Double fertilization means two separate fusion events happen in the same embryo sac at the same time. The question lists four possible descriptions and only one names the correct second partner for the male gametes.
- Fusion of two male gametes with an egg cell: Only one male gamete ever fuses with the egg cell. Sending both male gametes to the egg would leave nothing for the second fusion event, so this cannot be right.
- Fusion of one male gamete with an egg cell and other with synergids: Synergids sit beside the egg and guide the pollen tube in, but they are not fertilized. They degenerate after their job is done, so they are not the second partner.
- Fusion of one male gamete with egg cell and the second with antipodals: Antipodal cells sit at the opposite end of the embryo sac and take no part in fertilization; they usually degenerate later. So this pairing is also wrong.
- Fusion of one male gamete with egg cell and the second with central cell: One male gamete fuses with the egg cell to form the zygote (syngamy). The second male gamete fuses with the central cell, which carries two polar nuclei, to form the triploid primary endosperm nucleus (triple fusion). Together these two fusions are what double fertilization refers to.
Since only the central cell actually takes part in a fertilization event alongside the egg cell, the fourth option correctly describes double fertilization.
Let's summarize:
- Syngamy: male gamete plus egg cell, forms the zygote.
- Triple fusion: male gamete plus central cell (two polar nuclei), forms the endosperm nucleus.
- Synergids and antipodals play no part in fertilization.
So double fertilization is the fusion of one male gamete with the egg cell and the second with the central cell.