Biology · Sexual Reproduction in Flowering Plants · NEET
Self-pollination is easy and reliable, but if it continues over many generations it causes inbreeding depression, meaning the offspring become weaker and less able to survive. Cross-pollination mixes genes from two plants, which gives more variation and stronger, healthier offspring. So plants sacrifice the easy option to get the healthier one.
Both prevent autogamy (self-pollen in the same flower). Dichogamy is a TIMING device: the pollen and the stigma of the same flower mature at different times, so they never meet. Herkogamy is a POSITION device: the anther and the stigma are placed at different spots in the flower, so pollen cannot reach its own stigma. Remember: dichogamy = time, herkogamy = place.
These are two types of dichogamy. In protandry the anther matures and releases pollen BEFORE the stigma of the same flower becomes receptive. In protogyny the stigma becomes receptive BEFORE the pollen is released. In both cases the two are out of sync, so the flower cannot pollinate itself.
No. Monoecy (male and female flowers on the SAME plant, like castor and maize) prevents autogamy (self-pollen within one flower) but NOT geitonogamy, because pollen can still travel from a male flower to a female flower on the same plant. Only dioecy (male and female flowers on DIFFERENT plants, like papaya) prevents BOTH autogamy and geitonogamy. This exact point is a favourite NEET trap.
No. Self-incompatibility is a GENETIC mechanism, not a physical one. Even if self-pollen lands on the stigma, the pistil recognises it as 'self' and stops it, either by inhibiting pollen germination or by blocking pollen tube growth in the pistil. So fertilisation of the ovule does not happen.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
They are: (1) dichogamy (pollen release and stigma receptivity not synchronised), (2) herkogamy (anther and stigma at different positions), (3) self-incompatibility (a genetic block on self-pollen), and (4) unisexual flowers, that is monoecy and dioecy.
Dioecy, where male and female flowers are on separate plants (example: papaya). Monoecy prevents only autogamy, not geitonogamy.
It leads to inbreeding depression, where offspring become progressively weaker because there is no mixing of genes from different parents.
Monoecious: castor and maize (both sexes on the same plant). Dioecious: papaya (each plant is either male or female).
The pistil recognises self-pollen and inhibits its germination or blocks the growth of its pollen tube, so the self-pollen cannot fertilise the ovules.
