Question

Which of the following is prevented by unisexual flowers that is present on different plants?

• A. Autogamy and Geitonogamy
• B. Geitonogamy and Xenogamy
• C. Autogamy but not Geitonogamy
• D. Geitonogamy but not Xenogamy

Hint:

Dioecious plants (male & female on separate plants) promote outcrossing; they inherently prevent selfing and within-plant pollen transfer.
Differentiate autogamy (within same flower), geitonogamy (between flowers of same plant), and xenogamy (between different plants).
Dioecy = strong mechanism to ensure xenogamy.

Answer 1

The Correct Option is A

1. Unisexual flowers borne on different plants (dioecious condition) mean an individual plant bears only male or only female flowers.
2. Autogamy (self-pollination within the same flower) is impossible because there is no bisexual flower on a plant.
3. Geitonogamy (transfer of pollen between different flowers of the same plant) is also prevented because all flowers on a plant are of the same sex — a female plant has no pollen to donate and a male plant has no ovules to receive pollen.
4. Xenogamy (cross-pollination between different plants) is still possible and, in fact, required for fertilization in dioecious species.
5. Hence, unisexual flowers on different plants prevent both autogamy and geitonogamy. Therefore the correct answer is (1) Autogamy and Geitonogamy.