Fertilization Process

Double fertilization:

• Angiosperms are the most varied category of terrestrial plants, and they produce flowers. Flowers are the reproductive organs of angiosperms, containing separate male and female reproductive organs. Each contains gametes: sperm and egg cells, respectively. 
• S.G. Nawaschin and L.Guignard in 1898 and 1899, observed in Lilium and Fritillaria that both the male gametes released from a male gametophyte are involved in the fertilization. They fertilize two different components of the embryo sac.
• The fusion of one male gamete with an egg and that of another male gamete with two polar nuclei (secondary nucleus). Since both the male gametes are involved in fertilization, the phenomenon is called double fertilization.
• Pollination allows pollen grains to reach the stigma via style. Both sperm cells enter the ovule-synergid cell. This leads to fertilization. 
• In angiosperms, fertilization produces two structures: zygote and endosperm, hence the name "double fertilization." 
• Double fertilization is a complex process in which one of two sperm cells fuses with the egg cell and the other fuses with two polar nuclei, resulting in a diploid (2n) zygote and a triploid (3n) primary endosperm nucleus (PEN).
• The endosperm is referred to as triple fusion since it is made up of three haploid nuclei that fuse together. The main endosperm nucleus develops into the primary endosperm cell (PEC) and then into the endosperm.

Double fertilization

Significance of Double Fertilization:

• Unique to Angiosperms: It ensures that the parent plant invests in a seed with a food store only if the egg is fertilized.
• Diploid Zygote Formation: The diploid zygote develops into an embryo, which eventually becomes a new plant. The triploid PEN develops into nutritive endosperm tissue.
• Restoration of Diploid Condition: Syngamy restores the diploid condition by the fusion of a haploid male gamete with a haploid female gamete.
• Prevention of Polyembryony: It helps avoid polyembryony.

It consists of two processes:

1. Syngamy: One of the male gametes fuses with the egg nucleus (syngamy) to form Zygote which develops to form an embryo.
2. Triple fusion: The second gamete migrates to the central cell where it fuses with the polar nuclei or their fusion product, the secondary nucleus, and forms the primary endosperm nucleus (PEN). Since this involves the fusion of three nuclei, this phenomenon is called triple fusion. This act results in endosperm formation which forms the nutritive tissue for the embryo.
   

   Fertilization in Angiosperms