Question

A. Describe any two devices in a flowering plant which prevent both autogamy and geitonogamy.
B. Explain the events upto double fertilization after the pollen tube enters one of the synergids in an ovule of an angiosperm.

Answer 1

Sexual reproduction in flowering plants includes the production of male and female gametes and the transfer of male gamete to the female gamete via a process known as pollination.

Complete answer:
A. Autogamy (or self-fertilization) refers to the fertilization or pollination of a flower by its pollen grains, while geitonogamy refers to the pollination of a flower by the pollen grains of another flower situated on the same plant.
Two devices to prevent autogamy and geitonogamy involve Dichogamy and self-incompatibility.
1. Dichogamy: This is a cross-pollination promoting device; in this process bisexual flower promotes cross-pollination by maturing stamens and carpels at different times, which is mainly of two type’s i.e.
(i) Protandry- Stamens mature earlier than carpels for example in jasmine, sunflower, etc.
(ii) Protgyny- Carpels mature earlier than stamens for example in rose, Ashoka, etc.
2. Self-incompatibility: It is a mechanism that prevents flowers from self-fertilization by preventing pollen grains from one flower from fertilizing another flower of the same plant. This mechanism is genetically controlled and encourages outcross.

B. Events that occur upto Double-fertilization:
Pollen grains when falls on the stigma they start germinating and give rise to the pollen tube which passes through the style and finally enters into the female ovule. Then the pollen tube enters one of the synergids and releases out the male gametes. Out of these two male gametes, one male gamete fuses with the egg cell nucleus resulting in the formation of a zygote, and this process is known as syngamy. Other male gamete fuses with the two polar nuclei which are located in the central cell which forms a triploid primary endosperm nucleus or PEN. As this process involves the fusion of three haploid nuclei it is also known as triple fusion. Two types of fusions, syngamy and triple fusion in the embryo, lead to Double-fertilization.

Note: Cross-pollination or out-crossing allows more genetic variation or diversity and seeds which are produced are more viable than the self-fertilization as it leads to uniform progeny.