ICSE Class 9 Answered

Nature favours cross-pollination in the following ways:

(1) Unisexuality: Papaya and Cucumber have unisexual flowers, i.e. different male and female flowers, either borne on different plants or same plant.
(2) Dichogamy: Maturation of androecium and gynoecium occurs at different times. E.g. In apple, stigma matures before anthers (Protogyny).
(3) Self-sterility: This is a condition in which, even if the stigma receives pollen from the anthers of the same flower, the pollen fails to undergo further growth.