What is double fertilization? Describe it with the help of a neat and well labelled diagram. Give its importance.

Double fertilisation:

Male gametophyte produces two male gametes in angiosperms, out of them one unites with the female gamete and the other unites with the secondary nucleus. It is called double fertilisation as both gametes take part in fertilisation and fertilisation occurs twice. During double fertilisation, pollen tube with two male gametes and tube nucleus enters the embryo sac on reaching the ovule, through the micropylar end. After this, the tip of pollen tube bursts and two male gametes are discharged. Out of these two male gametes, one unites with the egg of female and the other unites with the secondary nucleus of the embryo sac, forming triploid or triple fusion nucleus, called the primary endosperm nucleus. The other male gamete fuses with the egg nucleus causing fertilisation, resulting in the formation of a diploid zygote or diploid oospore. This is called syngamy. The process involving the fusion of haploid nucleus of male gamete with the diploid secondary nucleus to form a triploid primary endosperm nucleus is called triple fusion.

The significance of double fertilisation:

(1) Syngamy results in formation of zygote which gives rise to the embryo
(2) Double fertilisation results in the formation of triploid endosperm which ensures, better nourishment of the developing embryo.
(3) Seeds are viable with a high percentage of germination.
(4) Double fertilisation brings about the recombination of characters.