Contrivances or Adaptations for Cross Pollination (Outbreeding devices)

1. Unisexuality: - In some plants, flowers are unisexual to prevent self-pollination. Examples – Date palm, papaya (Carica). 
2.  Dichogamy: - In many plants pollen release and stigma receptivity are not synchronized. Either the pollen is released before the stigma becomes receptive or stigma becomes receptive much before the release of pollen. Dichogamy is of two types – 
1.  Protandry: - In protandry, pollens are released before the stigma becomes receptive. Most of the Angiosperms are cross pollinated only because of protandrous condition. 
2.  Protogyny: - In protogyny, stigma becomes receptive much before the release of pollen. 
3.  Chasmogamy or Anthesis: - Opening [blooming] of the floral bud in the form of a flower is called anthesis. Chasmogamous flowers have exposed anthers and stigma. 
4.  Heterostyly: - In some plants anther and stigma are placed in different positions so that the pollen cannot come in contact with the stigma of same flower. Due to this reason, self-pollination is not possible in these plants e.g. Primula. 
5. Self-sterility or Self incompatibility: - This is a genetic mechanism and prevents self-pollen (from the same flower or other flower of the same plant) from fertilizing the ovules by inhibiting pollen germination or pollen tube growth in the pistil. e.g. tobacco.