Question

The pollen tube usually enters the embryo sac
A. Between the egg cell and synergid
B.By directly penetrating the egg
C. Between one synergids and antipodal cell
D. By knocking off the antipodals

Answer 1

Embryo sac is a thin-walled sac which has nucleus and eggs. It is a female part of a plant. They are divided into three types: monosporic, bisporic and tetrasporic.

Complete answer:
The embryo sac is a eight-nucleate, seven celled female gametophyte. It has a micropylar end in the lower region. Where there are two synergids and one female egg cell. On the top region, there is an egg apparatus, which has three antipodal cells. In the middle, there is one central cell, which has two polar nuclei.

Double fertilization occurs by two male gametes. One male gamete fuses with the female egg cell in the micropylar end. This is called syngamy. Another male gamete fuses with the central cell possessing two polar nuclei. This is called triple fusion.

The pollen tube which has male gametes enters the embryo sac through the nucleus and then from the micropylar end it penetrates , between the egg cell and synergid. First into the synergid and then into the egg cell. The filiform apparatus present in the synergid redirects the movement of the pollen tube into the egg cell. Therefore, the correct answer is between the egg cell and synergids.

Option B: The pollen tube does not directly penetrate the egg. Therefore, this is the incorrect option.
Option C: Synergids are at the lower end and antipodals at the upper. Therefore, this is the incorrect option.
Option D: The antipodals present in the upper region are not knocked off. Therefore, this is the incorrect option.

Hence the correct answer is option ‘A’.

Note: The three cells present at the chalazal end of the embryo sac are antipodal cells. The ploidy of embryo sac is haploid. The chalazal end is present in the upper region. Pollen tubes growing in the style perceive complex and essential signals sent from the gynoecium for nutrients.