(a) A pollen grain contains two male gametes. When a pollen grain falls on the stigma of the carpel, it grows a pollen tube downwards into the style. One of the male gametes reaches the ovary through pollen tube and fuses with egg to form zygote.
The other male gamete fuses with two polar nuclei one from each end of embryo sac to from endosperm nucleus. This fusion is called triple fusion. The zygote grows into an embryo and an endosperm nucleus grows into endosperm (the food storage tissue of seed).
(b) Sperms are produced much more in number than eggs because:
(i) Eggs are non-motile whereas sperms are motile and they may be misdirected while they are travelling towards eggs.
(ii) Sperms may not be able to survive under external environmental conditions for long.
(iii) To increase the probability of fertilisation of eggs.