Question

What is meant by double fertilisation?
A. Fusion between egg and male gamete
B. Fusion between secondary nucleus and male gamete
C. Both (A) and (B)
D. Formation of two zygotes

Answer 1

Hint:- After pollination, pollen grains germinate on stigma. They absorb moisture and sugar content from stigma and swell up. The inner layer of pollen grain called as intine grows out through pores of an outer layer called exine in the form of tube-like outgrowth which is called as pollen tube.
There are two nuclei present in the pollen grain at this time. During the formation of the pollen tube, the vegetative nucleus occupies the tip of the pollen tube. Meanwhile, the generative nucleus divides to form two male gametes.

Complete Answer:- The two male gametes produced by the generative nucleus are completely non-motile. When the pollen tube enters the ovary, at that time the ovule becomes matured. After the entrance inside the ovule, the pollen tube grows towards the egg apparatus because synergid cells secrete the chemicals to guide the tube.
Through the egg apparatus, a pollen tube enters into the embryo sac. The pollen tube releases all its contents including both male gametes inside the embryo sac. During fertilisation, the two polar nuclei of embryo sac fuse together to form a diploid nucleus (secondary nucleus).
Out of two, one male gamete fertilizes the egg cell to form a diploid zygote. This fusion is known as true fertilisation. The second male gamete gets fused with the diploid secondary nucleus. This fusion is known as triple fusion. Thus in angiosperms both true Fertilisation and triple fusion are present.
Double fertilisation and the triple fusion are the specific or universal characteristic of angiosperms. It is an event unique to flowering plants.

Thus the correct answer is option (C).

Note:- A zygote is formed by true fertilisation which develops into an embryo. A triploid primary endosperm nucleus is formed by triple fusion. The central cell after triple fusion becomes the primary endosperm cell and develops into the endosperm which is used as nutrition for growing embryos.
This type of fertilisation in which non-motile gametes are carried to the female gamete through pollen tube is called siphonogamy.