[5]
(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.

[2]
(a) Two devices in a flowering plant which prevents both autogamy and geitonogamy are
(i) Dioecy or production of unisexual flowers in different plants
(ii) Dichogamy, that is maturation of the male and female flowers at different times.


[3]
(b) The pollen tube releases two male gametes.
(i) One male gamete fuses with the egg cell (nuclei), resulting in a diploid zygote. This process is called syngamy. This results in the formation of a diploid cell, the zygote.
(ii) The other male gamete fuses with the polar nuclei (triple fusion), located in the central cell to form a triploid primary endosperm nucleus (PEN).
(iii) Since two types of fusions, syngamy and triple fusion take place in an embryo sac; the phenomenon is termed double fertilisation.