In which of the following plants are both autogamy and geitonogamy prevented?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Papaya
Autogamy is the transfer of pollen to the stigma of the same flower. Geitonogamy is the transfer of pollen to the stigma of another flower on the same plant. Both are prevented only when a plant bears flowers of one sex only, so that its own pollen has no stigma to reach on that plant.
Option (1) Wheat — Incorrect. Wheat bears bisexual flowers, with stamens and pistil in the same flower. Its pollen can reach the stigma of the same flower or of another flower on the same plant, so neither autogamy nor geitonogamy is prevented.
Option (2) Papaya — Correct. Papaya is dioecious: male and female flowers are borne on different plants, so each plant is either male or female. A male plant has no stigma and a female plant has no pollen, so both autogamy and geitonogamy are ruled out.
Option (3) Maize — Incorrect. Maize is monoecious: male and female flowers are on the same plant. Because the flowers are unisexual, autogamy cannot occur, but pollen can still reach a female flower of the same plant, so geitonogamy is possible.
Option (4) Castor — Incorrect. Castor is also monoecious, like maize. Its unisexual flowers prevent autogamy, but geitonogamy can still take place within the plant.
NCERT Extract:
Pollination – “… Another device to prevent self-pollination is the production of unisexual flowers. If both male and female flowers are present on the same plant such as castor and maize (monoecious), it prevents autogamy but not geitonogamy. In several species such as papaya, male and female flowers are present on different plants, that is each plant is either male or female (dioecy). This condition prevents both autogamy and geitonogamy.”