Question

A dioecious flowering plant prevents both
A. Cleistogamy and xenogamy
B. Autogamy and xenogamy
C. Autogamy and geitonogamy
D. Geitonogamy and xenogamy

Answer 1

Hint: Numerous sorts of pollination are seen in nature depending on the environment and plant requirements. Dioecious plants have male and female flowers on separate plants and thus, require cross-pollination.

Complete answer: Autogamy and geitonogamy are also prevented by dioecious plants. Autogamy is a mixing process between two gametes that are of the same flower, i.e., self-pollination. It is seen in many flowering plants, particularly in monoecious plants. Dioecious species, however, have male and female organs in two different flowers of two different plants that avoid autogamy. Geitonogamy, on the other hand, refers to the form of pollination that occurs on the same plant, but between two different flowers. It is also a type of autogamy-like self-pollination. In various species, dioecious plants possess their gametes and can thus also avoid geitonogamy. Cleistogamy occurs in some plants and uses non-opening flowers to propagate. It is really effective, so very few plant resources are needed to produce this form of pollination. Therefore, for undesirable places and adverse circumstances, it is fine.
Xenogamy refers to the type of pollination that occurs from a plant's anther to a separate plant's stigma. In dioecious plants, this happens and is responsible for producing genetic variations in offspring.
Hence, the correct answer is option C.

Note: Every method of pollination has its own environmental advantages. For instance, autogamy allows plants to reproduce even without the aid of pollinating agents. Cleistogamy helps in unfavourable conditions to effectively pollinate and, last but not least, xenogamy causes variations in the offspring that are helpful for the evolution process.