Stamen is the male reproductive organ and pistil is the female reproductive organ of a flower. Unisexual flowers are those flowers which possess either male or female reproductive organs. Since unisexual flowers do not have both type of reproductive organs, they cannot self-reproduce and hence show cross pollination. Fruit is a result of reproduction. Unisexual flowers with only stamen cannot reproduce and hence cannot produce fruits.
NOTE: Pollination is the process of transfer of pollens from one flower to another. Pollens are the male gametes. Cross-pollination is the transfer of pollens from one flower of a plant to another flower of a different plant. Self-pollination is the transfer of pollens from one flower of a plant to another flower of the same plant.
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