What are the various adaptations or contrivances met within angiosperms flowers which favour cross-pollination?
Contrivances favouring cross-pollination: In many plants, there are devices which completely or partially prevent self-pollination and encourage cross-pollination. 1. Unisexuality: The flowers are unisexual, i.e. stamens and carpels occur in different flowers. They may be present on the same plant, e.g. in Three cells are at the chalazal end and are called the antipodals. The large central cell has two polar nuclei. Thus, a typical angiosperm embryo sac at maturity is 7-celled and 8-nucleated. Structure of typical embryo sac. maize or on different plants e.g. in Papaya. The first condition is called monoecious and second is called dioecious. In monoecious plants, both self-and cross-pollination can occur but mostly there is cross-pollination as sex organs mature at different times. In dioecious only cross-pollination occurs. Dichogamy: When stamens and carpels of bisexual flowers mature at different times it is called dichogamy If stamens mature first, it is called a protandrous condition, or if carpels mature first, it is called protogynous condition. Dichogamy prevents self-pollination and ensures cross-pollination. 3. Self-sterility: In orchids, pollen grains of one flower does not germinate on the stigma of the same flower. This is called self-sterility but pollen grain germinates on the stigma of some other plant and thus ensures cross pollination. 4. Heterostyly: In primrose, there are different lengths of styles which ensure cross-pollination. 5. Herkogamy (Herko-barrier): In bisexual flowers, e.g. in Salvia and pea, peculiarities of petals ensure insect pollination, thus aids in cross-pollination.
