Explain why sexual reproduction in angiosperms is said to take place through double fertilization and triple fusion. Also draw a labelled diagram of the embryo sac to explain the phenomena.

Male sex organ:

The male sex organ in angiosperms is the stamen which consists of a slender filament with an anther at the tip. The microspore mother cells present inside anthers produce pollen grains after undergoing meiosis.

Female sex organ:

The female sex organ in angiosperms is the pistil or the carpel. The swollen base of the pistil is called the ovary, inside of which are present ovules. The embryo-sac is formed by meiosis in the megaspore mother cells. So, each of the cells of an embryo-sac is haploid. Each embryo-sac has a three-celled egg apparatus (consisting of one egg cell and two synergids), three antipodal cells and two polar nuclei.

Fertilisation:

Pollen grains are carried by wind or various other agencies to the stigma of a pistil. This is called pollination. The pollen grains germinate to form a pollen tube that grows through the stigma and style and reach the ovary. The pollen tubes enter the embryo-sac and two male gametes (formed by mitosis in the generative cell of the pollen grain) are discharged.

• One of the male gametes fuses with the egg cell to form a zygote (syngamy).
• The other male gamete fuses with the two polar nuclei to form a triploid primary endosperm nucleus (3n).

Double fertilisation:

The process involving two fusion events is called double fertilisation.

Triple fusion::

Since the formation of triploid primary endosperm nucleus requires the fusion of three haploid nuclei [diploid secondary nucleus (formed due to the fusion of two haploid polar nuclei) and a haploid male gamete], it is called triple fusion.

Diagram of embryo-sac: