Question

Write down the steps of double fertilization​

Answer 1

Double fertilization is a process unique to flowering plants (angiosperms) in which two sperm cells fertilize two different structures within the female reproductive system. The steps of double fertilization are:

1. Pollination: transfer of pollen grains from the male to the female flower
2. Germination: the pollen grain forms a pollen tube and grows down the style towards the ovule
3. Syngamy: the first sperm cell fuses with the egg cell to form a diploid zygote (2n)
4. Endosperm formation: the second sperm cell fuses with two polar nuclei to form a triploid cell (3n) which undergoes cell division to form the endosperm
5. Embryo formation: the zygote undergoes several rounds of cell division and differentiation to form the embryo

Double fertilization leads to the formation of both the embryo and endosperm within the ovule, which ultimately results in the development of a seed.

Answer 2

Steps of Double Fertilization.

• It was discovered by S.G. Nawaschin (1897) Lilium are Fertillaria sps.

1. Germination of pollen on stigma :

(i) By absorbing secretion of stigma pollen swells up and outer layer exine ruptures whereas inner lintine comes out in the form of pollen tube.

(ii) Branched polen tube is formed in amentiferae group.

(iii) In families malvaceae, cucurbitaceae, etc. polysiphonous condition (many pollen tubes from a single pollen) is found.

2. Path taken up by pollen tube :

(i) Pollen tube pierces through stigmatic papilla into the tissue of style.

(ii) Filiform apparatus of synergids secrete some chemotropically active substances, which promotes porogamy.

(iii) Obturator (special structure) act as a bridge for growth of pollen tube.

(iv) Cations are naturally occurring chemotropic agents in the pistil.

3. Entry of pollen tube into embryo sac :

(i) It enters into the ovule by either of the two methods; porogamy, chalazogamy or mesogamy.

(ii) Irrespective of the mode of entry of pollen tube into the ovule, the pollen tube always enters into embryo sac at micropylar end (Porogany).

(iii) If enters through chalaza (base) it is termed chalazogany.

(iv) If pierces through integuments, it is called mesogamy.

(v) Tip of pollen tube penetrates embryo sac and reaches egg apparatus.

4. Syngamy (Generative fertilization)

(i) Pollen tube burst as tube nucleus disorganises.

(ii) One of male gametes fuses with egg cell or oosphere.

(iii) Deploid oospore or zygote is formed.

5. Triple fusion (Vegetative fertilization) :

The other gamete fuses with secondary nucleus forming triploid primary endosperm (PEN) which later give rise to endosperm.