What is the difference between self-pollination and cross-pollination?

Self-pollination is the transfer of pollen from the anther to the stigma of the same flower or another flower on the same plant. Cross-pollination is the transfer of pollen from the anther of one flower to the stigma of a flower on a different plant of the same species. Self-pollination keeps the parent characters pure and does not depend on external agents, so it is more reliable, but over generations it can reduce vigour and variation. Cross-pollination needs an agent like wind, water or insects and is less certain, but its big advantage is that it mixes genes from two plants, producing more variation, healthier offspring and better adaptation. Plants often have features like unisexual flowers or different maturing times of stamens and stigma to encourage cross-pollination and avoid self-pollination.