self-unfruitful?

FAQ
Question: 

In regards to tree fruits, what is meant by the term self-unfruitful?

Answer: 

Pollination is the transfer of pollen from the anther to the stigma.  After pollination and fertilization, fruit set occurs.  There are two types of pollination.  Self-pollination occurs when the pollen is transferred from the anther to the stigma on the same flower, from another flower on the same plant, or from a flower on another plant of the same cultivar.  Self-pollinated plants are said to be self-fruitful.  Many plants cannot produce fruit from their own pollen and are considered self-unfruitful.  These plants require cross-pollination for fruit set.  Cross-pollination is the transfer of pollen from one plant to the flower of a genetically different plant or cultivar. 

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