Question

Describe the process of double fertilization.

Answer 1

Double fertilization:

1. The fusion of one male gamete with an egg and that of another male gamete with a secondary nucleus is called double fertilization.
   It is the characteristic feature of angiosperms.
   It was discovered by Nawaschin in the liliaceous plants like Lilium and Fritillaria.
2. When pollen grain reaches the surface of the stigma, it germinates and forms a pollen tube.
3. The pollen tube penetrates the stigma, style, and ovary chamber and then enters the ovule.
4. The growth of the pollen tube is guided by the chemicals secreted by the synergids.
5. Usually, when a pollen tube enters the ovule through the micropyle, it is termed porogamy.
   But in some cases, it enters through chalaza which is known as chalazogamy. In some plants, it enters by piercing the integuments which are called mesogamy.
6. A pollen tube penetrates the embryo sac of the ovule through its micropylar end.
7. The pollen tube carrying male gametes penetrates in one of the synergids.
8. Watery contents of synergid are absorbed by the pollen tube, due to which it ruptures and releases the contents, including the two non-motile male gametes.
9. As non-motile male gametes are carried through a hollow pollen tube, it is known as siphonogamy that ensures fertilization to take place.
10. Fertilization mainly involves two processes: Syngamy and Triple fusion.
    1. Syngamy:
       It is the fusion of haploid male gamete with a haploid female gamete (egg). It results in the formation of a diploid zygote which develops to form an embryo. Syngamy is a type of generative fertilization.
    2. Triple fusion:
       It is the fusion of a second haploid male gamete with the diploid secondary nucleus. It results in the formation of a Primary Endosperm Nucleus (PEN) which develops into triploid endosperm. Triple fusion is a type of vegetative fertilization.
11. In this process, both the male gametes participate, due to which fertilization occurs twice in the same embryo sac, hence it is described as double fertilization.