NCERT · Class 12 · Biology · Sexual Reproduction in Flowering PlantsCompare and contrast self-pollination (autogamy and geitonogamy) and cross-pollination (xenogamy). Discuss the various outbreeding devices that flowering plants have evolved to discourage self-pollination and encourage cross-pollination.

Introduction to Pollination Types\nPollination is the transfer of pollen grains from the anther to the stigma of a flower. Based on the source of pollen, pollination is broadly divided into self-pollination (transfer within the same plant) and cross-pollination (transfer between different plants of the same species).

Comparison of Pollination Types

• Autogamy (Self-Pollination within the same flower): Transfer of pollen grains from the anther to the stigma of the same flower. It requires synchrony in pollen release and stigma receptivity. Examples include Commelina, Oxalis, and Viola (which produce cleistogamous flowers).
• Geitonogamy: Transfer of pollen grains from the anther to the stigma of another flower of the same plant. Functionally, it is cross-pollination involving a pollinating agent, but genetically it is similar to self-pollination since the pollen comes from the same plant.
• Xenogamy (Cross-Pollination): Transfer of pollen grains from the anther to the stigma of a genetically different plant of the same species. It brings about genetic variations and introduces new combinations of characters in the offspring.

Outbreeding Devices (Mechanisms to Prevent Self-Pollination)\nSince continuous self-pollination leads to inbreeding depression, flowering plants have evolved several devices to discourage self-pollination:

• Non-synchronization (Dichogamy): The pollen release and stigma receptivity are not synchronized. Either the anthers mature before the stigma (protandry, e.g., sunflower) or the stigma becomes receptive before pollen release (protogyny, e.g., Gloriosa).
• Spatial Separation (Chasmogamy/Herkogamy): In some flowers, the anthers and stigma are placed at different positions, making self-pollination physically impossible without specialized agents.
• Self-Incompatibility: A genetic mechanism that prevents pollen germination or pollen tube growth in the pistil of the same flower or other flowers of the same plant. It is the most effective and widespread device.
• Production of Unisexual Flowers: If both male and female flowers are present on the same plant (monoecious, e.g., castor and maize), it prevents autogamy but not geitonogamy. If male and female flowers are on different plants (dioecious, e.g., papaya), both autogamy and geitonogamy are completely prevented.