(a) Autogamy – Pollination is achieved within the same flower. Tansfer of pollen grains from the anther to stigma of the same flowr. Autogamy is such flowers requires synchrony in pollen release and stigma respectively. 

Geitonogamy – Transfer of pollen grains from the anther to the stigma of another flower of the same plant. Although geiotonogamy is functionally cross-polination involving pollinating agent, genetically. It is similar to autogamy since the pollen grains come from the same plant.

Xenogamy – Transfer of pollen grains from the anther to the stigma of a different plant. This is the pollination which brings genetically different types of pollen grains to the stigma. 

(b) A special character of sexual reproduction in angiosperm is the interaction of pollengrain, male gametophyte with massive sporophytic tissue of pistil (stigma and style) before discharging the male gametes near the egg. Only after the elucidation of self-incompatibility, the importance of pollen-pistil interactions become apparent. All events from falling of pollengrains on stigma to the entering of pollentube into the ovule is called pollen-pistil interaction. The sequences of pollen pistil interaction are– 

Pollen on stigma 

      ↓ 

Pollen adhesion 

     ↓ 

Pollen hydration 

      ↓ 

Pollen germination 

      ↓ 

Pollen tube into the stigma 

           ↓ 

Pollen tube growth through the style 

           ↓ 

Pollen tube enters into the ovule