Q.List three strategies that a bisexual chasmogamous flower can evolve to prevent self pollination (autogamy).

A bisexual chasmogamous flower exposes its reproductive parts to the environment, so it must use physical or temporal barriers to avoid self-pollen landing on its own stigma. The NCERT textbook describes three clear strategies:

• Dichogamy – the anthers and stigma mature at different times. If the anthers release pollen before the stigma is receptive (protandry), or the stigma becomes receptive before the anthers dehisce (protogyny), self-pollination is mechanically prevented.
• Herkogamy – the anthers and stigma are placed at different positions within the same flower, so that pollen cannot easily fall or be transferred onto its own stigma. …

To understand these strategies, we first need to picture the flower’s situation. A bisexual (or hermaphrodite) flower has both male and female reproductive organs — stamens and pistil — in the same bloom. A chasmogamous flower is one that opens fully, exposing its reproductive parts to the environment. This openness is an invitation to pollinators and wind, but it also creates a risk: pollen from the same flower’s anthers can easily land on its own stigma. That is autogamy, or self-pollination. While self-pollination guarantees reproduction, it reduces genetic diversity and can lead to inbreeding depression. Evolution has therefore favoured mechanisms that reduce or prevent autogamy, while still allowing cross-pollination.

The first strategy is dichogamy, which means the male and female reproductive parts mature at different times. In a protandrous flower, the anthers release pollen before the stigma becomes receptive. By the time the stigma is ready, the flower’s own pollen is already gone — either shed or removed by pollinators. In a protogynous flower, the opposite happens: the stigma becomes receptive first, and only later do the anthers dehisce. In both cases, the flower cannot pollinate itself because the two sexual phases do not overlap. This is a simple, elegant temporal barrier.

The second strategy is herkogamy, a spatial separation. Here, the anthers and stigma are positioned in such a way that pollen from the same flower cannot easily reach its own stigma. For example, in some flowers the stigma is held far above the anthers, or the anthers are arranged in a ring around the style but the stigma is tucked away. Pollinators brushing against the flower may pick up pollen from the anthers and deposit it on the stigma of a different flower, but the flower’s own pollen cannot fall or be shaken onto its own stigma. This physical barrier is common in many familiar garden flowers.

The third and most sophisticated strategy is self-incompatibility (SI). This is a genetic mechanism, not a physical or temporal one. The flower’s pistil can recognise pollen that comes from the same plant (or from a genetically identical plant) and actively block its growth. If self-pollen lands on the stigma, the pollen tube either fails to germinate or is stopped before it reaches the ovule. This is controlled by a set of genes called the S-locus. Self-incompatibility is widespread in flowering plants — it is found in about half of all angiosperm families, including brassicas, solanaceae, and poppies. It is a fail-safe: even if self-pollen arrives, it is rejected. …