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

a) Dichogamy and self incompatibility help plants in preventing autogamy and geitonogamy . Dichogamy is the difference in maturation period of stamens and stigma.
b) After reaching the synergids, male gametes are released into the cytoplasm. One of the male gametes fuses with egg and the other fuses with polar nuclei leading to endosperm nucleus called triple fusion.